:- YAP Prolog User's Manual

@set VERSION: 5.1.1

This file documents the YAP Prolog System version No value for VERSION, a high-performance Prolog compiler developed at LIACC, Universidade do Porto. YAP is based on David H. D. Warren's WAM (Warren Abstract Machine), with several optimizations for better performance. YAP follows the Edinburgh tradition, and is largely compatible with DEC-10 Prolog, Quintus Prolog, and especially with C-Prolog.

This file contains extracts of the SWI-Prolog manual, as written by Jan Wielemaker. Our thanks to the author for his kind permission in allowing us to include his text in this document.

Introduction
1. Installing YAP		Installation
2. Running YAP
3. Syntax		The syntax of YAP
4. Loading Programs		Loading Prolog programs
5. The Module System		Using Modules in YAP
6. Built-In Predicates
7. Library Predicates
8. SWI-Prolog Emulation		SWI-Prolog emulation
9. Extensions to Prolog		Extensions to Standard YAP
9.1 Rational Trees		Working with Rational Trees
9.2 Coroutining		Changing the Execution of Goals
10. Attributed Variables		Using attributed Variables
11. Constraint Logic Programming over Reals		The CLP(R) System
12. CHR: Constraint Handling Rules		The CHR System
13. Logtalk		The Logtalk Object-Oriented System
14. Threads		Thread Library
15. Parallelism		Running in Or-Parallel
16. Tabling		Storing Intermediate Solutions of programs
18. Profiling the Abstract Machine		Profiling Abstract Machine Instructions
17. Tracing at Low Level		Tracing at Abstract Machine Level
19. Debugging		Using the Debugger
20. Indexing		Efficiency Considerations
21. C Language interface to YAP		Interfacing predicates written in C
22. Using YAP as a Library		Using YAP as a library in other programs
23. Compatibility with Other Prolog systems		Compatibility with other Prolog systems
Predicate Index		An item for each predicate
Concept Index		An item for each concept
Built In Predicates
6.1 Control Predicates		Controlling the execution of Prolog programs
6.2 Handling Undefined Procedures		Handling calls to Undefined Procedures
6.3 Predicates on terms		Predicates on Terms
6.4 Comparing Terms		Comparison of Terms
6.5 Arithmetic		Arithmetic in YAP
6.6 I/O Predicates		Input/Output with YAP
6.7 Using the Clausal Data Base		Modifying Prolog's Database
6.10 Collecting Solutions to a Goal		Finding All Possible Solutions
6.11 Grammar Rules
6.17 Predicate Information
6.12 Access to Operating System Functionality
6.13 Term Modification		Updating Prolog Terms
6.14 Profiling Prolog Programs		Profiling Prolog Execution
6.15 Counting Calls		Limiting the Maximum Number of Reductions
6.16 Arrays		Supporting Global and Local Arrays
6.17 Predicate Information		Information on Predicates
6.18 Miscellaneous		Miscellaneous Predicates
Subnodes of Running
2.1 Running Yap Interactively		Interacting with Yap
2.2 Running Prolog Files		Running Prolog files as scripts
Subnodes of Syntax
3.1 Syntax of Terms
3.2 Prolog Tokens		Syntax of Prolog tokens
Subnodes of Tokens
3.2.1 Numbers		Integer and Floating-Point Numbers
3.2.2 Character Strings		Sequences of Characters
3.2.3 Atoms		Atomic Constants
3.2.4 Variables		Logical Variables
3.2.5 Punctuation Tokens		Tokens that separate other tokens
3.2.6 Layout		Comments and Other Layout Rules
Subnodes of Numbers
3.2.1.1 Integers		How Integers are read and represented
3.2.1.2 Floating-point Numbers		Floating Point Numbers
Subnodes of Loading Programs
4.1 Program loading and updating		Program Loading and Updating
4.2 Changing the Compiler's Behavior		Changing the compiler's parameters
4.3 Saving and Loading Prolog States		Saving and Restoring Programs
Subnodes of Modules
5.1 Module Concepts		The Key Ideas in Modules
5.2 Defining a New Module		How To Define a New Module
5.3 Using Modules		How to Use a Module
5.4 Meta-Predicates in Modules		How to Handle New Meta-Predicates
Subnodes of Input/Output
6.6.1 Handling Streams and Files
6.6.2 Handling Streams and Files		C-Prolog Compatible File Handling
6.6.3 Handling Input/Output of Terms		Input/Output of terms
6.6.4 Handling Input/Output of Characters		Input/Output of Characters
6.6.5 Input/Output Predicates applied to Streams		Input/Output using Streams
6.6.6 Compatible C-Prolog predicates for Terminal I/O		C-Prolog compatible Character I/O to terminal
6.6.7 Controlling Input/Output		Controlling your Input/Output
6.6.8 Using Sockets From Yap		Using Sockets from YAP
Subnodes of Database
6.7.1 Modification of the Data Base		Asserting and Retracting
6.7.2 Looking at the Data Base		Finding out what is in the Data Base
6.7.3 Using Data Base References
6.8 Internal Data Base		YAP's Internal Database
6.9 The Blackboard		Storing and Fetching Terms in the BlackBoard
Subnodes of Library
7.1 Apply Macros		Apply a Predicate to a list or to sub-terms.
7.2 Association Lists		Binary Tree Implementation of Association Lists.
7.3 AVL Trees		Predicates to add and lookup balanced binary trees.
7.4 Heaps		Labelled binary tree where the key of each node is less than or equal to the keys of its children.
7.5 List Manipulation
7.6 Ordered Sets		Ordered Set Manipulation
7.7 Pseudo Random Number Integer Generator		Pseudo Random Numbers
7.8 Queues		Queue Manipulation
7.9 Random Number Generator		Random Numbers
7.10 Red-Black Trees		Predicates to add, lookup and delete in red-black binary trees.
7.11 Regular Expressions		Regular Expression Manipulation
7.12 Splay Trees
7.13 Reading From and Writing To Strings		Writing To and Reading From Strings
7.14 Calling The Operating System from YAP		System Utilities
7.15 Utilities On Terms		Utilities on Terms
7.16 Call Cleanup		Call With registered Cleanup Calls
7.17 Calls With Timeout		Call With Timeout
7.18 Updatable Binary Trees
7.19 Unweighted Graphs
7.20 Directed Graphs		Directed Graphs Implemented With Red-Black Trees
7.21 Undirected Graphs		Undirected Graphs Using DGraphs
Subnodes of Debugging
19.1 Debugging Predicates
19.2 Interacting with the debugger
Subnodes of Compatibility
23.1 Compatibility with the C-Prolog interpreter
23.2 Compatibility with the Quintus and SICStus Prolog systems
23.3 Compatibility with the ISO Prolog standard
Subnodes of Attributes
10.1 Attribute Declarations		Declaring New Attributes
10.2 Attribute Manipulation		Setting and Reading Attributes
10.3 Attributed Unification		Tuning the Unification Algorithm
10.4 Displaying Attributes		Displaying Attributes in User-Readable Form
10.5 Projecting Attributes		Obtaining the Attributes of Interest
10.6 Attribute Examples		Two Simple Examples of how to use Attributes.
Subnodes of SWI-Prolog
8.1 Invoking Predicates on all Members of a List		maplist and friends
8.2 Forall		forall built-in
8.3 hProlog and SWI-Prolog Attributed Variables		Emulating SWI-like attributed variables
8.4 SWI Global variables		Emulating SWI-like attributed variables
Subnodes of CLPR
11.1 Solver Predicates
11.2 Syntax of the predicate arguments
11.3 Use of unification
11.4 Non-Linear Constraints
Subnodes of CHR
12.1 Introduction
12.2 Syntax and Semantics
12.3 CHR in YAP Programs
12.4 Debugging
12.5 Examples
12.6 Compatibility with SICStus CHR
12.7 Guidelines
Subnodes of C-Interface
21.1 Terms		Primitives available to the C programmer
21.2 Unification		How to Unify Two Prolog Terms
21.3 Strings		From character arrays to Lists of codes and back
21.4 Memory Allocation		Stealing Memory From Yap
21.5 Controlling Yap Streams from C		Control How Yap sees Streams
21.6 From C back to Prolog		From C to Yap to C to Yap
21.7 Writing predicates in C		Writing Predicates in C
21.8 Loading Object Files
21.9 Saving and Restoring
21.10 Changes to the C-Interface in Yap4		Changes in Foreign Predicates Interface
Subnodes of C-Prolog
23.1.1 Major Differences between YAP and C-Prolog.
23.1.2 Yap predicates fully compatible with C-Prolog		Yap predicates fully compatible with
C-Prolog
23.1.3 Yap predicates not strictly compatible with C-Prolog		Yap predicates not strictly as C-Prolog
23.1.4 Yap predicates not available in C-Prolog
23.1.5 Yap predicates not available in C-Prolog		C-Prolog predicates not available in YAP
Subnodes of SICStus Prolog
23.2.1 Major Differences between YAP and SICStus Prolog.
23.2.2 Yap predicates fully compatible with SICStus Prolog		Yap predicates fully compatible with
SICStus Prolog
23.2.3 Yap predicates not strictly compatible with SICStus Prolog		Yap predicates not strictly as
SICStus Prolog
23.2.4 Yap predicates not available in SICStus Prolog
Tables
A. Summary of Yap Predefined Operators		Predefined operators

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Introduction

This document provides User information on version No value for VERSION of YAP (yet another prolog). The YAP Prolog System is a high-performance Prolog compiler developed at LIACC, Universidade do Porto. YAP provides several important features:

• Speed: YAP is widely considered one of the fastest available Prolog systems.
• Functionality: it supports stream I/O, sockets, modules, exceptions, Prolog debugger, C-interface, dynamic code, internal database, DCGs, saved states, co-routining, arrays.
• We explicitly allow both commercial and non-commercial use of YAP.

YAP is based on the David H. D. Warren's WAM (Warren Abstract Machine), with several optimizations for better performance. YAP follows the Edinburgh tradition, and was originally designed to be largely compatible with DEC-10 Prolog, Quintus Prolog, and especially with C-Prolog.

YAP implements most of the ISO-Prolog standard. We are striving at full compatibility, and the manual describes what is still missing. The manual also includes a (largely incomplete) comparison with SICStus Prolog.

The document is intended neither as an introduction to Prolog nor to the implementation aspects of the compiler. A good introduction to programming in Prolog is the book The Art of Prolog, by L. Sterling and E. Shapiro, published by "The MIT Press, Cambridge MA". Other references should include the classical Programming in Prolog, by W.F. Clocksin and C.S. Mellish, published by Springer-Verlag.

YAP 4.3 is known to build with many versions of gcc (<= gcc-2.7.2, >= gcc-2.8.1, >= egcs-1.0.1, gcc-2.95.*) and on a variety of Unixen: SunOS 4.1, Solaris 2.*, Irix 5.2, HP-UX 10, Dec Alpha Unix, Linux 1.2 and Linux 2.* (RedHat 4.0 thru 5.2, Debian 2.*) in both the x86 and alpha platforms. It has been built on Windows NT 4.0 using Cygwin from Cygnus Solutions (see README.nt) and using Visual C++ 6.0.

The overall copyright and permission notice for YAP4.3 can be found in the Artistic file in this directory. YAP follows the Perl Artistic license, and it is thus non-copylefted freeware.

If you have a question about this software, desire to add code, found a bug, want to request a feature, or wonder how to get further assistance, please send e-mail to yappers@ncc.up.pt. To subscribe to the mailing list, send a request to majordomo@ncc.up.pt with body "subscribe yappers".

Online documentation is available for YAP at:

http://www.ncc.up.pt/~vsc/Yap/

Recent versions of Yap, including both source and selected binaries, can be found from this same URL.

This manual was written by Vítor Santos Costa, Luís Damas, Rogério Reis, and Rúben Azevedo. The manual is largely based on the DECsystem-10 Prolog User's Manual by D.L. Bowen, L. Byrd, F. C. N. Pereira, L. M. Pereira, and D. H. D. Warren. We have also used comments from the Edinburgh Prolog library written by R. O'Keefe. We would also like to gratefully acknowledge the contributions from Ashwin Srinivasian.

We are happy to include in YAP several excellent packages developed under separate licenses. Our thanks to the authors for their kind authorization to include these packages.

The packages are, in alphabetical order:

• The CHR package developed by Tom Schrijvers, Christian Holzbaur, and Jan Wielemaker.
• The CLP(R) package developed Leslie De Koninck, Bart Demoen, Tom Schrijvers and Jan Wielemaker and based on the CLP(Q,R) implementation by Christian Holzbauer.
• The Logtalk Object-Oriented system is developed at the University of Beira Interior, Portugal, by Paulo Moura. The package is distributed under the Perl Artistic License. Instructions about loading this package are included in this document. The documentation on this package (including full installation instructions) is distributed separately from yap.tex.

  Copyright © 1998-2006 Paulo Moura

• The Pillow WEB library developed at Universidad Politecnica de Madrid by the CLIP group. This package is distributed under the FSF's LGPL. Documentation on this package is distributed separately from yap.tex.
• The yap2swi library implements some of the functionality of SWI's PL interface. Please do refer to the SWI-Prolog home page:

  http://www.swi-prolog.org

  for more information on SWI-Prolog and for a detailed description of its foreign interface.

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1. Installing YAP

1.1 Tuning the Functionality of YAP		Tuning the Functionality of YAP Machine
1.2 Tuning YAP for a Particular Machine and Compiler

To compile YAP it should be sufficient to:

1. mkdir ARCH.
2. cd ARCH.
3. ../configure ...options....

   Notice that by default configure gives you a vanilla configuration. For instance, in order to use coroutining and/or CLP you need to do

   ../configure --enable-coroutining ...options...

   Please see section Tuning the Functionality of YAP for extra options.

4. check the Makefile for any extensions or changes you want to make.

   YAP uses autoconf. Recent versions of Yap try to follow GNU conventions on where to place software.

   • The main executable is placed at BINDIR. This executable is actually a script that calls the Prolog engine, stored at LIBDIR.
   • LIBDIR is the directory where libraries are stored. YAPLIBDIR is a subdirectory that contains the Prolog engine and a Prolog library.
   • INCLUDEDIR is used if you want to use Yap as a library.
   • INFODIR is where to store info files. Usually /usr/local/info, /usr/info, or /usr/share/info.
5. make.
6. If the compilation succeeds, try ./yap.
7. If you feel satisfied with the result, do make install.
8. make install-info will create the info files in the standard info directory.
9. make html will create documentation in html format in the predefined directory.

   In most systems you will need to be superuser in order to do make install and make info on the standard directories.

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1.1 Tuning the Functionality of YAP

Compiling Yap with the standard options give you a plain vanilla Prolog. You can tune Yap to include extra functionality by calling configure with the appropriate options:

• --enable-rational-trees=yes gives you support for infinite rational trees.
• --enable-coroutining=yes gives you support for coroutining, including freezing of goals, attributed variables, and constraints. This will also enable support for infinite rational trees.
• --enable-depth-limit=yes allows depth limited evaluation, say for implementing iterative deepening.
• --enable-low-level-tracer=yes allows support for tracing all calls, retries, and backtracks in the system. This can help in debugging your application, but results in performance loss.
• --enable-wam-profile=yes allows profiling of abstract machine instructions. This is useful when developing YAP, should not be so useful for normal users.
• --enable-condor=yes allows using the Condor system that support High Throughput Computing (HTC) on large collections of distributively owned computing resources.
• --enable-tabling=yes allows tabling support. This option is still experimental.
• --enable-parallelism={env-copy,sba,a-cow} allows or-parallelism supported by one of these three forms. This option is still highly experimental.
• --with-gmp[=DIR] give a path to where one can find the GMP library if not installed in the default path.

Next follow machine dependent details:

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1.2 Tuning YAP for a Particular Machine and Compiler

The default options should give you best performance under GCC. Although the system is tuned for this compiler we have been able to compile versions of Yap under lcc in Linux, Sun's cc compiler, IBM's xlc, SGI's cc, and Microsoft's Visual C++ 6.0.

1.3 Tuning YAP for GCC.		Using the GNUCC compiler
1.3.1 Compiling Under Visual C++		Using Microsoft's Visual C++ environment
1.3.2 Compiling Under SGI's cc		Compiling Under SGI's cc

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1.3 Tuning YAP for GCC.

Yap has been developed to take advantage of GCC (but not to depend on it). The major advantage of GCC is threaded code and explicit register reservation.

YAP is set by default to compile with the best compilation flags we know. Even so, a few specific options reduce portability. The option

• --enable-max-performance=yes will try to support the best available flags for a specific architectural model. Currently, the option assumes a recent version of GCC.
• --enable-debug-yap compiles Yap so that it can be debugged by tools such as dbx or gdb.

Here follow a few hints:

On x86 machines the flags:

YAP_EXTRAS= ... -DBP_FREE=1

tells us to use the %bp register (frame-pointer) as the emulator's program counter. This seems to be stable and is now default.

On Sparc/Solaris2 use:

YAP_EXTRAS= ... -mno-app-regs -DOPTIMISE_ALL_REGS_FOR_SPARC=1

and YAP will get two extra registers! This trick does not work on SunOS 4 machines.

Note that versions of GCC can be tweaked to recognize different processors within the same instruction set, e.g. 486, Pentium, and PentiumPro for the x86; or Ultrasparc, and Supersparc for Sparc. Unfortunately, some of these tweaks do may make Yap run slower or not at all in other machines with the same instruction set, so they cannot be made default.

Last, the best options also depends on the version of GCC you are using, and it is a good idea to consult the GCC manual under the menus "Invoking GCC"/"Submodel Options". Specifically, you should check -march=XXX for recent versions of GCC/EGCS. In the case of GCC2.7 and other recent versions of GCC you can check:

486:

In order to take advantage of 486 specific optimizations in GCC 2.7.*:

YAP_EXTRAS= ... -m486 -DBP_FREE=1

Pentium:

YAP_EXTRAS= ... -m486 -malign-loops=2 -malign-jumps=2 \ -malign-functions=2

PentiumPro and other recent Intel and AMD machines:

PentiumPros are known not to require alignment. Check your version of GCC for the best -march option.

Super and UltraSparcs:

YAP_EXTRAS= ... -msupersparc

MIPS: if have a recent machine and you need a 64 bit wide address

space you can use the abi 64 bits or eabi option, as in:

CC="gcc -mabi=64" ./configure --...

Be careful. At least for some versions of GCC, compiling with -g seems to result in broken code.

WIN32: GCC is distributed in the MINGW32 and CYGWIN packages.

The Mingw32 environment is available from the URL:

http://www.mingw.org

You will need to install the msys and mingw packages. You should be able to do configure, make and make install.

If you use mingw32 you may want to search the contributed packages for the gmp multi-precision arithmetic library. If you do setup Yap with gmp note that libgmp.dll must be in the path, otherwise Yap will not be able to execute.

CygWin environment is available from the URL:

http://www.cygwin.com

and mirrors. We suggest using recent versions of the cygwin shell. The compilation steps under the cygwin shell are as follows:

mkdir cyg $YAPSRC/configure --enable-coroutining \\ --enable-depth-limit \\ --enable-max-performance make make install

By default, Yap will use the --enable-cygwin=no option to disable the use of the cygwin dll and to enable the mingw32 subsystem instead. Yap thus will not need the cygwin dll. It instead accesses the system's CRTDLL.DLL C run time library supplied with Win32 platforms through the mingw32 interface. Note that some older WIN95 systems may not have CRTDLL.DLL, in this case it should be sufficient to import the file from a newer WIN95 or WIN98 machine.

You should check the default installation path which is set to /Yap in the standard Makefile. This string will usually be expanded into c:\Yap by Windows.

The cygwin environment does not provide gmp. You can fetch a dll for the gmp library from http://www.sf.net/projects/mingwrep.

It is also possible to configure Yap to be a part of the cygwin environment. In this case you should use:

mkdir cyg $YAPSRC/configure --enable-coroutining \\ --enable-max-performance \\ --enable-cygwin=yes make make install

Yap will then compile using the cygwin library and will be installed in cygwin's /usr/local. You can use Yap from a cygwin console, or as a standalone application as long as it can find cygwin1.dll in its path.

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1.3.1 Compiling Under Visual C++

Yap compiles cleanly under Microsoft's Visual C++ release 6.0. We next give a step-by-step tutorial on how to compile Yap manually using this environment.

First, it is a good idea to build Yap as a DLL:

1. create a project named yapdll using File.New. The project will be a DLL project, initially empty.

   Notice that either the project is named yapdll or you must replace the preprocessors variable YAPDLL_EXPORTS to match your project names in the files YapInterface.h and c_interface.c.

2. add all .c files in the $YAPSRC/C directory and in the $YAPSRC\OPTYap directory to the Project's Source Files (use FileView).
3. add all .h files in the $YAPSRC/H directory, $YAPSRC\include directory and in the $YAPSRC\OPTYap subdirectory to the Project's Header Files.
4. Ideally, you should now use m4 to generate extra .h from .m4 files and use configure to create a config.h. Or, you can be lazy, and fetch these files from $YAPSRC\VC\include.
5. You may want to go to Build.Set Active Configuration and set Project Type to Release
6. To use Yap's own include directories you have to set the Project option Project.Project Settings.C/C++.Preprocessor.Additional Include Directories to include the directories $YAPSRC\H, $YAPSRC\VC\include, $YAPSRC\OPTYap and $YAPSRC\include. The syntax is:

   $YAPSRC\H, $YAPSRC\VC\include, $YAPSRC\OPTYap, $YAPSRC\include

7. Build: the system should generate an yapdll.dll and an yapdll.lib.
8. Copy the file yapdll.dll to your path. The file yapdll.lib should also be copied to a location where the linker can find it.

Now you are ready to create a console interface for Yap:

1. create a second project say wyap with File.New. The project will be a WIN32 console project, initially empty.
2. add $YAPSRC\console\yap.c to the Source Files.
3. add $YAPSRC\VC\include\config.h and the files in $YAPSRC\include to the Header Files.
4. You may want to go to Build.Set Active Configuration and set Project Type to Release.
5. you will eventually need to bootstrap the system by booting from boot.yap, so write:

   -b $YAPSRC\pl\boot.yap

   in Project.Project Settings.Debug.Program Arguments.

6. You need the sockets and yap libraries. Add

   ws2_32.lib yapdll.lib to

   to

   to Project.Project Settings.Link.Object/Library Modules

   You may also need to set the Link Path so that VC++ will find yapdll.lib.

7. set Project.Project Settings.C/C++.Preprocessor.Additional Include Directories to include the $YAPSRC/VC/include and $YAPSRC/include.

   The syntax is:

   $YAPSRC\VC\include, $YAPSRC\include

8. Build the system.
9. Use Build.Start Debug to boot the system, and then create the saved state with

   ['$YAPSRC\\pl\\init']. save_program(startup). ^Z

   That's it, you've got Yap and the saved state!

The $YAPSRC\VC directory has the make files to build Yap4.3.17 under VC++ 6.0.

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1.3.2 Compiling Under SGI's cc

YAP should compile under the Silicon Graphic's cc compiler, although we advise using the GNUCC compiler, if available.

64 bit

Support for 64 bits should work by using (under Bourne shell syntax):

CC="cc -64" $YAP_SRC_PATH/configure --...

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2. Running YAP

2.1 Running Yap Interactively		Interacting with Yap
2.2 Running Prolog Files		Running Prolog files as scripts

We next describe how to invoke Yap in Unix systems.

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2.1 Running Yap Interactively

Most often you will want to use Yap in interactive mode. Assuming that YAP is in the user's search path, the top-level can be invoked under Unix with the following command:

yap [-s n] [-h n] [-a n] [-c IP_HOST port ] [filename]

All the arguments and flags are optional and have the following meaning:

-?

print a short error message.

-s n

allocate n K bytes for local and global stacks

-h n

allocate n K bytes for heap and auxiliary stacks

-t n

allocate n K bytes for the trail stack

-l YAP_FILE

compile the Prolog file YAP_FILE before entering the top-level.

-L YAP_FILE

compile the Prolog file YAP_FILE and then halt. This option is useful for implementing scripts.

-g Goal

run the goal Goal before top-level. The goal is converted from an atom to a Prolog term.

-z Goal

run the goal Goal as top-level. The goal is converted from an atom to a Prolog term.

-b BOOT_FILE

boot code is in Prolog file BOOT_FILE. The filename must define the predicate '$live'/0.

-c IP_HOST port

connect standard streams to host IP_HOST at port port

filename

restore state saved in the given file

--

separator for arguments to Prolog code. These arguments are visible through the unix/1 built-in.

Note that YAP will output an error message on the following conditions:

• a file name was given but the file does not exist or is not a saved YAP state;
• the necessary amount of memory could not be allocated;
• the allocated memory is not enough to restore the state.

When restoring a saved state, YAP will allocate the same amount of memory as that in use when the state was saved, unless a different amount is specified by flags in the command line. By default, YAP restores the file `startup' from the current directory or from the YAP library.

• YAP usually boots from a saved state. The saved state will use the default installation directory to search for the YAP binary unless you define the environment variable YAPBINDIR.
• YAP always tries to find saved states from the current directory first. If it cannot it will use the environment variable YAPLIBDIR, if defined, or search the default library directory.
• YAP will try to find library files from the YAPSHAREDIR/library directory.

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2.2 Running Prolog Files

YAP can also be used to run Prolog files as scripts, at least in Unix-like environments. A simple example is shown next:

#!/usr/local/bin/yap -L -- # # Hello World script file using Yap # # put a dot because of syntax errors . :- write('Hello World'), nl.

The #! characters specify that the script should call the binary file Yap. Notice that many systems will require the complete path to the Yap binary. The -L flag indicates that YAP should consult the current file when booting and then halt. The remaining arguments are then passed to YAP. Note that YAP will skip the first lines if they start with # (the comment sign for Unix's shell). YAP will consult the file and execute any commands.

A slightly more sophisticated example is:

#!/usr/bin/yap -L -- # # Hello World script file using Yap # . :- initialization(main). main :- write('Hello World'), nl.

The initialization directive tells Yap to execute the goal main after consulting the file. Source code is thus compiled and main executed at the end. The . is useful while debugging the script as a Prolog program: it guarantees that the syntax error will not propagate to the Prolog code.

Notice that the -- is required so that the shell passes the extra arguments to YAP. As an example, consider the following script dump_args:

#!/usr/bin/yap -L -- #. main( [] ). main( [H|T] ) :- write( H ), nl, main( T ). :- unix( argv(AllArgs) ), main( AllArgs ).

If you this run this script with the arguments:

./dump_args -s 10000

the script will start an YAP process with stack size 10MB, and the list of arguments to the process will be empty.

Often one wants to run the script as any other program, and for this it is convenient to ignore arguments to YAP. This is possible by using L -- as in the next version of dump_args:

#!/usr/bin/yap -L -- main( [] ). main( [H|T] ) :- write( H ), nl, main( T ). :- unix( argv(AllArgs) ), main( AllArgs ).

The -- indicates the next arguments are not for YAP. Instead, they must be sent directly to the argv built-in. Hence, running

./dump_args test

will write test on the standard output.

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3. Syntax

We will describe the syntax of YAP at two levels. We first will describe the syntax for Prolog terms. In a second level we describe the tokens from which Prolog terms are built.

3.1 Syntax of Terms		Syntax of terms
3.2 Prolog Tokens		Syntax of Prolog tokens

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3.1 Syntax of Terms

Below, we describe the syntax of YAP terms from the different classes of tokens defined above. The formalism used will be BNF, extended where necessary with attributes denoting integer precedence or operator type.

term ----> subterm(1200) end_of_term_marker subterm(N) ----> term(M) [M <= N] term(N) ----> op(N, fx) subterm(N-1) | op(N, fy) subterm(N) | subterm(N-1) op(N, xfx) subterm(N-1) | subterm(N-1) op(N, xfy) subterm(N) | subterm(N) op(N, yfx) subterm(N-1) | subterm(N-1) op(N, xf) | subterm(N) op(N, yf) term(0) ----> atom '(' arguments ')' | '(' subterm(1200) ')' | '{' subterm(1200) '}' | list | string | number | atom | variable arguments ----> subterm(999) | subterm(999) ',' arguments list ----> '[]' | '[' list_expr ']' list_expr ----> subterm(999) | subterm(999) list_tail list_tail ----> ',' list_expr | ',..' subterm(999) | '|' subterm(999)

Notes:

• op(N,T) denotes an atom which has been previously declared with type T and base precedence N.
• Since ',' is itself a pre-declared operator with type xfy and precedence 1000, is subterm starts with a '(', op must be followed by a space to avoid ambiguity with the case of a functor followed by arguments, e.g.:

  + (a,b) [the same as '+'(','(a,b)) of arity one]

  versus

  +(a,b) [the same as '+'(a,b) of arity two]

• In the first rule for term(0) no blank space should exist between atom and '('.
• Each term to be read by the YAP parser must end with a single dot, followed by a blank (in the sense mentioned in the previous paragraph). When a name consisting of a single dot could be taken for the end of term marker, the ambiguity should be avoided by surrounding the dot with single quotes.

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3.2 Prolog Tokens

Prolog tokens are grouped into the following categories:

3.2.1 Numbers		Integer and Floating-Point Numbers
3.2.2 Character Strings		Sequences of Characters
3.2.3 Atoms		Atomic Constants
3.2.4 Variables		Logical Variables
3.2.5 Punctuation Tokens		Tokens that separate other tokens
3.2.6 Layout		Comments and Other Layout Rules

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3.2.1 Numbers

Numbers can be further subdivided into integer and floating-point numbers.

3.2.1.1 Integers		How Integers are read and represented
3.2.1.2 Floating-point Numbers		Floating Point Numbers

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3.2.1.1 Integers

Integer numbers are described by the following regular expression:

<integer> := {<digit>+<single-quote>|0{xXo}}<alpha_numeric_char>+

where {...} stands for optionality, + optional repetition (one or more times), <digit> denotes one of the characters 0 ... 9, | denotes or, and <single-quote> denotes the character "'". The digits before the <single-quote> character, when present, form the number basis, that can go from 0, 1 and up to 36. Letters from A to Z are used when the basis is larger than 10.

Note that if no basis is specified then base 10 is assumed. Note also that the last digit of an integer token can not be immediately followed by one of the characters 'e', 'E', or '.'.

Following the ISO standard, YAP also accepts directives of the form 0x to represent numbers in hexadecimal base and of the form 0o to represent numbers in octal base. For usefulness, YAP also accepts directives of the form 0X to represent numbers in hexadecimal base.

Example: the following tokens all denote the same integer

10 2'1010 3'101 8'12 16'a 36'a 0xa 0o12

Numbers of the form 0'a are used to represent character constants. So, the following tokens denote the same integer:

0'd 100

YAP (version No value for VERSION) supports integers that can fit the word size of the machine. This is 32 bits in most current machines, but 64 in some others, such as the Alpha running Linux or Digital Unix. The scanner will read larger or smaller integers erroneously.

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3.2.1.2 Floating-point Numbers

Floating-point numbers are described by:

<float> := <digit>+{<dot><digit>+} <exponent-marker>{<sign>}<digit>+ |<digit>+<dot><digit>+ {<exponent-marker>{<sign>}<digit>+}

where <dot> denotes the decimal-point character '.', <exponent-marker> denotes one of 'e' or 'E', and <sign> denotes one of '+' or '-'.

Examples:

10.0 10e3 10e-3 3.1415e+3

Floating-point numbers are represented as a double in the target machine. This is usually a 64-bit number.

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3.2.2 Character Strings

Strings are described by the following rules:

string --> '"' string_quoted_characters '"' string_quoted_characters --> '"' '"' string_quoted_characters string_quoted_characters --> '\' escape_sequence string_quoted_characters string_quoted_characters --> string_character string_quoted_characters escape_sequence --> 'a' | 'b' | 'r' | 'f' | 't' | 'n' | 'v' escape_sequence --> '\' | '"' | ''' | '`' escape_sequence --> at_most_3_octal_digit_seq_char '\' escape_sequence --> 'x' at_most_2_hexa_digit_seq_char '\'

where string_character in any character except the double quote and escape characters.

Examples:

"" "a string" "a double-quote:"""

The first string is an empty string, the last string shows the use of double-quoting. The implementation of YAP represents strings as lists of integers. Since Yap4.3.0 there is no static limit on string size.

Escape sequences can be used to include the non-printable characters a (alert), b (backspace), r (carriage return), f (form feed), t (horizontal tabulation), n (new line), and v (vertical tabulation). Escape sequences also be include the meta-characters \, ", ', and `. Last, one can use escape sequences to include the characters either as an octal or hexadecimal number.

The next examples demonstrates the use of escape sequences in YAP:

"\x0c\" "\01\" "\f" "\\"

The first three examples return a list including only character 12 (form feed). The last example escapes the escape character.

Escape sequences were not available in C-Prolog and in original versions of YAP up to 4.2.0. Escape sequences can be disable by using:

:- yap_flag(character_escapes,off).

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3.2.3 Atoms

Atoms are defined by one of the following rules:

atom --> solo-character atom --> lower-case-letter name-character* atom --> symbol-character+ atom --> single-quote single-quote atom --> ''' atom_quoted_characters ''' atom_quoted_characters --> ''' ''' atom_quoted_characters atom_quoted_characters --> '\' atom_sequence string_quoted_characters atom_quoted_characters --> character string_quoted_characters

where:

<solo-character> denotes one of: ! ; <symbol-character> denotes one of: # & * + - . / : < = > ? @ \ ^ ` ~ <lower-case-letter> denotes one of: a...z <name-character> denotes one of: _ a...z A...Z 0....9 <single-quote> denotes: '

and string_character denotes any character except the double quote and escape characters. Note that escape sequences in strings and atoms follow the same rules.

Examples:

a a12x '$a' ! => '1 2'

Version 4.2.0 of YAP removed the previous limit of 256 characters on an atom. Size of an atom is now only limited by the space available in the system.

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3.2.4 Variables

Variables are described by:

<variable-starter><variable-character>+

where

<variable-starter> denotes one of: _ A...Z <variable-character> denotes one of: _ a...z A...Z

If a variable is referred only once in a term, it needs not to be named and one can use the character _ to represent the variable. These variables are known as anonymous variables. Note that different occurrences of _ on the same term represent different anonymous variables.

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3.2.5 Punctuation Tokens

Punctuation tokens consist of one of the following characters:

( ) , [ ] { } |

These characters are used to group terms.

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3.2.6 Layout

Any characters with ASCII code less than or equal to 32 appearing before a token are ignored.

All the text appearing in a line after the character % is taken to be a comment and ignored (including %). Comments can also be inserted by using the sequence /* to start the comment and */ to finish it. In the presence of any sequence of comments or layout characters, the YAP parser behaves as if it had found a single blank character. The end of a file also counts as a blank character for this purpose.

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4. Loading Programs

Loading Programs 4.1 Program loading and updating  Program Loading and Updating 4.2 Changing the Compiler's Behavior  Changing the compiler's parameters 4.3 Saving and Loading Prolog States  Saving and Restoring Programs

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4.1 Program loading and updating

consult(+F)

Adds the clauses written in file F or in the list of files F to the program.

In YAP consult/1 does not remove previous clauses for the procedures defined in F. Moreover, note that all code in YAP is compiled.

reconsult(+F)

Updates the program replacing the previous definitions for the predicates defined in F.

[+F]

The same as consult(F).

[-+F]

The same as reconsult(F)

Example:

?- [file1, -file2, -file3, file4].

will consult file1 file4 and reconsult file2 and file3.

compile(+F)

In YAP, the same as reconsult/1.

ensure_loaded(+F) [ISO]

When the files specified by F are module files, ensure_loaded/1 loads them if they have note been previously loaded, otherwise advertises the user about the existing name clashes and prompts about importing or not those predicates. Predicates which are not public remain invisible.

When the files are not module files, ensure_loaded/1 loads them if they have not been loaded before, does nothing otherwise.

F must be a list containing the names of the files to load.

include(+F) [ISO]

The include directive includes the text files or sequence of text files specified by F into the file being currently consulted.

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4.2 Changing the Compiler's Behavior

This section presents a set of built-ins predicates designed to set the environment for the compiler.

source_mode(-O,+N)

The state of source mode can either be on or off. When the source mode is on, all clauses are kept both as compiled code and in a "hidden" database. O is unified with the previous state and the mode is set according to N.

source

After executing this goal, YAP keeps information on the source of the predicates that will be consulted. This enables the use of listing/0, listing/1 and clause/2 for those clauses.

The same as source_mode(_,on) or as declaring all newly defined static procedures as public.

no_source

The opposite to source.

The same as source_mode(_,off).

compile_expressions

After a call to this predicate, arithmetical expressions will be compiled. (see example below). This is the default behavior.

do_not_compile_expressions

After a call to this predicate, arithmetical expressions will not be compiled.

?- source, do_not_compile_expressions. yes ?- [user]. | p(X) :- X is 2 * (3 + 8). | :- end_of_file. ?- compile_expressions. yes ?- [user]. | q(X) :- X is 2 * (3 + 8). | :- end_of_file. :- listing. p(A):- A is 2 * (3 + 8). q(A):- A is 22.

hide(+Atom)

Make atom Atom invisible.

unhide(+Atom)

Make hidden atom Atom visible.

hide_predicate(+Pred)

Make predicate Pred invisible to current_predicate/2, listing, and friends.

expand_exprs(-O,+N)

Puts YAP in state N (on or off) and unify O with the previous state, where On is equivalent to compile_expressions and off is equivalent to do_not_compile_expressions. This predicate was kept to maintain compatibility with C-Prolog.

path(-D)

Unifies D with the current directory search-path of YAP. Note that this search-path is only used by YAP to find the files for consult/1, reconsult/1 and restore/1 and should not be taken for the system search path.

add_to_path(+D)

Adds D to the end of YAP's directory search path.

add_to_path(+D,+N)

Inserts D in the position, of the directory search path of YAP, specified by N. N must be either of first or last.

remove_from_path(+D)

Remove D from YAP's directory search path.

style_check(+X)

Turns on style checking according to the attribute specified by X, which must be one of the following:

single_var

Checks single occurrences of named variables in a clause.

discontiguous

Checks non-contiguous clauses for the same predicate in a file.

multiple

Checks the presence of clauses for the same predicate in more than one file when the predicate has not been declared as multifile

all

Performs style checking for all the cases mentioned above.

By default, style checking is disabled in YAP unless we are in sicstus or iso language mode.

The style_check/1 built-in is now deprecated. Please use the set_prolog_flag/1 instead.

no_style_check(+X)

Turns off style checking according to the attribute specified by X, which has the same meaning as in style_check/1.

The no_style_check/1 built-in is now deprecated. Please use the set_prolog_flag/1 instead.

multifile P [ISO]

Instructs the compiler about the declaration of a predicate P in more than one file. It must appear in the first of the loaded files where the predicate is declared, and before declaration of any of its clauses.

Multifile declarations affect reconsult/1 and compile/1: when a multifile predicate is reconsulted, only the clauses from the same file are removed.

Since Yap4.3.0 multifile procedures can be static or dynamic.

discontiguous(+G) [ISO]

Declare that the arguments are discontiguous procedures, that is, clauses for discontigous procedures may be separated by clauses from other procedures.

initialization(+G) [ISO]

The compiler will execute goals G after consulting the current file.

library_directory(+D)

Succeeds when D is a current library directory name. Library directories are the places where files specified in the form library(File) are searched by the predicates consult/1, reconsult/1, use_module/1 or ensure_loaded/1.

file_search_path(+NAME,-DIRECTORY)

Allows writing file names as compound terms. The NAME and DIRECTORY must be atoms. The predicate may generate multiple solutions. The predicate is originally defined as follows:

file_search_path(library,A) :- library_directory(A). file_search_path(system,A) :- prolog_flag(host_type,A).

Thus, [library(A)] will search for a file using library_directory/1 to obtain the prefix.

library_directory(+D)

Succeeds when D is a current library directory name. Library directories are the places where files specified in the form library(File) are searched by the predicates consult/1, reconsult/1, use_module/1 or ensure_loaded/1.

prolog_file_name(+Name,-FullPath)

Unify FullPath with the absolute path YAP would use to consult file Name.

public P [ISO]

Instructs the compiler that the source of a predicate of a list of predicates P must be kept. This source is then accessible through the clause/2 procedure and through the listing family of built-ins.

Note that all dynamic procedures are public. The source directive defines all new or redefined predicates to be public.

Since Yap4.3.0 multifile procedures can be static or dynamic.

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4.3 Saving and Loading Prolog States

save(+F)

Saves an image of the current state of YAP in file F. From Yap4.1.3 onwards, YAP saved states are executable files in the Unix ports.

save(+F,-OUT)

Saves an image of the current state of YAP in file F. From Yap4.1.3 onwards, YAP saved states are executable files in the Unix ports.

Unify OUT with 1 when saving the file and OUT with 0 when restoring the saved state.

save_program(+F)

Saves an image of the current state of the YAP database in file F.

save_program(+F, :G)

Saves an image of the current state of the YAP database in file F, and guarantee that execution of the restored code will start by trying goal G.

restore(+F)

Restores a previously saved state of YAP from file F.

YAP always tries to find saved states from the current directory first. If it cannot it will use the environment variable YAPLIBDIR, if defined, or search the default library directory.

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5. The Module System

Module systems are quite important for the development of large applications. YAP implements a module system compatible with the Quintus Prolog module system.

The YAP module system is predicate-based. This means a module consists of a set of predicates (or procedures), such that some predicates are public and the others are local to a module. Atoms and terms in general are global to the system. Moreover, the module system is flat, meaning that we do not support an hierarchy of modules. Modules can automatically import other modules, though. For compatibility with other module systems the YAP module system is non-strict, meaning both that there is both a way to access predicates private to a module and that is possible to declare predicates for a module from some other module.

YAP allows one to ignore the module system if one does not want to use it. Last note that using the module system does not introduce any significant overheads: only meta-calls that cross module boundaries are slowed down by the presence of modules.

5.1 Module Concepts		The Key Ideas in Modules
5.2 Defining a New Module		How To Define a New Module
5.3 Using Modules		How to Use a Module
5.4 Meta-Predicates in Modules		How to Handle New Meta-Predicates

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5.1 Module Concepts

The YAP module system applies to predicates. All predicates belong to a module. System predicates belong to the module primitives, and by default new predicates belong to the module user. Predicates from the module primitives are automatically visible to every module.

Every predicate must belong to a module. This module is called its source module.

By default, the source module for a clause occurring in a source file with a module declaration is the declared module. For goals typed in a source file without module declarations, their module is the module the file is being loaded into. If no module declarations exist, this is the current type-in module. The default type-in module is user, but one can set the current module by using the built-in module/1.

Note that in this module system one can explicitly specify the source mode for a clause by prefixing a clause with its module, say:

user:(a :- b).

In fact, to specify the source module for a clause it is sufficient to specify the source mode for the clause's head:

user:a :- b.

The rules for goals are similar. If a goal appears in a text file with a module declaration, the goal's source module is the declared module. Otherwise, it is the module the file is being loaded into or the type-in module.

One can override this rule by prefixing a goal with the module it is supposed to be executed into, say:

nasa:launch(apollo,13).

will execute the goal launch(apollo,13) as if the current source module was nasa.

Note that this rule breaks encapsulation and should be used with care.

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5.2 Defining a New Module

A new module is defined by a module declaration:

module(+M,+L)

This predicate defines the file where it appears as a module file; it must be the first declaration in the file. M must be an atom specifying the module name; L must be a list containing the module's public predicates specification, in the form [predicate_name/arity,...].

The public predicates of a module file can be made accessible by other files through the predicates consult/1, reconsult/1, ensure_loaded/1 or use_module/2. The non-public predicates of a module file are not visible by other files; they can, however, be accessed if the module name is prefixed to the file name through the :/2 operator.

The built-in module/1 sets the current source module:

module(+M,+L, +Options)

Similar to module/2, this predicate defines the file where it appears as a module file; it must be the first declaration in the file. M must be an atom specifying the module name; L must be a list containing the module's public predicates specification, in the form [predicate_name/arity,...].

The last argument Options must be a list of options, which can be:

filename

the filename for a module to import into the current module.

library(file)

a library file to import into the current module.

hide(Opt)

if Opt is false, keep source code for current module, if true, disable.

module(+M)

Defines M to be the current working or type-in module. All files which are not binded to a module are assumed to belong to the working module (also referred to as type-in module). To compile a non-module file into a module which is not the working one, prefix the file name with the module name, in the form Module:File, when loading the file.

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5.3 Using Modules

By default, all procedures to consult a file will load the modules defined therein. The two following declarations allow one to import a module explicitly. They differ on whether one imports all predicate declared in the module or not.

use_module(+F)

Loads the files specified by F, importing all their public predicates. Predicate name clashes are resolved by asking the user about importing or not the predicate. A warning is displayed when F is not a module file.

use_module(+F,+L)

Loads the files specified by F, importing the predicates specified in the list L. Predicate name clashes are resolved by asking the user about importing or not the predicate. A warning is displayed when F is not a module file.

use_module(?M,?F,+L)

If module M has been defined, import the procedures in L to the current module. Otherwise, load the files specified by F, importing the predicates specified in the list L.

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5.4 Meta-Predicates in Modules

The module system must know whether predicates operate on goals or clauses. Otherwise, such predicates would call a goal in the module they were defined, instead of calling it in the module they are currently executing. So, for instance:

:- module(example,[a/1]). ... a(G) :- call(G) ...

The expected behavior for this procedure is to execute goal G within the current module, that is, within example. On the other hand, when executing call/1 the system only knows where call/1 was defined, that is, it only knows of primitives. A similar problem arises for assert/1 and friends.

The meta_predicate/1 declaration informs the system that some arguments of a procedure are goals, clauses or clauses heads, and that these arguments must be expanded to receive the current source module:

meta_predicate G1,....,Gn

Each Gi is a mode specification. For example, a declaration for call/1 and setof/3 would be of the form:

:- meta_predicate call(:), setof(?,:,?).

If the argument is : or an integer, the argument is a call and must be expanded. Otherwise, the argument should not be expanded. Note that the system already includes declarations for all built-ins.

In the previous example, the only argument to call/1 must be expanded, resulting in the following code:

:- module(example,[a/1]). ... a(G) :- call(example:G) ...

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6. Built-In Predicates

Builtins, Debugging, Syntax, Top 6.1 Control Predicates  Controlling the Execution of Prolog Programs 6.2 Handling Undefined Procedures  Handling calls to Undefined Procedures 6.3 Predicates on terms  Predicates on Terms 6.4 Comparing Terms  Comparison of Terms 6.5 Arithmetic  Arithmetic in Yap 6.6 I/O Predicates  Input/Output with Yap 6.7 Using the Clausal Data Base  Modifying Prolog's Database 6.10 Collecting Solutions to a Goal  Finding All Possible Solutions 6.11 Grammar Rules  Grammar Rules 6.17 Predicate Information  Predicate Information 6.12 Access to Operating System Functionality  Access to Operating System Functionality 6.13 Term Modification  Updating Prolog Terms 6.14 Profiling Prolog Programs  Profiling Prolog Execution 6.15 Counting Calls  Limiting the Maximum Number of Reductions 6.16 Arrays  Supporting Global and Local Arrays 6.17 Predicate Information  Information on Predicates 6.18 Miscellaneous  Miscellaneous Predicates

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6.1 Control Predicates

This chapter describes the predicates for controlling the execution of Prolog programs.

In the description of the arguments of functors the following notation will be used:

• a preceding plus sign will denote an argument as an "input argument" - it cannot be a free variable at the time of the call;
• a preceding minus sign will denote an "output argument";
• an argument with no preceding symbol can be used in both ways.

+P, +Q [ISO]

Conjunction of goals (and).

Example:

p(X) :- q(X), r(X).

should be read as "p(X) if q(X) and r(X)".

+P ; +Q [ISO]

Disjunction of goals (or).

Example:

p(X) :- q(X); r(X).

should be read as "p(X) if q(X) or r(X)".

true [ISO]

Succeeds once.

fail [ISO]

Fails always.

false

The same as fail

! [ISO]

Read as "cut". Cuts any choices taken in the current procedure. When first found "cut" succeeds as a goal, but if backtracking should later return to it, the parent goal (the one which matches the head of the clause containing the "cut", causing the clause activation) will fail. This is an extra-logical predicate and cannot be explained in terms of the declarative semantics of Prolog.

example:

member(X,[X|_]). member(X,[_|L]) :- member(X,L).

With the above definition

?- member(X,[1,2,3]).

will return each element of the list by backtracking. With the following definition:

member(X,[X|_]) :- !. member(X,[_|L]) :- member(X,L).

the same query would return only the first element of the list, since backtracking could not "pass through" the cut.

\+ +P [ISO]

Goal P is not provable. The execution of this predicate fails if and only if the goal P finitely succeeds. It is not a true logical negation, which is impossible in standard Prolog, but "negation-by-failure".

This predicate might be defined as:

\+(P) :- P, !, fail. \+(_).

if P did not include "cuts".

not +P

Goal P is not provable. The same as '\+ P'.

This predicate is kept for compatibility with C-Prolog and previous versions of YAP. Uses of not/1 should be replace by (\+)/1, as YAP does not implement true negation.

+P -> +Q [ISO]

Read as "if-then-else" or "commit". This operator is similar to the conditional operator of imperative languages and can be used alone or with an else part as follows:

+P -> +Q

"if P then Q".

+P -> +Q; +R

"if P then Q else R".

These two predicates could be defined respectively in Prolog as:

(P -> Q) :- P, !, Q.

and

(P -> Q; R) :- P, !, Q. (P -> Q; R) :- R.

if there were no "cuts" in P, Q and R.

Note that the commit operator works by "cutting" any alternative solutions of P.

Note also that you can use chains of commit operators like:

P -> Q ; R -> S ; T.

Note that (->)/2 does not affect the scope of cuts in its arguments.

repeat [ISO]

Succeeds repeatedly.

In the next example, repeat is used as an efficient way to implement a loop. The next example reads all terms in a file:

a :- repeat, read(X), write(X), nl, X=end_of_file, !.

the loop is effectively terminated by the cut-goal, when the test-goal X=end succeeds. While the test fails, the goals read(X), write(X), and nl are executed repeatedly, because backtracking is caught by the repeat goal.

The built-in repeat/1 could be defined in Prolog by:

repeat. repeat :- repeat.

call(+P) [IS0]

If P is instantiated to an atom or a compound term, the goal call(P) is executed as if the value of P was found instead of the call to call/1, except that any "cut" occurring in P only cuts alternatives in the execution of P.

incore(+P)

The same as call/1.

call_with_args(+Name,...,?Ai,...)

Meta-call where Name is the name of the procedure to be called and the Ai are the arguments. The number of arguments varies between 0 and 10.

If Name is a complex term, then call_with_args/n behaves as call/n:

call(p(X1,...,Xm), Y1,...,Yn) :- p(X1,...,Xm,Y1,...,Yn).

+P

The same as call(P). This feature has been kept to provide compatibility with C-Prolog. When compiling a goal, YAP generates a call(X) whenever a variable X is found as a goal.

a(X) :- X.

is converted to:

a(X) :- call(X).

if(?G,?H,?I) [IS0]

Call goal H once per each solution of goal H. If goal H has no solutions, call goal I.

The built-in if/3 is similar to ->/3, with the difference that it will backtrack over the test goal. Consider the following small data-base:

a(1). b(a). c(x). a(2). b(b). c(y).

Execution of an if/3 query will proceed as follows:

?- if(a(X),b(Y),c(Z)). X = 1, Y = a ? ; X = 1, Y = b ? ; X = 2, Y = a ? ; X = 2, Y = b ? ; no

The system will backtrack over the two solutions for a/1 and the two solutions for b/1, generating four solutions.

Cuts are allowed inside the first goal G, but they will only prune over G.

If you want G to be deterministic you should use if-then-else, as it is both more efficient and more portable.

once(:G) [IS0]

Execute the goal G only once. The predicate is defined by:

once(G) :- call(G), !.

Note that cuts inside once/1 can only cut the other goals inside once/1.

abort

Abandons the execution of the current goal and returns to top level. All break levels (see break/0 below) are terminated. It is mainly used during debugging or after a serious execution error, to return to the top-level.

break

Suspends the execution of the current goal and creates a new execution level similar to the top level, displaying the following message:

[ Break (level <number>) ]

telling the depth of the break level just entered. To return to the previous level just type the end-of-file character or call the end_of_file predicate. This predicate is especially useful during debugging.

halt [ISO]

Halts Prolog, and exits to the calling application. In YAP, halt/0 returns the exit code 0.

halt(+ I) [ISO]

Halts Prolog, and exits to the calling application returning the code given by the integer I.

catch(+Goal,+Exception,+Action) [IS0]

The goal catch(Goal,Exception,Action) tries to execute goal Goal. If during its execution, Goal throws an exception E' and this exception unifies with Exception, the exception is considered to be caught and Action is executed. If the exception E' does not unify with Exception, control again throws the exception.

The top-level of YAP maintains a default exception handler that is responsible to capture uncaught exceptions.

throw(+Ball) [ISO]

The goal throw(Ball) throws an exception. Execution is stopped, and the exception is sent to the ancestor goals until reaching a matching catch/3, or until reaching top-level.

garbage_collect

The goal garbage_collect forces a garbage collection.

garbage_collect_atoms

The goal garbage_collect forces a garbage collection of the atoms in the data-base. Currently, only atoms are recovered.

gc

The goal gc enables garbage collection. The same as yap_flag(gc,on).

nogc

The goal nogc disables garbage collection. The same as yap_flag(gc,off).

grow_heap(+Size)

Increase heap size Size kilobytes.

grow_stack(+Size)

Increase stack size Size kilobytes.

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6.2 Handling Undefined Procedures

A predicate in a module is said to be undefined if there are no clauses defining the predicate, and if the predicate has not been declared to be dynamic. What YAP does when trying to execute undefined predicates can be specified through three different ways:

• By setting an YAP flag, through the yap_flag/2 or set_prolog_flag/2 built-ins. This solution generalizes the ISO standard.
• By using the unknown/2 built-in (this solution is compatible with previous releases of YAP).
• By defining clauses for the hook predicate user:unknown_predicate_handler/3. This solution is compatible with SICStus Prolog.

In more detail:

unknown(-O,+N)

Specifies an handler to be called is a program tries to call an undefined static procedure P.

The arity of N may be zero or one. If the arity is 0, the new action must be one of fail, warning, or error. If the arity is 1, P is an user-defined handler and at run-time, the argument to the handler P will be unified with the undefined goal. Note that N must be defined prior to calling unknown/2, and that the single argument to N must be unbound.

In YAP, the default action is to fail (note that in the ISO Prolog standard the default action is error).

After defining undefined/1 by:

undefined(A) :- format('Undefined predicate: ~w~n',[A]), fail.

and executing the goal:

unknown(U,undefined(X)).

a call to a predicate for which no clauses were defined will result in the output of a message of the form:

Undefined predicate: user:xyz(A1,A2)

followed by the failure of that call.

yap_flag(unknown,+SPEC)

Alternatively, one can use yap_flag/2, current_prolog_flag/2, or set_prolog_flag/2, to set this functionality. In this case, the first argument for the built-ins should be unknown, and the second argument should be either error, warning, fail, or a goal.

user:unknown_predicate_handler(+G,+M,?NG)

The user may also define clauses for user:unknown_predicate_handler/3 hook predicate. This user-defined procedure is called before any system processing for the undefined procedure, with the first argument G set to the current goal, and the second M set to the current module. The predicate G will be called from within the user module.

If user:unknown_predicate_handler/3 succeeds, the system will execute NG. If user:unknown_predicate_handler/3 fails, the system will execute default action as specified by unknown/2.

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6.3 Predicates on terms

var(T) [ISO]

Succeeds if T is currently a free variable, otherwise fails.

atom(T) [ISO]

Succeeds if and only if T is currently instantiated to an atom.

atomic(T) [ISO]

Checks whether T is an atomic symbol (atom or number).

compound(T) [ISO]

Checks whether T is a compound term.

db_reference(T)

Checks whether T is a database reference.

float(T) [ISO]

Checks whether T is a floating point number.

integer(T) [ISO]

Succeeds if and only if T is currently instantiated to an integer.

nonvar(T) [ISO]

The opposite of var(T).

number(T) [ISO]

Checks whether T is an integer or a float.

primitive(T)

Checks whether T is an atomic term or a database reference.

simple(T)

Checks whether T is unbound, an atom, or a number.

callable(T)

Checks whether T is a callable term, that is, an atom or a compound term.

name(A,L)

The predicate holds when at least one of the arguments is ground (otherwise, an error message will be displayed). The argument A will be unified with an atomic symbol and L with the list of the ASCII codes for the characters of the external representation of A.

name(yap,L).

will return:

L = [121,97,112].

and

name(3,L).

will return:

L = [51].

atom_chars(?A,?L) [ISO]

The predicate holds when at least one of the arguments is ground (otherwise, an error message will be displayed). The argument A must be unifiable with an atom, and the argument L with the list of the ASCII codes for the characters of the external representation of A.

The ISO-Prolog standard dictates that atom_chars/2 should unify the second argument with a list of one-char atoms, and not the character codes. For compatibility with previous versions of YAP, and with other Prolog implementations, YAP unifies the second argument with the character codes, as in atom_codes/2. Use the set_prolog_flag(to_chars_mode,iso) to obtain ISO standard compatibility.

atom_codes(?A,?L) [ISO]

The predicate holds when at least one of the arguments is ground (otherwise, an error message will be displayed). The argument A will be unified with an atom and L with the list of the ASCII codes for the characters of the external representation of A.

atom_concat(+As,?A)

The predicate holds when the first argument is a list of atoms, and the second unifies with the atom obtained by concatenating all the atoms in the first list.

atomic_concat(+As,?A)

The predicate holds when the first argument is a list of atoms, and the second unifies with the atom obtained by concatenating all the atomic terms in the first list. The first argument thus may contain atoms or numbers.

atom_concat(+A1,+A2,?A)

The predicate holds when the first argument and second argument are atoms, and the third unifies with the atom obtained by concatenating the first two arguments.

atom_length(+A,?I) [ISO]

The predicate holds when the first argument is an atom, and the second unifies with the number of characters forming that atom.

atom_concat(?A1,?A2,?A12) [ISO]

The predicate holds when the third argument unifies with an atom, and the first and second unify with atoms such that their representations concatenated are the representation for A12.

If A1 and A2 are unbound, the built-in will find all the atoms that concatenated give A12.

number_chars(?I,?L)

The predicate holds when at least one of the arguments is ground (otherwise, an error message will be displayed). The argument I must be unifiable with a number, and the argument L with the list of the ASCII codes for the characters of the external representation of I.

The ISO-Prolog standard dictates that number_chars/2 should unify the second argument with a list of one-char atoms, and not the character codes. For compatibility with previous versions of YAP, and with other Prolog implementations, YAP unifies the second argument with the character codes, as in number_codes/2. Use the set_prolog_flag(to_chars_mode,iso) to obtain ISO standard compatibility.

number_codes(?A,?L) [ISO]

The predicate holds when at least one of the arguments is ground (otherwise, an error message will be displayed). The argument A will be unified with a number and L with the list of the ASCII codes for the characters of the external representation of A.

number_atom(?I,?L)

The predicate holds when at least one of the arguments is ground (otherwise, an error message will be displayed). The argument I must be unifiable with a number, and the argument L must be unifiable with an atom representing the number.

char_code(?A,?I) [ISO]

The built-in succeeds with A bound to character represented as an atom, and I bound to the character code represented as an integer. At least, one of either A or I must be bound before the call.

sub_atom(+A,?Bef, ?Size, ?After, ?At_out) [ISO]

True when A and At_out are atoms such that the name of At_out has size Size and is a substring of the name of A, such that Bef is the number of characters before and After the number of characters afterwards.

Note that A must always be known, but At_out can be unbound when calling this built-in. If all the arguments for sub_atom/5 but A are unbound, the built-in will backtrack through all possible substrings of A.

numbervars(T,+N1,-Nn)

Instantiates each variable in term T to a term of the form: '$VAR'(I), with I increasing from N1 to Nn.

ground(T)

Succeeds if there are no free variables in the term T.

arg(+N,+T,A) [ISO]

Succeeds if the argument N of the term T unifies with A. The arguments are numbered from 1 to the arity of the term.

The current version will generate an error if T or N are unbound, if T is not a compound term, of if N is not a positive integer. Note that previous versions of YAP would fail silently under these errors.

functor(T,F,N)

The top functor of term T is named F and has arity N.

When T is not instantiated, F and N must be. If N is 0, F must be an atomic symbol, which will be unified with T. If N is not 0, then F must be an atom and T becomes instantiated to the most general term having functor F and arity N. If T is instantiated to a term then F and N are respectively unified with its top functor name and arity.

In the current version of YAP the arity N must be an integer. Previous versions allowed evaluable expressions, as long as the expression would evaluate to an integer. This feature is not available in the ISO Prolog standard.

T =.. L [ISO]

The list L is built with the functor and arguments of the term T. If T is instantiated to a variable, then L must be instantiated either to a list whose head is an atom, or to a list consisting of just a number.

X = Y [ISO]

Tries to unify terms X and Y.

X \= Y [ISO]

Succeeds if terms X and Y are not unifiable.

unify_with_occurs_check(?T1,?T2) [ISO]

Obtain the most general unifier of terms T1 and T2, if there is one.

This predicate implements the full unification algorithm. An example:n

unify_with_occurs_check(a(X,b,Z),a(X,A,f(B)).

will succeed with the bindings A = b and Z = f(B). On the other hand:

unify_with_occurs_check(a(X,b,Z),a(X,A,f(Z)).

would fail, because Z is not unifiable with f(Z). Note that (=)/2 would succeed for the previous examples, giving the following bindings A = b and Z = f(Z).

copy_term(?TI,-TF) [ISO]

Term TF is a variant of the original term TI, such that for each variable V in the term TI there is a new variable V' in term TF.

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6.4 Comparing Terms

The following predicates are used to compare and order terms, using the standard ordering:

• variables come before numbers, numbers come before atoms which in turn come before compound terms, i.e.: variables @< numbers @< atoms @< compound terms.
• variables are roughly ordered by "age" (the "oldest" variable is put first);
• floating point numbers are sorted in increasing order;
• Integers are sorted in increasing order;
• atoms are sorted in lexicographic order;
• compound terms are ordered first by name, then by arity of the main functor, and finally by their arguments in left-to-right order.

compare(C,X,Y)

As a result of comparing X and Y, C may take one of the following values:

• = if X and Y are identical;
• < if X precedes Y in the defined order;
• > if Y precedes X in the defined order;

X == Y [ISO]

Succeeds if terms X and Y are strictly identical. The difference between this predicate and =/2 is that, if one of the arguments is a free variable, it only succeeds when they have already been unified.

?- X == Y.

fails, but,

?- X = Y, X == Y.

succeeds.

?- X == 2.

fails, but,

?- X = 2, X == 2.

succeeds.

X \== Y [ISO]

Terms X and Y are not strictly identical.

X @< Y [ISO]

Term X precedes term Y in the standard order.

X @=< Y [ISO]

Term X does not follow term Y in the standard order.

X @> Y [ISO]

Term X follows term Y in the standard order.

X @>= Y [ISO]

Term X does not precede term Y in the standard order.

sort(+L,-S)

Unifies S with the list obtained by sorting L and merging identical (in the sense of ==) elements.

keysort(+L,S)

Assuming L is a list of the form Key-Value, keysort(+L,S) unifies S with the list obtained from L, by sorting its elements according to the value of Key.

?- keysort([3-a,1-b,2-c,1-a,1-b],S).

would return:

S = [1-b,1-a,1-b,2-c,3-a]

length(?L,?S)

Unify the well-defined list L with its length. The procedure can be used to find the length of a pre-defined list, or to build a list of length S.

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6.5 Arithmetic

Arithmetic expressions in YAP may use the following operators or evaluable predicates:

+X

The value of X itself.

-X [ISO]

Symmetric value.

X+Y [ISO]

Sum.

X-Y [ISO]

Difference.

X*Y [ISO]

Product.

X/Y [ISO]

Quotient.

X//Y [ISO]

Integer quotient.

X mod Y [ISO]

Integer remainder.

X rem Y

Integer remainder, the same as mod.

exp(X) [ISO]

Natural exponential.

log(X) [ISO]

Natural logarithm.

log10(X)

Decimal logarithm.

sqrt(X) [ISO]

Square root.

sin(X) [ISO]

Sine.

cos(X) [ISO]

Cosine.

tan(X)

Tangent.

asin(X)

Arc sine.

acos(X)

Arc cosine.

atan(X) [ISO]

Arc tangent.

atan2(X)

Four-quadrant arc tangent.

sinh(X)

Hyperbolic sine.

cosh(X)

Hyperbolic cosine.

tanh(X)

Hyperbolic tangent.

asinh(X)

Hyperbolic arc sine.

acosh(X)

Hyperbolic arc cosine.

atanh(X)

Hyperbolic arc tangent.

integer(X) [ISO]

If X evaluates to a float, the integer between the value of X and 0 closest to the value of X, else if X evaluates to an integer, the value of X.

float(X) [ISO]

If X evaluates to an integer, the corresponding float, else the float itself.

float_fractional_part(X) [ISO]

The fractional part of the floating point number X, or 0.0 if X is an integer. In the iso language mode, X must be an integer.

float_integer_part(X) [ISO]

The float giving the integer part of the floating point number X, or X if X is an integer. In the iso language mode, X must be an integer.

abs(X) [ISO]

The absolute value of X.

ceiling(X) [ISO]

The float that is the smallest integral value not smaller than X.

In iso language mode the argument must be a floating point-number and the result is an integer.

floor(X) [ISO]

The float that is the greatest integral value not greater than X.

In iso language mode the argument must be a floating point-number and the result is an integer.

round(X) [ISO]

The nearest integral value to X. If X is equidistant to two integers, it will be rounded to the closest even integral value.

In iso language mode the argument must be a floating point-number, the result is an integer and it the float is equidistant it is rounded up, that is, to the least integer greater than X.

sign(X) [ISO]

Return 1 if the X evaluates to a positive integer, 0 it if evaluates to 0, and -1 if it evaluates to a negative integer. If X evaluates to a floating-point number return 1.0 for a positive X, 0.0 for 0.0, and -1.0 otherwise.

truncate(X)

The float that is the integral value between X and 0 closest to X.

max(X,Y)

The greater value of X and Y.

min(X,Y)

The lesser value of X and Y.

X ^ Y

X raised to the power of Y, (from the C-Prolog syntax).

exp(X,Y)

X raised to the power of Y, (from the Quintus Prolog syntax).

X ** Y [ISO]

X raised to the power of Y (from ISO).

X /\ Y [ISO]

Integer bitwise conjunction.

X \/ Y [ISO]

Integer bitwise disjunction.

X # Y [ISO]

Integer bitwise exclusive disjunction.

X << Y

Integer bitwise left logical shift of X by Y places.

X >> Y [ISO]

Integer bitwise right logical shift of X by Y places.

\ X [ISO]

Integer bitwise negation.

gcd(X,Y)

The greatest common divisor of the two integers X and Y.

msb(X)

The most significant bit of the integer X.

[X]

Evaluates to X for expression X. Useful because character strings in Prolog are lists of character codes.

X is Y*10+C-"0"

is the same as

X is Y*10+C-[48].

which would be evaluated as:

X is Y*10+C-48.

Besides numbers and the arithmetic operators described above, certain atoms have a special meaning when present in arithmetic expressions:

pi

The value of pi, the ratio of a circle's circumference to its diameter.

e

The base of the natural logarithms.

inf

Infinity according to the IEEE Floating-Point standard. Note that evaluating this term will generate a domain error in the iso language mode.

nan

Not-a-number according to the IEEE Floating-Point standard. Note that evaluating this term will generate a domain error in the iso language mode.

cputime

CPU time in seconds, since YAP was invoked.

heapused

Heap space used, in bytes.

local

Local stack in use, in bytes.

global

Global stack in use, in bytes.

random

A "random" floating point number between 0 and 1.

The primitive YAP predicates involving arithmetic expressions are:

X is +Y [2]

This predicate succeeds iff the result of evaluating the expression Y unifies with X. This is the predicate normally used to perform evaluation of arithmetic expressions:

X is 2+3*4

succeeds with X = 14.

+X < +Y [ISO]

The value of the expression X is less than the value of expression Y.

+X =< +Y [ISO]

The value of the expression X is less than or equal to the value of expression Y.

+X > +Y [ISO]

The value of the expression X is greater than the value of expression Y.

+X >= +Y [ISO]

The value of the expression X is greater than or equal to the value of expression Y.

+X =:= +Y [ISO]

The value of the expression X is equal to the value of expression Y.

+X =\= +Y [ISO]

The value of the expression X is different from the value of expression Y.

srandom(+X)

Use the argument X as a new seed for YAP's random number generator. The argument should be an integer, but floats are acceptable.

Notes:

• In contrast to previous versions of Yap, Yap4 does not convert automatically between integers and floats.
• arguments to trigonometric functions are expressed in radians.
• if a (non-instantiated) variable occurs in an arithmetic expression YAP will generate an exception. If no error handler is available, execution will be thrown back to the top-level.

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6.6 I/O Predicates

Some of the I/O predicates described below will in certain conditions provide error messages and abort only if the file_errors flag is set. If this flag is cleared the same predicates will just fail. Details on setting and clearing this flag are given under 7.7.

Subnodes of Input/Output 6.6.1 Handling Streams and Files  Handling Streams and Files 6.6.2 Handling Streams and Files  C-Prolog Compatible File Handling 6.6.3 Handling Input/Output of Terms  Input/Output of terms 6.6.4 Handling Input/Output of Characters  Input/Output of Characters 6.6.5 Input/Output Predicates applied to Streams  Input/Output using Streams 6.6.6 Compatible C-Prolog predicates for Terminal I/O  C-Prolog compatible Character I/O to terminal 6.6.7 Controlling Input/Output  Controlling your Input/Output 6.6.8 Using Sockets From Yap  Using Sockets from Yap

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6.6.1 Handling Streams and Files

open(+F,+M,-S) [ISO]

Opens the file with name F in mode M ('read', 'write' or 'append'), returning S unified with the stream name.

At most, there are 17 streams opened at the same time. Each stream is either an input or an output stream but not both. There are always 3 open streams: user_input for reading, user_output for writing and user_error for writing. If there is no ambiguity, the atoms user_input and user_output may be referred to as user.

The file_errors flag controls whether errors are reported when in mode 'read' or 'append' the file F does not exist or is not readable, and whether in mode 'write' or 'append' the file is not writable.

open(+F,+M,-S,+Opts) [ISO]

Opens the file with name F in mode M ('read', 'write' or 'append'), returning S unified with the stream name, and following these options:

type(+T)

Specify whether the stream is a text stream (default), or a binary stream.

reposition(+Bool)

Specify whether it is possible to reposition the stream (true), or not (false). By default, YAP enables repositioning for all files, except terminal files and sockets.

eof_action(+Action)

Specify the action to take if attempting to input characters from a stream where we have previously found an end-of-file. The possible actions are error, that raises an error, reset, that tries to reset the stream and is used for tty type files, and eof_code, which generates a new end-of-file (default for non-tty files).

alias(+Name)

Specify an alias to the stream. The alias Name must be an atom. The alias can be used instead of the stream descriptor for every operation concerning the stream.

The operation will fail and give an error if the alias name is already in use. YAP allows several aliases for the same file, but only one is returned by stream_property/2

close(+S) [ISO]

Closes the stream S. If S does not stand for a stream currently opened an error is reported. The streams user_input, user_output, and user_error can never be closed.

By default, give a file name, close/1 will also try to close a corresponding open stream. This feature is not available in ISO or SICStus languages mode and is deprecated.

close(+S,+O) [ISO]

Closes the stream S, following options O.

The only valid options are force(true) and force(false). YAP currently ignores these options.

absolute_file_name(+Name,-FullPath)

Give the path a full path FullPath Yap would use to consult a file named Name. Unify FullPath with user if the file name is user.

current_stream(F,M,S)

Defines the relation: The stream S is opened on the file F in mode M. It might be used to obtain all open streams (by backtracking) or to access the stream for a file F in mode M, or to find properties for a stream S.

flush_output [ISO]

Send all data in the output buffer to current output stream.

flush_output(+S) [ISO]

Send all data in the output buffer to stream S.

set_input(+S)

Set stream S as the current input stream. Predicates like read/1 and get/1 will start using stream S.

set_output(+S)

Set stream S as the current output stream. Predicates like write/1 and put/1 will start using stream S.

stream_select(+STREAMS,+TIMEOUT,-READSTREAMS)

Given a list of open STREAMS openeded in read mode and a TIMEOUT return a list of streams who are now available for reading.

If the TIMEOUT is instantiated to off, stream_select/3 will wait indefinitely for a stream to become open. Otherwise the timeout must be of the form SECS:USECS where SECS is an integer gives the number of seconds to wait for a timeout and USECS adds the number of micro-seconds.

This built-in is only defined if the system call select is available in the system.

current_input(-S) [ISO]

Unify S with the current input stream.

current_output(-S) [ISO]

Unify S with the current output stream.

at_end_of_stream [ISO]

Succeed if the current stream has stream position end-of-stream or past-end-of-stream.

at_end_of_stream(+S) [ISO]

Succeed if the stream S has stream position end-of-stream or past-end-of-stream. Note that S must be a readable stream.

set_stream_position(+S, +POS) [ISO]

Given a stream position POS for a stream S, set the current stream position for S to be POS.

stream_property(?Stream,?Prop) [ISO]

Obtain the properties for the open streams. If the first argument is unbound, the procedure will backtrack through all open streams. Otherwise, the first argument must be a stream term (you may use current_stream to obtain a current stream given a file name).

The following properties are recognized:

file_name(P)

An atom giving the file name for the current stream. The file names are user_input, user_output, and user_error for the standard streams.

mode(P)

The mode used to open the file. It may be one of append, read, or write.

input

The stream is readable.

output

The stream is writable.

alias(A)

ISO-Prolog primitive for stream aliases. Yap returns one of the existing aliases for the stream.

position(P)

A term describing the position in the stream.

end_of_stream(E)

Whether the stream is at the end of stream, or it has found the end of stream and is past, or whether it has not yet reached the end of stream.

eof_action(A)

The action to take when trying to read after reaching the end of stream. The action may be one of error, generate an error, eof_code, return character code -1, or reset the stream.

reposition(B)

Whether the stream can be repositioned or not, that is, whether it is seekable.

type(T)

Whether the stream is a text stream or a binary stream.

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6.6.2 Handling Streams and Files

tell(+S)

If S is a currently opened stream for output, it becomes the current output stream. If S is an atom it is taken to be a filename. If there is no output stream currently associated with it, then it is opened for output, and the new output stream created becomes the current output stream. If it is not possible to open the file, an error occurs. If there is a single opened output stream currently associated with the file, then it becomes the current output stream; if there are more than one in that condition, one of them is chosen.

Whenever S is a stream not currently opened for output, an error may be reported, depending on the state of the file_errors flag. The predicate just fails, if S is neither a stream nor an atom.

telling(-S)

The current output stream is unified with S.

told

Closes the current output stream, and the user's terminal becomes again the current output stream. It is important to remember to close streams after having finished using them, as the maximum number of simultaneously opened streams is 17.

see(+S)

If S is a currently opened input stream then it is assumed to be the current input stream. If S is an atom it is taken as a filename. If there is no input stream currently associated with it, then it is opened for input, and the new input stream thus created becomes the current input stream. If it is not possible to open the file, an error occurs. If there is a single opened input stream currently associated with the file, it becomes the current input stream; if there are more than one in that condition, then one of them is chosen.

When S is a stream not currently opened for input, an error may be reported, depending on the state of the file_errors flag. If S is neither a stream nor an atom the predicates just fails.

seeing(-S)

The current input stream is unified with S.

seen

Closes the current input stream (see 6.7.).

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6.6.3 Handling Input/Output of Terms

read(-T) [ISO]

Reads the next term from the current input stream, and unifies it with T. The term must be followed by a dot ('.') and any blank-character as previously defined. The syntax of the term must match the current declarations for operators (see op). If the end-of-stream is reached, T is unified with the atom end_of_file. Further reads from of the same stream may cause an error failure (see open/3).

read_term(-T,+Options) [ISO]

Reads term T from the current input stream with execution controlled by the following options:

singletons(-Names)

Unify Names with a list of the form Name=Var, where Name is the name of a non-anonymous singleton variable in the original term, and Var is the variable's representation in YAP.

syntax_errors(+Val)

Control action to be taken after syntax errors. See yap_flag/2 for detailed information.

variable_names(-Names)

Unify Names with a list of the form Name=Var, where Name is the name of a non-anonymous variable in the original term, and Var is the variable's representation in YAP.

variables(-Names)

Unify Names with a list of the variables in term T.

char_conversion(+IN,+OUT) [ISO]

While reading terms convert unquoted occurrences of the character IN to the character OUT. Both IN and OUT must be bound to single characters atoms.

Character conversion only works if the flag char_conversion is on. This is default in the iso and sicstus language modes. As an example, character conversion can be used for instance to convert characters from the ISO-LATIN-1 character set to ASCII.

If IN is the same character as OUT, char_conversion/2 will remove this conversion from the table.

current_char_conversion(?IN,?OUT) [ISO]

If IN is unbound give all current character translations. Otherwise, give the translation for IN, if one exists.

write(T) [ISO]

The term T is written to the current output stream according to the operator declarations in force.

display(+T)

Displays term T on the current output stream. All Prolog terms are written in standard parenthesized prefix notation.

write_canonical(+T) [ISO]

Displays term T on the current output stream. Atoms are quoted when necessary, and operators are ignored, that is, the term is written in standard parenthesized prefix notation.

write_term(+T, +Opts) [ISO]

Displays term T on the current output stream, according to the following options:

quoted(+Bool)

If true, quote atoms if this would be necessary for the atom to be recognized as an atom by YAP's parser. The default value is false.

ignore_ops(+Bool)

If true, ignore operator declarations when writing the term. The default value is false.

numbervars(+Bool)

If true, output terms of the form '$VAR'(N), where N is an integer, as a sequence of capital letters. The default value is false.

portrayed(+Bool)

If true, use portray/1 to portray bound terms. The default value is false.

max_depth(+Depth)

If Depth is a positive integer, use Depth as the maximum depth to portray a term. The default is 0, that is, unlimited depth.

writeq(T) [ISO]

Writes the term T, quoting names to make the result acceptable to the predicate 'read' whenever necessary.

print(T)

Prints the term T to the current output stream using write/1 unless T is bound and a call to the user-defined predicate portray/1 succeeds. To do pretty printing of terms the user should define suitable clauses for portray/1 and use print/1.

format(+T,+L)

Print formatted output to the current output stream. The arguments in list L are output according to the string or atom T.

A control sequence is introduced by a w. The following control sequences are available in YAP:

'~~'

Print a single tilde.

'~a'

The next argument must be an atom, that will be printed as if by write.

'~Nc'

The next argument must be an integer, that will be printed as a character code. The number N is the number of times to print the character (default 1).

'~Ne'

'~NE'

'~Nf'

'~Ng'

'~NG'

The next argument must be a floating point number. The float F, the number N and the control code c will be passed to printf as:

printf("%s.Nc", F)

As an example:

?- format("~8e, ~8E, ~8f, ~8g, ~8G~w", [3.14,3.14,3.14,3.14,3.14,3.14]). 3.140000e+00, 3.140000E+00, 3.140000, 3.14, 3.143.14

'~Nd'

The next argument must be an integer, and N is the number of digits after the decimal point. If N is 0 no decimal points will be printed. The default is N = 0.

?- format("~2d, ~d",[15000, 15000]). 150.00, 15000

'~ND'

Identical to '~Nd', except that commas are used to separate groups of three digits.

?- format("~2D, ~D",[150000, 150000]). 1,500.00, 150,000

'~i'

Ignore the next argument in the list of arguments:

?- format('The ~i met the boregrove',[mimsy]). The met the boregrove

'~k'

Print the next argument with write_canonical:

?- format("Good night ~k",a+[1,2]). Good night +(a,[1,2])

'~Nn'

Print N newlines (where N defaults to 1).

'~NN'

Print N newlines if at the beginning of the line (where N defaults to 1).

'~Nr'

The next argument must be an integer, and N is interpreted as a radix, such that 2 <= N <= 36 (the default is 8).

?- format("~2r, 0x~16r, ~r", [150000, 150000, 150000]). 100100100111110000, 0x249f0, 444760

Note that the letters a-z denote digits larger than 9.

'~NR'

Similar to '~NR'. The next argument must be an integer, and N is interpreted as a radix, such that 2 <= N <= 36 (the default is 8).

?- format("~2r, 0x~16r, ~r", [150000, 150000, 150000]). 100100100111110000, 0x249F0, 444760

The only difference is that letters A-Z denote digits larger than 9.

'~p'

Print the next argument with print/1:

?- format("Good night ~p",a+[1,2]). Good night a+[1,2]

'~q'

Print the next argument with writeq/1:

?- format("Good night ~q",'Hello'+[1,2]). Good night 'Hello'+[1,2]

'~Ns'

The next argument must be a list of character codes. The system then outputs their representation as a string, where N is the maximum number of characters for the string (N defaults to the length of the string).

?- format("The ~s are ~4s",["woods","lovely"]). The woods are love

'~w'

Print the next argument with write/1:

?- format("Good night ~w",'Hello'+[1,2]). Good night Hello+[1,2]

The number of arguments, N, may be given as an integer, or it may be given as an extra argument. The next example shows a small procedure to write a variable number of a characters:

write_many_as(N) :- format("~*c",[N,0'a]).

The format/2 built-in also allows for formatted output. One can specify column boundaries and fill the intermediate space by a padding character:

'~N|'

Set a column boundary at position N, where N defaults to the current position.

'~N+'

Set a column boundary at N characters past the current position, where N defaults to 8.

'~Nt'

Set padding for a column, where N is the fill code (default is SPC).

The next example shows how to align columns and padding. We first show left-alignment:

?- format("~n*Hello~16+*~n",[]). *Hello *

Note that we reserve 16 characters for the column.

The following example shows how to do right-alignment:

?- format("*~tHello~16+*~n",[]). * Hello*

The ~t escape sequence forces filling before Hello.

We next show how to do centering:

?- format("*~tHello~t~16+*~n",[]). * Hello *

The two ~t escape sequence force filling both before and after Hello. Space is then evenly divided between the right and the left sides.

format(+S,+T,+L)

Print formatted output to stream S.

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6.6.4 Handling Input/Output of Characters

put(+N)

Outputs to the current output stream the character whose ASCII code is N. The character N must be a legal ASCII character code, an expression yielding such a code, or a list in which case only the first element is used.

put_byte(+N) [ISO]

Outputs to the current output stream the character whose code is N. The current output stream must be a binary stream.

put_char(+N) [ISO]

Outputs to the current output stream the character who is used to build the representation of atom A. The current output stream must be a text stream.

put_code(+N) [ISO]

Outputs to the current output stream the character whose ASCII code is N. The current output stream must be a text stream. The character N must be a legal ASCII character code, an expression yielding such a code, or a list in which case only the first element is used.

get(-C)

The next non-blank character from the current input stream is unified with C. Blank characters are the ones whose ASCII codes are not greater than 32. If there are no more non-blank characters in the stream, C is unified with -1. If end_of_stream has already been reached in the previous reading, this call will give an error message.

get0(-C)

The next character from the current input stream is consumed, and then unified with C. There are no restrictions on the possible values of the ASCII code for the character, but the character will be internally converted by YAP.

get_byte(-C) [ISO]

If C is unbound, or is a character code, and the current stream is a binary stream, read the next byte from the current stream and unify its code with C.

get_char(-C) [ISO]

If C is unbound, or is an atom representation of a character, and the current stream is a text stream, read the next character from the current stream and unify its atom representation with C.

get_code(-C) [ISO]

If C is unbound, or is the code for a character, and the current stream is a text stream, read the next character from the current stream and unify its code with C.

peek_byte(-C) [ISO]

If C is unbound, or is a character code, and the current stream is a binary stream, read the next byte from the current stream and unify its code with C, while leaving the current stream position unaltered.

peek_char(-C) [ISO]

If C is unbound, or is an atom representation of a character, and the current stream is a text stream, read the next character from the current stream and unify its atom representation with C, while leaving the current stream position unaltered.

peek_code(-C) [ISO]

If C is unbound, or is the code for a character, and the current stream is a text stream, read the next character from the current stream and unify its code with C, while leaving the current stream position unaltered.

skip(+N)

Skips input characters until the next occurrence of the character with ASCII code N. The argument to this predicate can take the same forms as those for put (see 6.11).

tab(+N)

Outputs N spaces to the current output stream.

nl [ISO]

Outputs a new line to the current output stream.

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6.6.5 Input/Output Predicates applied to Streams

read(+S,-T) [ISO]

Reads term T from the stream S instead of from the current input stream.

read_term(+S,-T,+Options) [ISO]

Reads term T from stream S with execution controlled by the same options as read_term/2.

write(+S,T) [ISO]

Writes term T to stream S instead of to the current output stream.

write_canonical(+S,+T) [ISO]

Displays term T on the stream S. Atoms are quoted when necessary, and operators are ignored.

write_term(+S, +T, +Opts) [ISO]

Displays term T on the current output stream, according to the same options used by write_term/3.

writeq(+S,T) [ISO]

As writeq/1, but the output is sent to the stream S.

display(+S,T)

Like display/1, but using stream S to display the term.

print(+S,T)

Prints term T to the stream S instead of to the current output stream.

put(+S,+N)

As put(N), but to stream S.

put_byte(+S,+N) [ISO]

As put_byte(N), but to binary stream S.

put_char(+S,+A) [ISO]

As put_char(A), but to text stream S.

put_code(+S,+N) [ISO]

As put_code(N), but to text stream S.

get(+S,-C)

The same as get(C), but from stream S.

get0(+S,-C)

The same as get0(C), but from stream S.

get_byte(+S,-C) [ISO]

If C is unbound, or is a character code, and the stream S is a binary stream, read the next byte from that stream and unify its code with C.

get_char(+S,-C) [ISO]

If C is unbound, or is an atom representation of a character, and the stream S is a text stream, read the next character from that stream and unify its representation as an atom with C.

get_code(+S,-C) [ISO]

If C is unbound, or is a character code, and the stream S is a text stream, read the next character from that stream and unify its code with C.

peek_byte(+S,-C) [ISO]

If C is unbound, or is a character code, and S is a binary stream, read the next byte from the current stream and unify its code with C, while leaving the current stream position unaltered.

peek_char(+S,-C) [ISO]

If C is unbound, or is an atom representation of a character, and the stream S is a text stream, read the next character from that stream and unify its representation as an atom with C, while leaving the current stream position unaltered.

peek_code(+S,-C) [ISO]

If C is unbound, or is an atom representation of a character, and the stream S is a text stream, read the next character from that stream and unify its representation as an atom with C, while leaving the current stream position unaltered.

skip(+S,-C)

Like skip/1, but using stream S instead of the current input stream.

tab(+S,+N)

The same as tab/1, but using stream S.

nl(+S)

Outputs a new line to stream S.

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6.6.6 Compatible C-Prolog predicates for Terminal I/O

ttyput(+N)

As put(N) but always to user_output.

ttyget(-C)

The same as get(C), but from stream user_input.

ttyget0(-C)

The same as get0(C), but from stream user_input.

ttyskip(-C)

Like skip/1, but always using stream user_input. stream.

ttytab(+N)

The same as tab/1, but using stream user_output.

ttynl

Outputs a new line to stream user_output.

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6.6.7 Controlling Input/Output

exists(+F)

Checks if file F exists in the current directory.

nofileerrors

Switches off the file_errors flag, so that the predicates see/1, tell/1, open/3 and close/1 just fail, instead of producing an error message and aborting whenever the specified file cannot be opened or closed.

fileerrors

Switches on the file_errors flag so that in certain error conditions I/O predicates will produce an appropriated message and abort.

write_depth(T,L,A)

Unifies T with the value of the maximum depth of a term to be written, L with the maximum length of a list to write, and A with the maximum number of arguments of a compound term to write. The setting will be used by write/1 or write/2. The default value for all arguments is 0, meaning unlimited depth and length.

?- write_depth(3,5,5). yes ?- write(a(b(c(d(e(f(g))))))). a(b(c(....))) yes ?- write([1,2,3,4,5,6,7,8]). [1,2,3,4,5,...] yes ?- write(a(1,2,3,4,5,6,7,8)). a(1,2,3,4,5,...) yes

write_depth(T,L)

Same as write_depth(T,L,_). Unifies T with the value of the maximum depth of a term to be written, and L with the maximum length of a list to write. The setting will be used by write/1 or write/2. The default value for all arguments is 0, meaning unlimited depth and length.

?- write_depth(3,5,5). yes ?- write(a(b(c(d(e(f(g))))))). a(b(c(....))) yes ?- write([1,2,3,4,5,6,7,8]). [1,2,3,4,5,...] yes

always_prompt_user

Force the system to prompt the user even if the user_input stream is not a terminal. This command is useful if you want to obtain interactive control from a pipe or a socket.

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6.6.8 Using Sockets From Yap

YAP includes a SICStus Prolog compatible socket interface. This is a low level interface that provides direct access to the major socket system calls. These calls can be used both to open a new connection in the network or connect to a networked server. Socket connections are described as read/write streams, and standard I/O built-ins can be used to write on or read from sockets. The following calls are available:

socket(+DOMAIN,+TYPE,+PROTOCOL,-SOCKET)

Corresponds to the BSD system call socket. Create a socket for domain DOMAIN of type TYPE and protocol PROTOCOL. Both DOMAIN and TYPE should be atoms, whereas PROTOCOL must be an integer. The new socket object is accessible through a descriptor bound to the variable SOCKET.

The current implementation of YAP only accepts two socket domains: 'AF_INET' and 'AF_UNIX'. Socket types depend on the underlying operating system, but at least the following types are supported: 'SOCK_STREAM' and 'SOCK_DGRAM'.

socket(+DOMAIN,-SOCKET)

Call socket/4 with TYPE bound to 'SOCK_STREAM' and PROTOCOL bound to 0.

socket_close(+SOCKET)

Close socket SOCKET. Note that sockets used in socket_connect (that is, client sockets) should not be closed with socket_close, as they will be automatically closed when the corresponding stream is closed with close/1 or close/2.

socket_bind(+SOCKET, ?PORT)

Interface to system call bind, as used for servers: bind socket to a port. Port information depends on the domain:

'AF_UNIX'(+FILENAME)

'AF_FILE'(+FILENAME)

use file name FILENAME for UNIX or local sockets.

'AF_INET'(?HOST,?PORT)

If HOST is bound to an atom, bind to host HOST, otherwise if unbound bind to local host (HOST remains unbound). If port PORT is bound to an integer, try to bind to the corresponding port. If variable PORT is unbound allow operating systems to choose a port number, which is unified with PORT.

socket_connect(+SOCKET, +PORT, -STREAM)

Interface to system call connect, used for clients: connect socket SOCKET to PORT. The connection results in the read/write stream STREAM.

Port information depends on the domain:

'AF_UNIX'(+FILENAME)

'AF_FILE'(+FILENAME)

connect to socket at file FILENAME.

'AF_INET'(+HOST,+PORT)

Connect to socket at host HOST and port PORT.

socket_listen(+SOCKET, +LENGTH)

Interface to system call listen, used for servers to indicate willingness to wait for connections at socket SOCKET. The integer LENGTH gives the queue limit for incoming connections, and should be limited to 5 for portable applications. The socket must be of type SOCK_STREAM or SOCK_SEQPACKET.

socket_accept(+SOCKET, -STREAM)

socket_accept(+SOCKET, -CLIENT, -STREAM)

Interface to system call accept, used for servers to wait for connections at socket SOCKET. The stream descriptor STREAM represents the resulting connection. If the socket belongs to the domain 'AF_INET', CLIENT unifies with an atom containing the IP address for the client in numbers and dots notation.

socket_accept(+SOCKET, -STREAM)

Accept a connection but do not return client information.

socket_buffering(+SOCKET, -MODE, -OLD, +NEW)

Set buffering for SOCKET in read or write MODE. OLD is unified with the previous status, and NEW receives the new status which may be one of unbuf or fullbuf.

socket_select(+SOCKETS, -NEWSTREAMS, +TIMEOUT, +STREAMS, -READSTREAMS)

Interface to system call select, used for servers to wait for connection requests or for data at sockets. The variable SOCKETS is a list of form KEY-SOCKET, where KEY is an user-defined identifier and SOCKET is a socket descriptor. The variable TIMEOUT is either off, indicating execution will wait until something is available, or of the form SEC-USEC, where SEC and USEC give the seconds and microseconds before socket_select/5 returns. The variable SOCKETS is a list of form KEY-STREAM, where KEY is an user-defined identifier and STREAM is a stream descriptor

Execution of socket_select/5 unifies READSTREAMS from STREAMS with readable data, and NEWSTREAMS with a list of the form KEY-STREAM, where KEY was the key for a socket with pending data, and STREAM the stream descriptor resulting from accepting the connection.

current_host(?HOSTNAME)

Unify HOSTNAME with an atom representing the fully qualified hostname for the current host. Also succeeds if HOSTNAME is bound to the unqualified hostname.

hostname_address(?HOSTNAME,?IP_ADDRESS)

HOSTNAME is an host name and IP_ADDRESS its IP address in number and dots notation.

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6.7 Using the Clausal Data Base

Predicates in YAP may be dynamic or static. By default, when consulting or reconsulting, predicates are assumed to be static: execution is faster and the code will probably use less space. Static predicates impose some restrictions: in general there can be no addition or removal of clauses for a procedure if it is being used in the current execution.

Dynamic predicates allow programmers to change the Clausal Data Base with the same flexibility as in C-Prolog. With dynamic predicates it is always possible to add or remove clauses during execution and the semantics will be the same as for C-Prolog. But the programmer should be aware of the fact that asserting or retracting are still expensive operations, and therefore he should try to avoid them whenever possible.

dynamic +P

Declares predicate P or list of predicates [P1,...,Pn] as a dynamic predicate. P must be written in form: name/arity.

:- dynamic god/1.

a more convenient form can be used:

:- dynamic son/3, father/2, mother/2.

or, equivalently,

:- dynamic [son/3, father/2, mother/2].

Note:

a predicate is assumed to be dynamic when asserted before being defined.

dynamic_predicate(+P,+Semantics)

Declares predicate P or list of predicates [P1,...,Pn] as a dynamic predicate following either logical or immediate semantics.

Subnodes of Database 6.7.1 Modification of the Data Base  Asserting and Retracting 6.7.2 Looking at the Data Base  Finding out what is in the Data Base 6.7.3 Using Data Base References  Using Data Base References 6.8 Internal Data Base  Yap's Internal Database 6.9 The Blackboard  Storing and Fetching Terms in the BlackBoard

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6.7.1 Modification of the Data Base

These predicates can be used either for static or for dynamic predicates:

assert(+C)

Adds clause C to the program. If the predicate is undefined, declare it as dynamic.

Most Prolog systems only allow asserting clauses for dynamic predicates. This is also as specified in the ISO standard. YAP allows asserting clauses for static predicates, as long as the predicate is not in use and the language flag is cprolog. Note that this feature is deprecated, if you want to assert clauses for static procedures you should use assert_static/1.

asserta(+C) [ISO]

Adds clause C to the beginning of the program. If the predicate is undefined, declare it as dynamic.

assertz(+C) [ISO]

Adds clause C to the end of the program. If the predicate is undefined, declare it as dynamic.

Most Prolog systems only allow asserting clauses for dynamic predicates. This is also as specified in the ISO standard. YAP allows asserting clauses for static predicates. The current version of YAP supports this feature, but this feature is deprecated and support may go away in future versions.

abolish(+PredSpec) [ISO]

Deletes the predicate given by PredSpec from the database. If PredSpec is an unbound variable, delete all predicates for the current module. The specification must include the name and arity, and it may include module information. Under iso language mode this built-in will only abolish dynamic procedures. Under other modes it will abolish any procedures.

abolish(+P,+N)

Deletes the predicate with name P and arity N. It will remove both static and dynamic predicates.

assert_static(:C)

Adds clause C to a static procedure. Asserting a static clause for a predicate while choice-points for the predicate are available has undefined results.

asserta_static(:C)

Adds clause C to the beginning of a static procedure.

assertz_static(:C)

Adds clause C to the end of a static procedure. Asserting a static clause for a predicate while choice-points for the predicate are available has undefined results.

The following predicates can be used for dynamic predicates and for static predicates, if source mode was on when they were compiled:

clause(+H,B) [ISO]

A clause whose head matches H is searched for in the program. Its head and body are respectively unified with H and B. If the clause is a unit clause, B is unified with true.

This predicate is applicable to static procedures compiled with source active, and to all dynamic procedures.

clause(+H,B,-R)

The same as clause/2, plus R is unified with the reference to the clause in the database. You can use instance/2 to access the reference's value. Note that you may not use erase/1 on the reference on static procedures.

nth_clause(+H,I,-R)

Find the Ith clause in the predicate defining H, and give a reference to the clause. Alternatively, if the reference R is given the head H is unified with a description of the predicate and I is bound to its position.

The following predicates can only be used for dynamic predicates:

retract(+C) [ISO]

Erases the first clause in the program that matches C. This predicate may also be used for the static predicates that have been compiled when the source mode was on. For more information on source/0 (see section Changing the Compiler's Behavior).

retractall(+G)

Retract all the clauses whose head matches the goal G. Goal G must be a call to a dynamic predicate.

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6.7.2 Looking at the Data Base

listing

Lists in the current output stream all the clauses for which source code is available (these include all clauses for dynamic predicates and clauses for static predicates compiled when source mode was on).

listing(+P)

Lists predicate P if its source code is available.

portray_clause(+C)

Write clause C as if written by listing/0.

portray_clause(+S,+C)

Write clause C on stream S as if written by listing/0.

current_atom(A)

Checks whether A is a currently defined atom. It is used to find all currently defined atoms by backtracking.

current_predicate(F) [ISO]

F is the predicate indicator for a currently defined user or library predicate. F is of the form Na/Ar, where the atom Na is the name of the predicate, and Ar its arity.

current_predicate(A,P)

Defines the relation: P is a currently defined predicate whose name is the atom A.

system_predicate(A,P)

Defines the relation: P is a built-in predicate whose name is the atom A.

predicate_property(P,Prop)

For the predicates obeying the specification P unify Prop with a property of P. These properties may be:

built_in

true for built-in predicates,

dynamic

true if the predicate is dynamic

static

true if the predicate is static

meta_predicate(M)

true if the predicate has a meta_predicate declaration M.

multifile

true if the predicate was declared to be multifile

imported_from(Mod)

true if the predicate was imported from module Mod.

exported

true if the predicate is exported in the current module.

public

true if the predicate is public; note that all dynamic predicates are public.

tabled

true if the predicate is tabled; note that only static predicates can be tabled in YAP.

source

true if source for the predicate is available.

number_of_clauses(ClauseCount)

Number of clauses in the predicate definition. Always one if external or built-in.

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6.7.3 Using Data Base References

Data Base references are a fast way of accessing terms. The predicates erase/1 and instance/1 also apply to these references and may sometimes be used instead of retract/1 and clause/2.

assert(+C,-R)

The same as assert(C) (see section Modification of the Data Base) but unifies R with the database reference that identifies the new clause, in a one-to-one way. Note that asserta/2 only works for dynamic predicates. If the predicate is undefined, it will automatically be declared dynamic.

asserta(+C,-R)

The same as asserta(C) but unifying R with the database reference that identifies the new clause, in a one-to-one way. Note that asserta/2 only works for dynamic predicates. If the predicate is undefined, it will automatically be declared dynamic.

assertz(+C,-R)

The same as assertz(C) but unifying R with the database reference that identifies the new clause, in a one-to-one way. Note that asserta/2 only works for dynamic predicates. If the predicate is undefined, it will automatically be declared dynamic.

retract(+C,-R)

Erases from the program the clause C whose database reference is R. The predicate must be dynamic.

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6.8 Internal Data Base

Some programs need global information for, e.g. counting or collecting data obtained by backtracking. As a rule, to keep this information, the internal data base should be used instead of asserting and retracting clauses (as most novice programmers do), . In YAP (as in some other Prolog systems) the internal data base (i.d.b. for short) is faster, needs less space and provides a better insulation of program and data than using asserted/retracted clauses. The i.d.b. is implemented as a set of terms, accessed by keys that unlikely what happens in (non-Prolog) data bases are not part of the term. Under each key a list of terms is kept. References are provided so that terms can be identified: each term in the i.d.b. has a unique reference (references are also available for clauses of dynamic predicates).

recorda(+K,T,-R)

Makes term T the first record under key K and unifies R with its reference.

recordz(+K,T,-R)

Makes term T the last record under key K and unifies R with its reference.

recorda_at(+R0,T,-R)

Makes term T the record preceding record with reference R0, and unifies R with its reference.

recordz_at(+R0,T,-R)

Makes term T the record following record with reference R0, and unifies R with its reference.

recordaifnot(+K,T,-R)

If a term equal to T up to variable renaming is stored under key K fail. Otherwise, make term T the first record under key K and unify R with its reference.

recordzifnot(+K,T,-R)

If a term equal to T up to variable renaming is stored under key K fail. Otherwise, make term T the first record under key K and unify R with its reference.

recorded(+K,T,R)

Searches in the internal database under the key K, a term that unifies with T and whose reference matches R. This built-in may be used in one of two ways:

• K may be given, in this case the built-in will return all elements of the internal data-base that match the key.
• R may be given, if so returning the key and element that match the reference.

nth_instance(?K,?Index,T,?R)

Fetches the Indexnth entry in the internal database under the key K. Entries are numbered from one. If the key K are the Index are bound, a reference is unified with R. Otherwise, the reference R must be given, and the term the system will find the matching key and index.

erase(+R)

The term referred to by R is erased from the internal database. If reference R does not exist in the database, erase just fails.

erased(+R)

Succeeds if the object whose database reference is R has been erased.

instance(+R,-T)

If R refers to a clause or a recorded term, T is unified with its most general instance. If R refers to an unit clause C, then T is unified with C :- true. When R is not a reference to an existing clause or to a recorded term, this goal fails.

eraseall(+K)

All terms belonging to the key K are erased from the internal database. The predicate always succeeds.

current_key(?A,?K)

Defines the relation: K is a currently defined database key whose name is the atom A. It can be used to generate all the keys for the internal data-base.

key_statistics(+K,-Entries,-Size,-IndexSize)

Returns several statistics for a key K. Currently, it says how many entries we have for that key, Entries, what is the total size spent on entries, Size, and what is the amount of space spent in indices.

key_statistics(+K,-Entries,-TotalSize)

Returns several statistics for a key K. Currently, it says how many entries we have for that key, Entries, what is the total size spent on this key.

get_value(+A,-V)

In YAP, atoms can be associated with constants. If one such association exists for atom A, unify the second argument with the constant. Otherwise, unify V with [].

This predicate is YAP specific.

set_value(+A,+C)

Associate atom A with constant C.

The set_value and get_value built-ins give a fast alternative to the internal data-base. This is a simple form of implementing a global counter.

read_and_increment_counter(Value) :- get_value(counter, Value), Value1 is Value+1, set_value(counter, Value1).

This predicate is YAP specific.

recordzifnot(+K,T,-R)

If a variant of T is stored under key K fail. Otherwise, make term T the last record under key K and unify R with its reference.

This predicate is YAP specific.

recordaifnot(+K,T,-R)

If a variant of T is stored under key K fail. Otherwise, make term T the first record under key K and unify R with its reference.

This predicate is YAP specific.

There is a strong analogy between the i.d.b. and the way dynamic predicates are stored. In fact, the main i.d.b. predicates might be implemented using dynamic predicates:

recorda(X,T,R) :- asserta(idb(X,T),R). recordz(X,T,R) :- assertz(idb(X,T),R). recorded(X,T,R) :- clause(idb(X,T),R).

We can take advantage of this, the other way around, as it is quite easy to write a simple Prolog interpreter, using the i.d.b.:

asserta(G) :- recorda(interpreter,G,_). assertz(G) :- recordz(interpreter,G,_). retract(G) :- recorded(interpreter,G,R), !, erase(R). call(V) :- var(V), !, fail. call((H :- B)) :- !, recorded(interpreter,(H :- B),_), call(B). call(G) :- recorded(interpreter,G,_).

In YAP, much attention has been given to the implementation of the i.d.b., especially to the problem of accelerating the access to terms kept in a large list under the same key. Besides using the key, YAP uses an internal lookup function, transparent to the user, to find only the terms that might unify. For instance, in a data base containing the terms

b b(a) c(d) e(g) b(X) e(h)

stored under the key k/1, when executing the query

:- recorded(k(_),c(_),R).

recorded would proceed directly to the third term, spending almost the time as if a(X) or b(X) was being searched. The lookup function uses the functor of the term, and its first three arguments (when they exist). So, recorded(k(_),e(h),_) would go directly to the last term, while recorded(k(_),e(_),_) would find first the fourth term, and then, after backtracking, the last one.

This mechanism may be useful to implement a sort of hierarchy, where the functors of the terms (and eventually the first arguments) work as secondary keys.

In the YAP's i.d.b. an optimized representation is used for terms without free variables. This results in a faster retrieval of terms and better space usage. Whenever possible, avoid variables in terms in terms stored in the i.d.b.

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6.9 The Blackboard

YAP implements a blackboard in the style of the SICStus Prolog blackboard. The blackboard uses the same underlying mechanism as the internal data-base but has several important differences:

• It is module aware, in contrast to the internal data-base.
• Keys can only be atoms or integers, and not compound terms.
• A single term can be stored per key.
• An atomic update operation is provided; this is useful for parallelism.

bb_put(+Key,?Term)

Store term table Term in the blackboard under key Key. If a previous term was stored under key Key it is simply forgotten.

bb_get(+Key,?Term)

Unify Term with a term stored in the blackboard under key Key, or fail silently if no such term exists.

bb_delete(+Key,?Term)

Delete any term stored in the blackboard under key Key and unify it with Term. Fail silently if no such term exists.

bb_update(+Key,?Term,?New)

Atomically unify a term stored in the blackboard under key Key with Term, and if the unification succeeds replace it by New. Fail silently if no such term exists or if unification fails.

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6.10 Collecting Solutions to a Goal

When there are several solutions to a goal, if the user wants to collect all the solutions he may be led to use the data base, because backtracking will forget previous solutions.

YAP allows the programmer to choose from several system predicates instead of writing his own routines. findall/3 gives you the fastest, but crudest solution. The other built-in predicates post-process the result of the query in several different ways:

findall(T,+G,-L) [ISO]

Unifies L with a list that contains all the instantiations of the term T satisfying the goal G.

With the following program:

a(2,1). a(1,1). a(2,2).

the answer to the query

findall(X,a(X,Y),L).

would be:

X = _32 Y = _33 L = [2,1,2]; no

findall(T,+G,+L,-L0)

Similar to findall/3, but appends all answers to list L0.

all(T,+G,-L)

Similar to findall(T,G,L) but eliminating repeated elements. Thus, assuming the same clauses as in the above example, the reply to the query

all(X,a(X,Y),L).

would be:

X = _32 Y = _33 L = [2,1]; no

bagof(T,+G,-L) [ISO]

For each set of possible instances of the free variables occurring in G but not in T, generates the list L of the instances of T satisfying G. Again, assuming the same clauses as in the examples above, the reply to the query

bagof(X,a(X,Y),L). would be: X = _32 Y = 1 L = [2,1]; X = _32 Y = 2 L = [2]; no

setof(X,+P,-B) [ISO]

Similar to bagof(T,G,L) but sorting list L and keeping only one copy of each element. Again, assuming the same clauses as in the examples above, the reply to the query

setof(X,a(X,Y),L).

would be:

X = _32 Y = 1 L = [1,2]; X = _32 Y = 2 L = [2]; no

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6.11 Grammar Rules

Grammar rules in Prolog are both a convenient way to express definite clause grammars and an extension of the well known context-free grammars.

A grammar rule is of the form:

head --> body

where both head and body are sequences of one or more items linked by the standard conjunction operator ','.

Items can be:

• a non-terminal symbol may be either a complex term or an atom.
• a terminal symbol may be any Prolog symbol. Terminals are written as Prolog lists.
• an empty body is written as the empty list '[ ]'.
• extra conditions may be inserted as Prolog procedure calls, by being written inside curly brackets '{' and '}'.
• the left side of a rule consists of a nonterminal and an optional list of terminals.
• alternatives may be stated in the right-hand side of the rule by using the disjunction operator ';'.
• the cut and conditional symbol ('->') may be inserted in the right hand side of a grammar rule

Grammar related built-in predicates:

expand_term(T,-X)

This predicate is used by YAP for preprocessing each top level term read when consulting a file and before asserting or executing it. It rewrites a term T to a term X according to the following rules: first try to use the user defined predicate term_expansion/2. If this call fails then the translating process for DCG rules is applied, together with the arithmetic optimizer whenever the compilation of arithmetic expressions is in progress.

user:goal_expansion(+G,+M,-NG)

Yap now supports goal_expansion/3. This is an user-defined procedure that is called after term expansion when compiling or asserting goals for each sub-goal in a clause. The first argument is bound to the goal and the second to the module under which the goal G will execute. If goal_expansion/3 succeeds the new sub-goal NG will replace G and will be processed in the same way. If goal_expansion/3 fails the system will use the default rules.

phrase(+P,L,R)

This predicate succeeds when the difference list L-R is a phrase of type P.

phrase(+P,L)

This predicate succeeds when L is a phrase of type P. The same as phrase(P,L,[]).

Both this predicate and the previous are used as a convenient way to start execution of grammar rules.

'C'(S1,T,S2)

This predicate is used by the grammar rules compiler and is defined as 'C'([H|T],H,T).

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6.12 Access to Operating System Functionality

The following built-in predicates allow access to underlying Operating System functionality:

cd(+D)

Changes the current directory (on UNIX environments).

environ(+E,-S)

Given an environment variable E this predicate unifies the second argument S with its value.

getcwd(-D)

Unify the current directory, represented as an atom, with the argument D.

putenv(+E,+S)

Set environment variable E to the value S. If the environment variable E does not exist, create a new one. Both the environment variable and the value must be atoms.

rename(+F,+G)

Renames file F to G.

sh

Creates a new shell interaction.

system(+S)

Passes command S to the Bourne shell (on UNIX environments) or the current command interpreter in WIN32 environments.

unix(+S)

Access to Unix-like functionality:

argv/1

Return a list of arguments to the program. These are the arguments that follow a --, as in the usual Unix convention.

cd/0

Change to home directory.

cd/1

Change to given directory. Acceptable directory names are strings or atoms.

environ/2

If the first argument is an atom, unify the second argument with the value of the corresponding environment variable.

getcwd/1

Unify the first argument with an atom representing the current directory.

putenv/2

Set environment variable E to the value S. If the environment variable E does not exist, create a new one. Both the environment variable and the value must be atoms.

shell/1

Execute command under current shell. Acceptable commands are strings or atoms.

system/1

Execute command with /bin/sh. Acceptable commands are strings or atoms.

shell/0

Execute a new shell.

alarm(+Seconds,+Callable,+OldAlarm)

Arranges for YAP to be interrupted in Seconds seconds. When interrupted, YAP will execute Callable and then return to the previous execution. If Seconds is 0, no new alarm is scheduled. In any event, any previously set alarm is canceled.

The variable OldAlarm unifies with the number of seconds remaining until any previously scheduled alarm was due to be delivered, or with 0 if there was no previously scheduled alarm.

Note that execution of Callable will wait if YAP is executing built-in predicates, such as Input/Output operations.

The next example shows how alarm/3 can be used to implement a simple clock:

loop :- loop. ticker :- write('.'), flush_output, get_value(tick, yes), alarm(1,ticker,_). :- set_value(tick, yes), alarm(1,ticker,_), loop.

The clock, ticker, writes a dot and then checks the flag tick to see whether it can continue ticking. If so, it calls itself again. Note that there is no guarantee that the each dot corresponds a second: for instance, if the YAP is waiting for user input, ticker will wait until the user types the entry in.

The next example shows how alarm/3 can be used to guarantee that a certain procedure does not take longer than a certain amount of time:

loop :- loop. :- catch((alarm(10, throw(ball), _),loop), ball, format('Quota exhausted.~n',[])).

In this case after 10 seconds our loop is interrupted, ball is thrown, and the handler writes Quota exhausted. Execution then continues from the handler.

Note that in this case loop/0 always executes until the alarm is sent. Often, the code you are executing succeeds or fails before the alarm is actually delivered. In this case, you probably want to disable the alarm when you leave the procedure. The next procedure does exactly so:

once_with_alarm(Time,Goal,DoOnAlarm) :- catch(execute_once_with_alarm(Time, Goal), alarm, DoOnAlarm). execute_once_with_alarm(Time, Goal) :- alarm(Time, alarm, _), ( call(Goal) -> alarm(0, alarm, _) ; alarm(0, alarm, _), fail).

The procedure has three arguments: the Time before the alarm is sent; the Goal to execute; and the goal DoOnAlarm to execute if the alarm is sent. It uses catch/3 to handle the case the alarm is sent. Then it starts the alarm, calls the goal Goal, and disables the alarm on success or failure.

on_signal(+Signal,?OldAction,+Callable)

Set the interrupt handler for soft interrupt Signal to be Callable. OldAction is unified with the previous handler.

Only a subset of the software interrupts (signals) can have their handlers manipulated through on_signal/3. Their POSIX names, YAP names and default behavior is given below. The "YAP name" of the signal is the atom that is associated with each signal, and should be used as the first argument to on_signal/3. It is chosen so that it matches the signal's POSIX name.

on_signal/3 succeeds, unless when called with an invalid signal name or one that is not supported on this platform. No checks are made on the handler provided by the user.

sig_up (Hangup)

SIGHUP in Unix/Linux; Reconsult the initialization files ~/.yaprc, ~/.prologrc and ~/prolog.ini.

sig_usr1 and sig_usr2 (User signals)

SIGUSR1 and SIGUSR2 in Unix/Linux; Print a message and halt.

A special case is made, where if Callable is bound to default, then the default handler is restored for that signal.

A call in the form on_signal(S,H,H) can be used to retrieve a signal's current handler without changing it.

It must be noted that although a signal can be received at all times, the handler is not executed while Yap is waiting for a query at the prompt. The signal will be, however, registered and dealt with as soon as the user makes a query.

Please also note, that neither POSIX Operating Systems nor Yap guarantee that the order of delivery and handling is going to correspond with the order of dispatch.

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6.13 Term Modification

It is sometimes useful to change the value of instantiated variables. Although, this is against the spirit of logic programming, it is sometimes useful. As in other Prolog systems, YAP has several primitives that allow updating Prolog terms. Note that these primitives are also backtrackable.

The setarg/3 primitive allows updating any argument of a Prolog compound terms. The mutable family of predicates provides mutable variables. They should be used instead of setarg/3, as they allow the encapsulation of accesses to updatable variables. Their implementation can also be more efficient for long deterministic computations.

setarg(+I,+S,?T)

Set the value of the Ith argument of term S to term T.

create_mutable(+D,-M)

Create new mutable variable M with initial value D.

get_mutable(?D,+M)

Unify the current value of mutable term M with term D.

is_mutable(?D)

Holds if D is a mutable term.

get_mutable(?D,+M)

Unify the current value of mutable term M with term D.

update_mutable(+D,+M)

Set the current value of mutable term M to term D.

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6.14 Profiling Prolog Programs

Predicates compiled with YAP's flag profiling set to on, keep information on the number of times the predicate was called. This information can be used to detect what are the most commonly called predicates in the program.

The YAP profiling sub-system is currently under-development. Functionality for this sub-system will increase with newer implementation.

Notes:

• Profiling works for both static and dynamic predicates.
• Currently only information on entries and retries to a predicate are maintained. This may change in the future.
• As an example, the following user-level program gives a list of the most often called procedures in a program. The procedure list_profile shows all procedures, irrespective of module, and the procedure list_profile/1 shows the procedures being used in a specific module.

  list_profile :- % get number of calls for each profiled procedure setof(D-[M:P|D1],(current_module(M),profile_data(M:P,calls,D),profile_data(M:P,retries,D1)),LP), % output so that the most often called % predicates will come last: write_profile_data(LP). list_profile(Module) :- % get number of calls for each profiled procedure setof(D-[Module:P|D1],(profile_data(Module:P,calls,D),profile_data(Module:P,retries,D1)),LP), % output so that the most often called % predicates will come last: write_profile_data(LP). write_profile_data([]). write_profile_data([D-[M:P|R]|SLP]) :- % swap the two calls if you want the most often % called predicates first. format('~a:~w: ~32+~t~d~12+~t~d~12+~n', [M,P,D,R]), write_profile_data(SLP).

These are the current predicates to access and clear profiling data:

profile_data(?Na/Ar, ?Parameter, -Data)

Give current profile data on Parameter for a predicate described by the predicate indicator Na/Ar. If any of Na/Ar or Parameter are unbound, backtrack through all profiled predicates or stored parameters. Current parameters are:

calls

Number of times a procedure was called.

retries

Number of times a call to the procedure was backtracked to and retried.

profile_reset

Reset all profiling information.

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6.15 Counting Calls

Predicates compiled with YAP's flag call_counting set to on update counters on the numbers of calls and of retries. Counters are actually decreasing counters, so that they can be used as timers. Three counters are available:

• calls: number of predicate calls since execution started or since system was reset;
• retries: number of retries for predicates called since execution started or since counters were reset;
• calls_and_retries: count both on predicate calls and retries.

These counters can be used to find out how many calls a certain goal takes to execute. They can also be used as timers.

The code for the call counters piggybacks on the profiling code. Therefore, activating the call counters also activates the profiling counters.

These are the predicates that access and manipulate the call counters:

call_count_data(-Calls, -Retries, -CallsAndRetries)

Give current call count data. The first argument gives the current value for the Calls counter, next the Retries counter, and last the CallsAndRetries counter.

call_count_reset

Reset call count counters. All timers are also reset.

call_count(?CallsMax, ?RetriesMax, ?CallsAndRetriesMax)

Set call count counter as timers. YAP will generate an exception if one of the instantiated call counters decreases to 0. YAP will ignore unbound arguments:

• CallsMax: throw the exception call_counter when the counter calls reaches 0;
• RetriesMax: throw the exception retry_counter when the counter retries reaches 0;
• CallsAndRetriesMax: throw the exception call_and_retry_counter when the counter calls_and_retries reaches 0.

Next, we show a simple example of how to use call counters:

?- yap_flag(call_counting,on), [-user]. l :- l. end_of_file. yap_flag(call_counting,off). yes yes ?- catch((call_count(10000,_,_),l),call_counter,format("limit_exceeded.~n",[])). limit_exceeded. yes

Notice that we first compile the looping predicate l/0 with call_counting on. Next, we catch/3 to handle an exception when l/0 performs more than 10000 reductions.

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6.16 Arrays

The YAP system includes experimental support for arrays. The support is enabled with the option YAP_ARRAYS.

There are two very distinct forms of arrays in YAP. The dynamic arrays are a different way to access compound terms created during the execution. Like any other terms, any bindings to these terms and eventually the terms themselves will be destroyed during backtracking. Our goal in supporting dynamic arrays is twofold. First, they provide an alternative to the standard arg/3 built-in. Second, because dynamic arrays may have name that are globally visible, a dynamic array can be visible from any point in the program. In more detail, the clause

g(X) :- array_element(a,2,X).

will succeed as long as the programmer has used the built-in array/2 to create an array term with at least 3 elements in the current environment, and the array was associated with the name a. The element X is a Prolog term, so one can bind it and any such bindings will be undone when backtracking. Note that dynamic arrays do not have a type: each element may be any Prolog term.

The static arrays are an extension of the database. They provide a compact way for manipulating data-structures formed by characters, integers, or floats imperatively. They can also be used to provide two-way communication between YAP and external programs through shared memory.

In order to efficiently manage space elements in a static array must have a type. Currently, elements of static arrays in YAP should have one of the following predefined types:

• byte: an 8-bit signed character.
• unsigned_byte: an 8-bit unsigned character.
• int: Prolog integers. Size would be the natural size for the machine's architecture.
• float: Prolog floating point number. Size would be equivalent to a double in C.
• atom: a Prolog atom.
• dbref: an internal database reference.
• term: a generic Prolog term. Note that this will term will not be stored in the array itself, but instead will be stored in the Prolog internal database.

Arrays may be named or anonymous. Most arrays will be named, that is associated with an atom that will be used to find the array. Anonymous arrays do not have a name, and they are only of interest if the TERM_EXTENSIONS compilation flag is enabled. In this case, the unification and parser are extended to replace occurrences of Prolog terms of the form X[I] by run-time calls to array_element/3, so that one can use array references instead of extra calls to arg/3. As an example:

g(X,Y,Z,I,J) :- X[I] is Y[J]+Z[I].

should give the same results as:

G(X,Y,Z,I,J) :- array_element(X,I,E1), array_element(Y,J,E2), array_element(Z,I,E3), E1 is E2+E3.

Note that the only limitation on array size are the stack size for dynamic arrays; and, the heap size for static (not memory mapped) arrays. Memory mapped arrays are limited by available space in the file system and in the virtual memory space.

The following predicates manipulate arrays:

array(+Name, +Size)

Creates a new dynamic array. The Size must evaluate to an integer. The Name may be either an atom (named array) or an unbound variable (anonymous array).

Dynamic arrays work as standard compound terms, hence space for the array is recovered automatically on backtracking.

static_array(+Name, +Size, +Type)

Create a new static array with name Name. Note that the Name must be an atom (named array). The Size must evaluate to an integer. The Type must be bound to one of types mentioned previously.

static_array_location(+Name, -Ptr)

Give the location for a static array with name Name.

static_array_properties(?Name, ?Size, ?Type)

Show the properties size and type of a static array with name Name. Can also be used to enumerate all current static arrays.

This built-in will silently fail if the there is no static array with that name.

static_array_to_term(?Name, ?Term)

Convert a static array with name Name to a compound term of name Name.

This built-in will silently fail if the there is no static array with that name.

mmapped_array(+Name, +Size, +Type, +File)

Similar to static_array/3, but the array is memory mapped to file File. This means that the array is initialized from the file, and that any changes to the array will also be stored in the file.

This built-in is only available in operating systems that support the system call mmap. Moreover, mmapped arrays do not store generic terms (type term).

close_static_array(+Name)

Close an existing static array of name Name. The Name must be an atom (named array). Space for the array will be recovered and further accesses to the array will return an error.

resize_static_array(+Name, -OldSize, +NewSize)

Expand or reduce a static array, The Size must evaluate to an integer. The Name must be an atom (named array). The Type must be bound to one of int, dbref, float or atom.

Note that if the array is a mmapped array the size of the mmapped file will be actually adjusted to correspond to the size of the array.

array_element(+Name, +Index, ?Element)

Unify Element with Name[Index]. It works for both static and dynamic arrays, but it is read-only for static arrays, while it can be used to unify with an element of a dynamic array.

update_array(+Name, +Index, ?Value)

Attribute value Value to Name[Index]. Type restrictions must be respected for static arrays. This operation is available for dynamic arrays if MULTI_ASSIGNMENT_VARIABLES is enabled (true by default). Backtracking undoes update_array/3 for dynamic arrays, but not for static arrays.

Note that update_array/3 actually uses setarg/3 to update elements of dynamic arrays, and setarg/3 spends an extra cell for every update. For intensive operations we suggest it may be less expensive to unify each element of the array with a mutable terms and to use the operations on mutable terms.

add_to_array_element(+Name, +Index, , +Number, ?NewValue)

Add Number Name[Index] and unify NewValue with the incremented value. Observe that Name[Index] must be an number. If Name is a static array the type of the array must be int or float. If the type of the array is int you only may add integers, if it is float you may add integers or floats. If Name corresponds to a dynamic array the array element must have been previously bound to a number and Number can be any kind of number.

The add_to_array_element/3 built-in actually uses setarg/3 to update elements of dynamic arrays. For intensive operations we suggest it may be less expensive to unify each element of the array with a mutable terms and to use the operations on mutable terms.

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6.17 Predicate Information

Built-ins that return information on the current predicates and modules:

current_module(M)

Succeeds if M are defined modules. A module is defined as soon as some predicate defined in the module is loaded, as soon as a goal in the module is called, or as soon as it becomes the current type-in module.

current_module(M,F)

Succeeds if M are current modules associated to the file F.

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6.18 Miscellaneous

statistics/0

Send to the current user error stream general information on space used and time spent by the system.

?- statistics. memory (total) 4784124 bytes program space 3055616 bytes: 1392224 in use, 1663392 free 2228132 max stack space 1531904 bytes: 464 in use, 1531440 free global stack: 96 in use, 616684 max local stack: 368 in use, 546208 max trail stack 196604 bytes: 8 in use, 196596 free 0.010 sec. for 5 code, 2 stack, and 1 trail space overflows 0.130 sec. for 3 garbage collections which collected 421000 bytes 0.000 sec. for 0 atom garbage collections which collected 0 bytes 0.880 sec. runtime 1.020 sec. cputime 25.055 sec. elapsed time

The example shows how much memory the system spends. Memory is divided into Program Space, Stack Space and Trail. In the example we have 3MB allocated for program spaces, with less than half being actually used. Yap also shows the maximum amount of heap space having been used which was over 2MB.

The stack space is divided into two stacks which grow against each other. We are in the top level so very little stack is being used. On the other hand, the system did use a lot of global and local stack during the previous execution (we refer the reader to a WAM tutorial in order to understand what are the global and local stacks).

Yap also shows information on how many memory overflows and garbage collections the system executed, and statistics on total execution time. Cputime includes all running time, runtime excludes garbage collection and stack overflow time.

statistics(?Param,-Info)

Gives statistical information on the system parameter given by first argument:

cputime

[Time since Boot,Time From Last Call to Cputime]
This gives the total cputime in milliseconds spent executing Prolog code, garbage collection and stack shifts time included.

garbage_collection

[Number of GCs,Total Global Recovered,Total Time Spent]
Number of garbage collections, amount of space recovered in kbytes, and total time spent doing garbage collection in milliseconds. More detailed information is available using yap_flag(gc_trace,verbose).

global_stack

[Global Stack Used,Execution Stack Free]
Space in kbytes currently used in the global stack, and space available for expansion by the local and global stacks.

local_stack

[Local Stack Used,Execution Stack Free]
Space in kbytes currently used in the local stack, and space available for expansion by the local and global stacks.

heap

[Heap Used,Heap Free]
Total space in kbytes not recoverable in backtracking. It includes the program code, internal data base, and, atom symbol table.

program

[Program Space Used,Program Space Free]
Equivalent to heap.

runtime

[Time since Boot,Time From Last Call to Runtime]
This gives the total cputime in milliseconds spent executing Prolog code, not including garbage collections and stack shifts. Note that until Yap4.1.2 the runtime statistics would return time spent on garbage collection and stack shifting.

stack_shifts

[Number of Heap Shifts,Number of Stack Shifts,Number of Trail Shifts]
Number of times YAP had to expand the heap, the stacks, or the trail. More detailed information is available using yap_flag(gc_trace,verbose).

trail

[Trail Used,Trail Free]
Space in kbytes currently being used and still available for the trail.

walltime

[Time since Boot,Time From Last Call to Runtime]
This gives the clock time in milliseconds since starting Prolog.

yap_flag(?Param,?Value)

Set or read system properties for Param:

argv

Read-only flag. It unifies with a list of atoms that gives the arguments to Yap after --.

bounded [ISO]

Read-only flag telling whether integers are bounded. The value depends on whether YAP uses the GMP library or not.

profiling

If off (default) do not compile call counting information for procedures. If on compile predicates so that they calls and retries to the predicate may be counted. Profiling data can be read through the call_count_data/3 built-in.

char_conversion [ISO]

Writable flag telling whether a character conversion table is used when reading terms. The default value for this flag is off except in sicstus and iso language modes, where it is on.

character_escapes [ISO]

Writable flag telling whether a character escapes are enables, on, or disabled, off. The default value for this flag is on.

debug [ISO]

If Value is unbound, tell whether debugging is on or off. If Value is bound to on enable debugging, and if it is bound to off disable debugging.

discontiguous_warnings

If Value is unbound, tell whether warnings for discontiguous predicates are on or off. If Value is bound to on enable these warnings, and if it is bound to off disable them. The default for YAP is off, unless we are in sicstus or iso mode.

dollar_as_lower_case

If off (default) consider the character '$' a control character, if on consider '$' a lower case character.

double_quotes [ISO]

If Value is unbound, tell whether a double quoted list of characters token is converted to a list of atoms, chars, to a list of integers, codes, or to a single atom, atom. If Value is bound, set to the corresponding behavior. The default value is codes.

fast

If on allow fast machine code, if off (default) disable it. Only available in experimental implementations.

fileerrors

If on fileerrors is on, if off (default) fileerrors is disabled.

float_format

C-library printf() format specification used by write/1 and friends to determine how floating point numbers are printed. The default is %.15g. The specified value is passed to printf() without further checking. For example, if you want less digits printed, %g will print all floats using 6 digits instead of the default 15.

gc

If on allow garbage collection (default), if off disable it.

gc_margin

Set or show the minimum free stack before starting garbage collection. The default depends on total stack size.

gc_trace

If off (default) do not show information on garbage collection and stack shifts, if on inform when a garbage collection or stack shift happened, if verbose give detailed information on garbage collection and stack shifts. Last, if very_verbose give detailed information on data-structures found during the garbage collection process, namely, on choice-points.

host_type

Return configure system information, including the machine-id for which Yap was compiled and Operating System information.

index

If on allow indexing (default), if off disable it.

informational_messages

If on allow printing of informational messages, such as the ones that are printed when consulting. If off disable printing these messages. It is on by default except if Yap is booted with the -L flag.

integer_rounding_function [ISO]

Read-only flag telling the rounding function used for integers. Takes the value down for the current version of YAP.

language

Choose whether YAP is closer to C-Prolog, cprolog, iso-prolog, iso or SICStus Prolog, sicstus. The current default is cprolog. This flag affects update semantics, leashing mode, style_checking, handling calls to undefined procedures, how directives are interpreted, when to use dynamic, character escapes, and how files are consulted.

max_arity [ISO]

Read-only flag telling the maximum arity of a functor. Takes the value unbounded for the current version of YAP.

max_integer [ISO]

Read-only flag telling the maximum integer in the implementation. Depends on machine and Operating System architecture, and on whether YAP uses the GMP multi-precision library. If bounded is false, requests for max_integer will fail.

max_tagged_integer

Read-only flag telling the maximum integer we can store as a single word. Depends on machine and Operating System architecture. It can be used to find the word size of the current machine.

min_integer [ISO]

Read-only flag telling the minimum integer in the implementation. Depends on machine and Operating System architecture, and on whether YAP uses the GMP multi-precision library. If bounded is false, requests for min_integer will fail.

min_tagged_integer

Read-only flag telling the minimum integer we can store as a single word. Depends on machine and Operating System architecture.

n_of_integer_keys_in_bb

Read or set the size of the hash table that is used for looking up the blackboard when the key is an integer.

n_of_integer_keys_in_db

Read or set the size of the hash table that is used for looking up the internal data-base when the key is an integer.

profiling

If off (default) do not compile profiling information for procedures. If on compile predicates so that they will output profiling information. Profiling data can be read through the profile_data/3 built-in.

redefine_warnings

If Value is unbound, tell whether warnings for procedures defined in several different files are on or off. If Value is bound to on enable these warnings, and if it is bound to off disable them. The default for YAP is off, unless we are in sicstus or iso mode.

single_var_warnings

If Value is unbound, tell whether warnings for singleton variables are on or off. If Value is bound to on enable these warnings, and if it is bound to off disable them. The default for YAP is off, unless we are in sicstus or iso mode.

strict_iso

If Value is unbound, tell whether strict ISO compatibility mode is on or off. If Value is bound to on set language mode to iso and enable strict mode. If Value is bound to off disable strict mode, and keep the current language mode. The default for YAP is off.

Under strict ISO prolog mode all calls to non-ISO built-ins generate an error. Compilation of clauses that would call non-ISO built-ins will also generate errors. Pre-processing for grammar rules is also disabled. Module expansion is still performed.

Arguably, ISO Prolog does not provide all the functionality required from a modern Prolog system. Moreover, because most Prolog implementations do not fully implement the standard and because the standard itself gives the implementor latitude in a few important questions, such as the unification algorithm and maximum size for numbers there is not guarantee that programs compliant with this mode will work the same way in every Prolog and in every platform. We thus believe this mode is mostly useful when investigating how a program depends on a Prolog's platform specific features.

stack_dump_on_error

If on show a stack dump when Yap finds an error. The default is off.

syntax_errors

Control action to be taken after syntax errors while executing read/1, read/2, or read_term/3:

dec10

Report the syntax error and retry reading the term.

fail

Report the syntax error and fail (default).

error

Report the syntax error and generate an error.

quiet

Just fail

system_options

This read only flag tells which options were used to compile Yap. Currently it informs whether the system supports coroutining, depth_limit, the low_level_tracer, or-parallelism, rational_trees, tabling, threads, or the wam_profiler.

to_chars_mode

Define whether YAP should follow quintus-like semantics for the atom_chars/1 or number_chars/1 built-in, or whether it should follow the ISO standard (iso option).

+

toplevel_hook

+If bound, set the argument to a goal to be executed before entering the top-level. If unbound show the current goal or true if none is presented. Only the first solution is considered and the goal is not backtracked into.

typein_module

If bound, set the current working or type-in module to the argument, which must be an atom. If unbound, unify the argument with the current working module.

unknown [ISO]

Corresponds to calling the unknown/2 built-in.

update_semantics

Define whether YAP should follow immediate update semantics, as in C-Prolog (default), logical update semantics, as in Quintus Prolog, SICStus Prolog, or in the ISO standard. There is also an intermediate mode, logical_assert, where dynamic procedures follow logical semantics but the internal data base still follows immediate semantics.

user_error

If the second argument is bound to a stream, set user_error to this stream. If the second argument is unbound, unify the argument with the current user_error stream.

By default, the user_error stream is set to a stream corresponding to the Unix stderr stream.

The next example shows how to use this flag:

?- open( '/dev/null', append, Error, [alias(mauri_tripa)] ). Error = '$stream'(3) ? ; no ?- set_prolog_flag(user_error, mauri_tripa). close(mauri_tripa). yes ?-

We execute three commands. First, we open a stream in write mode and give it an alias, in this case mauri_tripa. Next, we set user_error to the stream via the alias. Note that after we did so prompts from the system were redirected to the stream mauri_tripa. Last, we close the stream. At this point, YAP automatically redirects the user_error alias to the original stderr.

user_input

If the second argument is bound to a stream, set user_input to this stream. If the second argument is unbound, unify the argument with the current user_input stream.

By default, the user_input stream is set to a stream corresponding to the Unix stdin stream.

user_output

If the second argument is bound to a stream, set user_output to this stream. If the second argument is unbound, unify the argument with the current user_output stream.

By default, the user_output stream is set to a stream corresponding to the Unix stdout stream.

version

Read-only flag that giving the current version of Yap.

write_strings

Writable flag telling whether the system should write lists of integers that are writable character codes using the list notation. It is on if enables or off if disabled. The default value for this flag is off.

current_prolog_flag(?Flag,-Value) [ISO]

Obtain the value for a YAP Prolog flag. Equivalent to calling yap_flag/2 with the second argument unbound, and unifying the returned second argument with Value.

prolog_flag(?Flag,-OldValue,+NewValue)

Obtain the value for a YAP Prolog flag and then set it to a new value. Equivalent to first calling current_prolog_flag/2 with the second argument OldValue unbound and then calling set_prolog_flag/2 with the third argument NewValue.

set_prolog_flag(+Flag,+Value) [ISO]

Set the value for YAP Prolog flag Flag. Equivalent to calling yap_flag/2 with both arguments bound.

op(+P,+T,+A) [ISO]

Defines the operator A or the list of operators A with type T (which must be one of xfx, xfy,yfx, xf, yf, fx or fy) and precedence P (see appendix iv for a list of predefined operators).

Note that if there is a preexisting operator with the same name and type, this operator will be discarded. Also, ',' may not be defined as an operator, and it is not allowed to have the same for an infix and a postfix operator.

current_op(P,T,F) [ISO]

Defines the relation: P is a currently defined operator of type T and precedence P.

prompt(-A,+B)

Changes YAP input prompt from A to B.

initialization

Execute the goals defined by initialization/1. Only the first answer is considered.

prolog_initialization(G)

Add a goal to be executed on system initialization. This is compatible with SICStus Prolog's initialization/1.

version

Write YAP's boot message.

version(-Message)

Add a message to be written when yap boots or after aborting. It is not possible to remove messages.

prolog_load_context(?Key, ?Value)

Obtain information on what is going on in the compilation process. The following keys are available:

directory

Full name for the directory where YAP is currently consulting the file.

file

Full name for the file currently being consulted. Notice that included filed are ignored.

module

Current source module.

source

Full name for the file currently being read in, which may be consulted, reconsulted, or included.

stream

Stream currently being read in.

term_position

Stream position at the stream currently being read in.

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7. Library Predicates

Library files reside in the library_directory path (set by the LIBDIR variable in the Makefile for YAP). Currently, most files in the library are from the Edinburgh Prolog library.

Library, Extensions, Builtins, Top 7.1 Apply Macros  Apply a Predicate to a list or to sub-terms. 7.2 Association Lists  Binary Tree Implementation of Association Lists. 7.3 AVL Trees  Predicates to add and lookup balanced binary trees. 7.4 Heaps  Labelled binary tree where the key of each node is less than or equal to the keys of its children. 7.5 List Manipulation  List Manipulation 7.6 Ordered Sets  Ordered Set Manipulation 7.7 Pseudo Random Number Integer Generator  Pseudo Random Numbers 7.8 Queues  Queue Manipulation 7.9 Random Number Generator  Random Numbers 7.10 Red-Black Trees  Predicates to add, lookup and delete in red-black binary trees. 7.11 Regular Expressions  Regular Expression Manipulation 7.12 Splay Trees  Splay Trees 7.13 Reading From and Writing To Strings  Writing To and Reading From Strings 7.14 Calling The Operating System from YAP  System Utilities 7.15 Utilities On Terms  Utilities on Terms 7.16 Call Cleanup  Call With registered Cleanup Calls 7.17 Calls With Timeout  Call With Timeout 7.18 Updatable Binary Trees  Updatable Binary Trees 7.19 Unweighted Graphs  Unweighted Graphs 7.20 Directed Graphs  Directed Graphs Implemented With Red-Black Trees 7.21 Undirected Graphs  Undirected Graphs Using DGraphs

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7.1 Apply Macros

This library provides a set of utilities for applying a predicate to all elements of a list or to all sub-terms of a term. They allow to easily perform the most common do-loop constructs in Prolog. To avoid performance degradation due to apply/2, each call creates an equivalent Prolog program, without meta-calls, which is executed by the Prolog engine instead. Note that if the equivalent Prolog program already exists, it will be simply used. The library is based on code by Joachim Schimpf.

The following routines are available once included with the use_module(library(apply_macros)) command.

maplist(+Pred, ?ListIn, ?ListOut)

Creates ListOut by applying the predicate Pred to all elements of ListIn.

checklist(+Pred, +List)

Succeeds if the predicate Pred succeeds on all elements of List.

selectlist(+Pred, +ListIn, ?ListOut)

Creates ListOut of all list elements of ListIn that pass a given test

convlist(+Pred, +ListIn, ?ListOut)

A combination of maplist and selectlist: creates ListOut by applying the predicate Pred to all list elements on which Pred succeeds

sumlist(+Pred, +List, ?AccIn, ?AccOut)

Calls Pred on all elements of List and collects a result in Accumulator.

mapargs(+Pred, ?TermIn, ?TermOut)

Creates TermOut by applying the predicate Pred to all arguments of TermIn

sumargs(+Pred, +Term, ?AccIn, ?AccOut)

Calls the predicate Pred on all arguments of Term and collects a result in Accumulator

mapnodes(+Pred, +TermIn, ?TermOut)

Creates TermOut by applying the predicate Pred to all sub-terms of TermIn (depth-first and left-to-right order)

checknodes(+Pred, +Term)

Succeeds if the predicate Pred succeeds on all sub-terms of Term (depth-first and left-to-right order)

sumnodes(+Pred, +Term, ?AccIn, ?AccOut)

Calls the predicate Pred on all sub-terms of Term and collect a result in Accumulator (depth-first and left-to-right order)

Examples:

%given plus(X,Y,Z) :- Z is X + Y. plus_if_pos(X,Y,Z) :- Y > 0, Z is X + Y. vars(X, Y, [X|Y]) :- var(X), !. vars(_, Y, Y). trans(TermIn, TermOut) :- (compound(TermIn) ; atom(TermIn)), TermIn =.. [p|Args], TermOut =..[q|Args], !. trans(X,X). %success maplist(plus(1), [1,2,3,4], [2,3,4,5]). checklist(var, [X,Y,Z]). selectlist(<(0), [-1,0,1], [1]). convlist(plus_if_pos(1), [-1,0,1], [2]). sumlist(plus, [1,2,3,4], 1, 11). mapargs(number_atom,s(1,2,3), s('1','2','3')). sumargs(vars, s(1,X,2,Y), [], [Y,X]). mapnodes(trans, p(a,p(b,a),c), q(a,q(b,a),c)). checknodes(\==(T), p(X,p(Y,X),Z)). sumnodes(vars, [c(X), p(X,Y), q(Y)], [], [Y,Y,X,X]). % another one maplist(mapargs(number_atom),[c(1),s(1,2,3)],[c('1'),s('1','2','3')]).

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7.2 Association Lists

The following association list manipulation predicates are available once included with the use_module(library(assoc)) command. The original library used Richard O'Keefe's implementation, on top of unbalanced binary trees. The current code utilises code from the red-black trees library and emulates the SICStus Prolog interface.

assoc_to_list(+Assoc,?List)

Given an association list Assoc unify List with a list of the form Key-Val, where the elements Key are in ascending order.

del_assoc(+Key, +Assoc, ?Val, ?NewAssoc)

Succeeds if NewAssoc is an association list, obtained by removing the element with Key and Val from the list Assoc.

del_max_assoc(+Assoc, ?Key, ?Val, ?NewAssoc)

Succeeds if NewAssoc is an association list, obtained by removing the largest element of the list, with Key and Val from the list Assoc.

del_min_assoc(+Assoc, ?Key, ?Val, ?NewAssoc)

Succeeds if NewAssoc is an association list, obtained by removing the smallest element of the list, with Key and Val from the list Assoc.

empty_assoc(+Assoc)

Succeeds if association list Assoc is empty.

gen_assoc(+Assoc,?Key,?Value)

Given the association list Assoc, unify Key and Value with two associated elements. It can be used to enumerate all elements in the association list.

get_assoc(+Key,+Assoc,?Value)

If Key is one of the elements in the association list Assoc, return the associated value.

get_assoc(+Key,+Assoc,?Value,+NAssoc,?NValue)

If Key is one of the elements in the association list Assoc, return the associated value Value and a new association list NAssoc where Key is associated with NValue.

get_prev_assoc(+Key,+Assoc,?Next,?Value)

If Key is one of the elements in the association list Assoc, return the previous key, Next, and its value, Value.

get_next_assoc(+Key,+Assoc,?Next,?Value)

If Key is one of the elements in the association list Assoc, return the next key, Next, and its value, Value.

is_assoc(+Assoc)

Succeeds if Assoc is an association list, that is, if it is a red-black tree.

list_to_assoc(+List,?Assoc)

Given a list List such that each element of List is of the form Key-Val, and all the Keys are unique, Assoc is the corresponding association list.

map_assoc(+Pred,+Assoc)

Succeeds if the unary predicate name Pred(Val) holds for every element in the association list.

map_assoc(+Pred,+Assoc,?New)

Given the binary predicate name Pred and the association list Assoc, New in an association list with keys in Assoc, and such that if Key-Val is in Assoc, and Key-Ans is in New, then Pred(Val,Ans) holds.

max_assoc(+Assoc,-Key,?Value)

Given the association list Assoc, Key in the largest key in the list, and Value the associated value.

min_assoc(+Assoc,-Key,?Value)

Given the association list Assoc, Key in the smallest key in the list, and Value the associated value.

ord_list_to_assoc(+List,?Assoc)

Given an ordered list List such that each element of List is of the form Key-Val, and all the Keys are unique, Assoc is the corresponding association list.

put_assoc(+Key,+Assoc,+Val,+New)

The association list New includes and element of association key with Val, and all elements of Assoc that did not have key Key.

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7.3 AVL Trees

AVL trees are balanced search binary trees. They are named after their inventors, Adelson-Velskii and Landis, and they were the first dynamically balanced trees to be proposed. The YAP AVL tree manipulation predicates library uses code originally written by Martin van Emdem and published in the Logic Programming Newsletter, Autumn 1981. A bug in this code was fixed by Philip Vasey, in the Logic Programming Newsletter, Summer 1982. The library currently only includes routines to insert and lookup elements in the tree. Please try red-black trees if you need deletion.

avl_insert(+Key,?Value,+T0,+TF)

Add an element with key Key and Value to the AVL tree T0 creating a new AVL tree TF. Duplicated elements are allowed.

avl_lookup(+Key,-Value,+T)

Lookup an element with key Key in the AVL tree T, returning the value Value.

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7.4 Heaps

A heap is a labelled binary tree where the key of each node is less than or equal to the keys of its sons. The point of a heap is that we can keep on adding new elements to the heap and we can keep on taking out the minimum element. If there are N elements total, the total time is O(NlgN). If you know all the elements in advance, you are better off doing a merge-sort, but this file is for when you want to do say a best-first search, and have no idea when you start how many elements there will be, let alone what they are.

The following heap manipulation routines are available once included with the use_module(library(heaps)) command.

add_to_heap(+Heap,+key,+Datum,-NewHeap)

Inserts the new Key-Datum pair into the heap. The insertion is not stable, that is, if you insert several pairs with the same Key it is not defined which of them will come out first, and it is possible for any of them to come out first depending on the history of the heap.

empty_heap(?Heap)

Succeeds if Heap is an empty heap.

get_from_heap(+Heap,-key,-Datum,-Heap)

Returns the Key-Datum pair in OldHeap with the smallest Key, and also a Heap which is the OldHeap with that pair deleted.

heap_size(+Heap, -Size)

Reports the number of elements currently in the heap.

heap_to_list(+Heap, -List)

Returns the current set of Key-Datum pairs in the Heap as a List, sorted into ascending order of Keys.

list_to_heap(+List, -Heap)

Takes a list of Key-Datum pairs (such as keysort could be used to sort) and forms them into a heap.

min_of_heap(+Heap, -Key, -Datum)

Returns the Key-Datum pair at the top of the heap (which is of course the pair with the smallest Key), but does not remove it from the heap.

min_of_heap(+Heap, -Key1, -Datum1,

-Key2, -Datum2) Returns the smallest (Key1) and second smallest (Key2) pairs in the heap, without deleting them.

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7.5 List Manipulation

The following list manipulation routines are available once included with the use_module(library(lists)) command.

append(?Prefix,?Suffix,?Combined)

True when all three arguments are lists, and the members of Combined are the members of Prefix followed by the members of Suffix. It may be used to form Combined from a given Prefix, Suffix or to take a given Combined apart.

delete(+List, ?Element, ?Residue)

True when List is a list, in which Element may or may not occur, and Residue is a copy of List with all elements identical to Element deleted.

flatten(+List, ?FlattenedList)

Flatten a list of lists List into a single list FlattenedList.

?- flatten([[1],[2,3],[4,[5,6],7,8]],L). L = [1,2,3,4,5,6,7,8] ? ; no

is_list(+List)

True when List is a proper list. That is, List is bound to the empty list (nil) or a term with functor '.' and arity 2.

last(+List,?Last)

True when List is a list and Last is identical to its last element.

list_concat(+Lists,?List)

True when Lists is a list of lists and List is the concatenation of Lists.

member(?Element, ?Set)

True when Set is a list, and Element occurs in it. It may be used to test for an element or to enumerate all the elements by backtracking.

memberchk(+Element, +Set)

As member/2, but may only be used to test whether a known Element occurs in a known Set. In return for this limited use, it is more efficient when it is applicable.

nth0(?N, ?List, ?Elem)

True when Elem is the Nth member of List, counting the first as element 0. (That is, throw away the first N elements and unify Elem with the next.) It can only be used to select a particular element given the list and index. For that task it is more efficient than member/2

nth(?N, ?List, ?Elem)

The same as nth0/3, except that it counts from 1, that is nth(1, [H|_], H).

nth0(?N, ?List, ?Elem, ?Rest)

Unifies Elem with the Nth element of List, counting from 0, and Rest with the other elements. It can be used to select the Nth element of List (yielding Elem and Rest), or to insert Elem before the Nth (counting from 1) element of Rest, when it yields List, e.g. nth0(2, List, c, [a,b,d,e]) unifies List with [a,b,c,d,e]. nth/4 is the same except that it counts from 1. nth0/4 can be used to insert Elem after the Nth element of Rest.

nth(?N, ?List, ?Elem, ?Rest)

Unifies Elem with the Nth element of List, counting from 1, and Rest with the other elements. It can be used to select the Nth element of List (yielding Elem and Rest), or to insert Elem before the Nth (counting from 1) element of Rest, when it yields List, e.g. nth(1, List, c, [a,b,d,e]) unifies List with [a,b,c,d,e]. nth/4 can be used to insert Elem after the Nth element of Rest.

permutation(+List,?Perm)

True when List and Perm are permutations of each other.

remove_duplicates(+List, ?Pruned)

Removes duplicated elements from List. Beware: if the List has non-ground elements, the result may surprise you.

reverse(+List, ?Reversed)

True when List and Reversed are lists with the same elements but in opposite orders.

same_length(?List1, ?List2)

True when List1 and List2 are both lists and have the same number of elements. No relation between the values of their elements is implied. Modes same_length(-,+) and same_length(+,-) generate either list given the other; mode same_length(-,-) generates two lists of the same length, in which case the arguments will be bound to lists of length 0, 1, 2, ...

select(?Element, ?Set, ?Residue)

True when Set is a list, Element occurs in Set, and Residue is everything in Set except Element (things stay in the same order).

sublist(?Sublist, ?List)

True when both append(_,Sublist,S) and append(S,_,List) hold.

suffix(?Suffix, ?List)

Holds when append(_,Suffix,List) holds.

sum_list(?Numbers, ?Total)

True when Numbers is a list of numbers, and Total is their sum.

sumlist(?Numbers, ?Total)

True when Numbers is a list of integers, and Total is their sum. The same as sum_list/2, please do use sum_list/2 instead.

max_list(?Numbers, ?Max)

True when Numbers is a list of numbers, and Max is the maximum.

min_list(?Numbers, ?Min)

True when Numbers is a list of numbers, and Min is the minimum.

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7.6 Ordered Sets

The following ordered set manipulation routines are available once included with the use_module(library(ordsets)) command. An ordered set is represented by a list having unique and ordered elements. Output arguments are guaranteed to be ordered sets, if the relevant inputs are. This is a slightly patched version of Richard O'Keefe's original library.

list_to_ord_set(+List, ?Set)

Holds when Set is the ordered representation of the set represented by the unordered representation List.

merge(+List1, +List2, -Merged)

Holds when Merged is the stable merge of the two given lists.

Notice that merge/3 will not remove duplicates, so merging ordered sets will not necessarily result in an ordered set. Use ord_union/3 instead.

ord_add_element(+Set1, +Element, ?Set2)

Inserting Element in Set1 returns Set2. It should give exactly the same result as merge(Set1, [Element], Set2), but a bit faster, and certainly more clearly. The same as ord_insert/3.

ord_del_element(+Set1, +Element, ?Set2)

Removing Element from Set1 returns Set2.

ord_disjoint(+Set1, +Set2)

Holds when the two ordered sets have no element in common.

ord_member(+Element, +Set)

Holds when Element is a member of Set.

ord_insert(+Set1, +Element, ?Set2)

Inserting Element in Set1 returns Set2. It should give exactly the same result as merge(Set1, [Element], Set2), but a bit faster, and certainly more clearly. The same as ord_add_element/3.

ord_intersect(+Set1, +Set2)

Holds when the two ordered sets have at least one element in common.

ord_intersection(+Set1, +Set2, ?Intersection)

Holds when Intersection is the ordered representation of Set1 and Set2.

ord_intersection(+Set1, +Set2, ?Intersection, ?Diff)

Holds when Intersection is the ordered representation of Set1 and Set2. Diff is the difference between Set2 and Set1.

ord_seteq(+Set1, +Set2)

Holds when the two arguments represent the same set.

ord_setproduct(+Set1, +Set2, -Set)

If Set1 and Set2 are ordered sets, Product will be an ordered set of x1-x2 pairs.

ord_subset(+Set1, +Set2)

Holds when every element of the ordered set Set1 appears in the ordered set Set2.

ord_subtract(+Set1, +Set2, ?Difference)

Holds when Difference contains all and only the elements of Set1 which are not also in Set2.

ord_symdiff(+Set1, +Set2, ?Difference)

Holds when Difference is the symmetric difference of Set1 and Set2.

ord_union(+Sets, ?Union)

Holds when Union is the union of the lists Sets.

ord_union(+Set1, +Set2, ?Union)

Holds when Union is the union of Set1 and Set2.

ord_union(+Set1, +Set2, ?Union, ?Diff)

Holds when Union is the union of Set1 and Set2 and Diff is the difference.

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7.7 Pseudo Random Number Integer Generator

The following routines produce random non-negative integers in the range 0 .. 2^(w-1) -1, where w is the word size available for integers, e.g. 32 for Intel machines and 64 for Alpha machines. Note that the numbers generated by this random number generator are repeatable. This generator was originally written by Allen Van Gelder and is based on Knuth Vol 2.

rannum(-I)

Produces a random non-negative integer I whose low bits are not all that random, so it should be scaled to a smaller range in general. The integer I is in the range 0 .. 2^(w-1) - 1. You can use:

rannum(X) :- yap_flag(max_integer,MI), rannum(R), X is R/MI.

to obtain a floating point number uniformly distributed between 0 and 1.

ranstart

Initialize the random number generator using a built-in seed. The ranstart/0 built-in is always called by the system when loading the package.

ranstart(+Seed)

Initialize the random number generator with user-defined Seed. The same Seed always produces the same sequence of numbers.

ranunif(+Range,-I)

ranunif/2 produces a uniformly distributed non-negative random integer I over a caller-specified range R. If range is R, the result is in 0 .. R-1.

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7.8 Queues

The following queue manipulation routines are available once included with the use_module(library(queues)) command. Queues are implemented with difference lists.

make_queue(+Queue)

Creates a new empty queue. It should only be used to create a new queue.

join_queue(+Element, +OldQueue, -NewQueue)

Adds the new element at the end of the queue.

list_join_queue(+List, +OldQueue, -NewQueue)

Ads the new elements at the end of the queue.

jump_queue(+Element, +OldQueue, -NewQueue)

Adds the new element at the front of the list.

list_jump_queue(+List, +OldQueue, +NewQueue)

Adds all the elements of List at the front of the queue.

head_queue(+Queue, ?Head)

Unifies Head with the first element of the queue.

serve_queue(+OldQueue, +Head, -NewQueue)

Removes the first element of the queue for service.

empty_queue(+Queue)

Tests whether the queue is empty.

length_queue(+Queue, -Length)

Counts the number of elements currently in the queue.

list_to_queue(+List, -Queue)

Creates a new queue with the same elements as List.

queue_to_list(+Queue, -List)

Creates a new list with the same elements as Queue.

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7.9 Random Number Generator

The following random number operations are included with the use_module(library(random)) command. Since Yap-4.3.19 Yap uses the O'Keefe public-domain algorithm, based on the "Applied Statistics" algorithm AS183.

getrand(-Key)

Unify Key with a term of the form rand(X,Y,Z) describing the current state of the random number generator.

random(-Number)

Unify Number with a floating-point number in the range [0...1).

random(+LOW, +HIGH, -NUMBER)

Unify Number with a number in the range [LOW...HIGH). If both LOW and HIGH are integers then NUMBER will also be an integer, otherwise NUMBER will be a floating-point number.

randseq(+LENGTH, +MAX, -Numbers)

Unify Numbers with a list of LENGTH unique random integers in the range [1�...MAX).

randset(+LENGTH, +MAX, -Numbers)

Unify Numbers with an ordered list of LENGTH unique random integers in the range [1�...MAX).

setrand(+Key)

Use a term of the form rand(X,Y,Z) to set a new state for the random number generator. The integer X must be in the range [1...30269), the integer Y must be in the range [1...30307), and the integer Z must be in the range [1...30323).

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7.10 Red-Black Trees

Red-Black trees are balanced search binary trees. They are named because nodes can be classified as either red or black. The code we include is based on "Introduction to Algorithms", second edition, by Cormen, Leiserson, Rivest and Stein. The library includes routines to insert, lookup and delete elements in the tree.

rb_new(?T)

Create a new tree.

rb_empty(?T)

Succeeds if tree T is empty.

is_rbtree(+T)

Check whether T is a valid red-black tree.

rb_insert(+T0,+Key,?Value,+TF)

Add an element with key Key and Value to the tree T0 creating a new red-black tree TF. Duplicated elements are not allowed.

rb_lookup(+Key,-Value,+T)

Backtrack through all elements with key Key in the red-black tree T, returning for each the value Value.

rb_lookupall(+Key,-Value,+T)

Lookup all elements with key Key in the red-black tree T, returning the value Value.

rb_delete(+T,+Key,-TN)

Delete element with key Key from the tree T, returning a new tree TN.

rb_delete(+T,+Key,-Val,-TN)

Delete element with key Key from the tree T, returning the value Val associated with the key and a new tree TN.

rb_del_min(+T,-Key,-Val,-TN)

Delete the least element from the tree T, returning the key Key, the value Val associated with the key and a new tree TN.

rb_del_max(+T,-Key,-Val,-TN)

Delete the largest element from the tree T, returning the key Key, the value Val associated with the key and a new tree TN.

rb_update(+T,+Key,+NewVal,-TN)

Tree TN is tree T, but with value for Key associated with NewVal. Fails if it cannot find Key in T.

rb_apply(+T,+Key,+G,-TN)

If the value associated with key Key is Val0 in T, and if call(G,Val0,ValF) holds, then TN differs from T only in that Key is associated with value ValF in tree TN. Fails if it cannot find Key in T, or if call(G,Val0,ValF) is not satisfiable.

rb_visit(+T,-Pairs)

Pairs is an infix visit of tree T, where each element of Pairs is of the form K-Val.

rb_size(+T,-Size)

Size is the number of elements in T.

rb_keys(+T,+Keys)

Keys is an infix visit with all keys in tree T. Keys will be sorted, but may be duplicate.

rb_map(+T,+G,-TN)

For all nodes Key in the tree T, if the value associated with key Key is Val0 in tree T, and if call(G,Val0,ValF) holds, then the value associated with Key in TN is ValF. Fails if or if call(G,Val0,ValF) is not satisfiable for all Var0.

rb_partial_map(+T,+Keys,+G,-TN)

For all nodes Key in Keys, if the value associated with key Key is Val0 in tree T, and if call(G,Val0,ValF) holds, then the value associated with Key in TN is ValF. Fails if or if call(G,Val0,ValF) is not satisfiable for all Var0. Assumes keys are not repeated.

rb_clone(+T,+NT,+Nodes)

"Clone" the red-back tree into a new tree with the same keys as the original but with all values set to unbound values. Nodes is a list containing all new nodes as pairs K-V.

rb_min(+T,-Key,-Value)

Key is the minimum key in T, and is associated with Val.

rb_max(+T,-Key,-Value)

Key is the maximal key in T, and is associated with Val.

rb_next(+T, +Key,-Next,-Value)

Next is the next element after Key in T, and is associated with Val.

rb_previous(+T, +Key,-Previous,-Value)

Previous is the previous element after Key in T, and is associated with Val.

list_to_rbtree(+L, -T)

T is the red-black tree corresponding to the mapping in list L.

ord_list_to_rbtree(+L, -T)

T is the red-black tree corresponding to the mapping in ordered list L.

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7.11 Regular Expressions

This library includes routines to determine whether a regular expression matches part or all of a string. The routines can also return which parts parts of the string matched the expression or subexpressions of it. This library relies on Henry Spencer's C-package and is only available in operating systems that support dynamic loading. The C-code has been obtained from the sources of FreeBSD-4.0 and is protected by copyright from Henry Spencer and from the Regents of the University of California (see the file library/regex/COPYRIGHT for further details).

Much of the description of regular expressions below is copied verbatim from Henry Spencer's manual page.

A regular expression is zero or more branches, separated by "|". It matches anything that matches one of the branches.

A branch is zero or more pieces, concatenated. It matches a match for the first, followed by a match for the second, etc.

A piece is an atom possibly followed by "*", "+", or "?". An atom followed by "*" matches a sequence of 0 or more matches of the atom. An atom followed by "+" matches a sequence of 1 or more matches of the atom. An atom followed by "?" matches a match of the atom, or the null string.

An atom is a regular expression in parentheses (matching a match for the regular expression), a range (see below), "." (matching any single character), "^" (matching the null string at the beginning of the input string), "$" (matching the null string at the end of the input string), a "\" followed by a single character (matching that character), or a single character with no other significance (matching that character).

A range is a sequence of characters enclosed in "[]". It normally matches any single character from the sequence. If the sequence begins with "^", it matches any single character not from the rest of the sequence. If two characters in the sequence are separated by "-", this is shorthand for the full list of ASCII characters between them (e.g. "[0-9]" matches any decimal digit). To include a literal "]" in the sequence, make it the first character (following a possible "^"). To include a literal "-", make it the first or last character.

regexp(+RegExp,+String,+Opts)

Match regular expression RegExp to input string String according to options Opts. The options may be:

• nocase: Causes upper-case characters in String to be treated as lower case during the matching process.

regexp(+RegExp,+String,+Opts,SubMatchVars)

Match regular expression RegExp to input string String according to options Opts. The variable SubMatchVars should be originally a list of unbound variables all will contain a sequence of matches, that is, the head of SubMatchVars will contain the characters in String that matched the leftmost parenthesized subexpression within RegExp, the next head of list will contain the characters that matched the next parenthesized subexpression to the right in RegExp, and so on.

The options may be:

• nocase: Causes upper-case characters in String to be treated as lower case during the matching process.
• indices: Changes what is stored in SubMatchVars. Instead of storing the matching characters from String, each variable will contain a term of the form IO-IF giving the indices in String of the first and last characters in the matching range of characters.

In general there may be more than one way to match a regular expression to an input string. For example, consider the command

regexp("(a*)b*","aabaaabb", [], [X,Y])

Considering only the rules given so far, X and Y could end up with the values "aabb" and "aa", "aaab" and "aaa", "ab" and "a", or any of several other combinations. To resolve this potential ambiguity regexp chooses among alternatives using the rule "first then longest". In other words, it considers the possible matches in order working from left to right across the input string and the pattern, and it attempts to match longer pieces of the input string before shorter ones. More specifically, the following rules apply in decreasing order of priority:

1. If a regular expression could match two different parts of an input string then it will match the one that begins earliest.
2. If a regular expression contains "|" operators then the leftmost matching sub-expression is chosen.
3. In *, +, and ? constructs, longer matches are chosen in preference to shorter ones.
4. In sequences of expression components the components are considered from left to right.

In the example from above, "(a*)b*" matches "aab": the "(a*)" portion of the pattern is matched first and it consumes the leading "aa"; then the "b*" portion of the pattern consumes the next "b". Or, consider the following example:

regexp("(ab|a)(b*)c", "abc", [], [X,Y,Z])

After this command X will be "abc", Y will be "ab", and Z will be an empty string. Rule 4 specifies that "(ab|a)" gets first shot at the input string and Rule 2 specifies that the "ab" sub-expression is checked before the "a" sub-expression. Thus the "b" has already been claimed before the "(b*)" component is checked and (b*) must match an empty string.

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7.12 Splay Trees

Splay trees are explained in the paper "Self-adjusting Binary Search Trees", by D.D. Sleator and R.E. Tarjan, JACM, vol. 32, No.3, July 1985, p. 668. They are designed to support fast insertions, deletions and removals in binary search trees without the complexity of traditional balanced trees. The key idea is to allow the tree to become unbalanced. To make up for this, whenever we find a node, we move it up to the top. We use code by Vijay Saraswat originally posted to the Prolog mailing-list.

splay_access(-Return,+Key,?Val,+Tree,-NewTree)

If item Key is in tree Tree, return its Val and unify Return with true. Otherwise unify Return with null. The variable NewTree unifies with the new tree.

splay_delete(+Key,?Val,+Tree,-NewTree)

Delete item Key from tree Tree, assuming that it is present already. The variable Val unifies with a value for key Key, and the variable NewTree unifies with the new tree. The predicate will fail if Key is not present.

splay_init(-NewTree)

Initialize a new splay tree.

splay_insert(+Key,?Val,+Tree,-NewTree)

Insert item Key in tree Tree, assuming that it is not there already. The variable Val unifies with a value for key Key, and the variable NewTree unifies with the new tree. In our implementation, Key is not inserted if it is already there: rather it is unified with the item already in the tree.

splay_join(+LeftTree,+RighTree,-NewTree)

Combine trees LeftTree and RighTree into a single treeNewTree containing all items from both trees. This operation assumes that all items in LeftTree are less than all those in RighTree and destroys both LeftTree and RighTree.

splay_split(+Key,?Val,+Tree,-LeftTree,-RightTree)

Construct and return two trees LeftTree and RightTree, where LeftTree contains all items in Tree less than Key, and RightTree contains all items in Tree greater than Key. This operations destroys Tree.

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7.13 Reading From and Writing To Strings

From Version 4.3.2 onwards YAP implements SICStus Prolog compatible String I/O. The library allows users to read from and write to a memory buffer as if it was a file. The memory buffer is built from or converted to a string of character codes by the routines in library. Therefore, if one wants to read from a string the string must be fully instantiated before the library built-in opens the string for reading. These commands are available through the use_module(library(charsio)) command.

format_to_chars(+Form, +Args, -Result)

Execute the built-in procedure format/2 with form Form and arguments Args outputting the result to the string of character codes Result.

format_to_chars(+Form, +Args, -Result0, -Result)

Execute the built-in procedure format/2 with form Form and arguments Args outputting the result to the difference list of character codes Result-Result0.

write_to_chars(+Term, -Result)

Execute the built-in procedure write/1 with argument Term outputting the result to the string of character codes Result.

write_to_chars(+Term, -Result0, -Result)

Execute the built-in procedure write/1 with argument Term outputting the result to the difference list of character codes Result-Result0.

atom_to_chars(+Atom, -Result)

Convert the atom Atom to the string of character codes Result.

atom_to_chars(+Atom, -Result0, -Result)

Convert the atom Atom to the difference list of character codes Result-Result0.

number_to_chars(+Number, -Result)

Convert the number Number to the string of character codes Result.

number_to_chars(+Number, -Result0, -Result)

Convert the atom Number to the difference list of character codes Result-Result0.

read_from_chars(+Chars, -Term)

Parse the list of character codes Chars and return the result in the term Term. The character codes to be read must terminate with a dot character such that either (i) the dot character is followed by blank characters; or (ii) the dot character is the last character in the string.

open_chars_stream(+Chars, -Stream)

Open the list of character codes Chars as a stream Stream.

with_output_to_chars(?Goal, -Chars)

Execute goal Goal such that its standard output will be sent to a memory buffer. After successful execution the contents of the memory buffer will be converted to the list of character codes Chars.

with_output_to_chars(?Goal, ?Chars0, -Chars)

Execute goal Goal such that its standard output will be sent to a memory buffer. After successful execution the contents of the memory buffer will be converted to the difference list of character codes Chars-Chars0.

with_output_to_chars(?Goal, -Stream, ?Chars0, -Chars)

Execute goal Goal such that its standard output will be sent to a memory buffer. After successful execution the contents of the memory buffer will be converted to the difference list of character codes Chars-Chars0 and Stream receives the stream corresponding to the memory buffer.

The implementation of the character IO operations relies on three YAP built-ins:

charsio:open_mem_read_stream(+String, -Stream)

Store a string in a memory buffer and output a stream that reads from this memory buffer.

charsio:open_mem_write_stream(-Stream)

Create a new memory buffer and output a stream that writes to it.

charsio:peek_mem_write_stream(-Stream, L0, L)

Convert the memory buffer associated with stream Stream to the difference list of character codes L-L0.

These built-ins are initialized to belong to the module charsio in init.yap. Novel procedures for manipulating strings by explicitly importing these built-ins.

YAP does not currently support opening a charsio stream in append mode, or seeking in such a stream.

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7.14 Calling The Operating System from YAP

Yap now provides a library of system utilities compatible with the SICStus Prolog system library. This library extends and to some point replaces the functionality of Operating System access routines. The library includes Unix/Linux and Win32 C code. They are available through the use_module(library(system)) command.

datime(datime(-Year, -Month, -DayOfTheMonth,

-Hour, -Minute, -Second) The datime/1 procedure returns the current date and time, with information on Year, Month, DayOfTheMonth, Hour, Minute, and Second. The Hour is returned on local time. This function uses the WIN32 GetLocalTime function or the Unix localtime function.

?- datime(X). X = datime(2001,5,28,15,29,46) ?

mktime(datime(+Year, +Month, +DayOfTheMonth,

+Hour, +Minute, +Second), -Seconds) The mktime/1 procedure returns the number of Seconds elapsed since 00:00:00 on January 1, 1970, Coordinated Universal Time (UTC). The user provides information on Year, Month, DayOfTheMonth, Hour, Minute, and Second. The Hour is given on local time. This function uses the WIN32 GetLocalTime function or the Unix mktime function.

?- mktime(datime(2001,5,28,15,29,46),X). X = 991081786 ? ;

delete_file(+File)

The delete_file/1 procedure removes file File. If File is a directory, remove the directory and all its subdirectories.

?- delete_file(x).

delete_file(+File,+Opts)

The delete_file/2 procedure removes file File according to options Opts. These options are directory if one should remove directories, recursive if one should remove directories recursively, and ignore if errors are not to be reported.

This example is equivalent to using the delete_file/1 predicate:

?- delete_file(x, [recursive]).

directory_files(+Dir,+List)

Given a directory Dir, directory_files/2 procedures a listing of all files and directories in the directory:

?- directory_files('.',L), writeq(L). ['Makefile.~1~','sys.so','Makefile','sys.o',x,..,'.']

The predicates uses the dirent family of routines in Unix environments, and findfirst in WIN32.

file_exists(+File)

The atom File corresponds to an existing file.

file_exists(+File,+Permissions)

The atom File corresponds to an existing file with permissions compatible with Permissions. YAP currently only accepts for permissions to be described as a number. The actual meaning of this number is Operating System dependent.

file_property(+File,?Property)

The atom File corresponds to an existing file, and Property will be unified with a property of this file. The properties are of the form type(Type), which gives whether the file is a regular file, a directory, a fifo file, or of unknown type; size(Size), with gives the size for a file, and mod_time(Time), which gives the last time a file was modified according to some Operating System dependent timestamp; mode(mode), gives the permission flags for the file, and linkto(FileName), gives the file pointed to by a symbolic link. Properties can be obtained through backtracking:

?- file_property('Makefile',P). P = type(regular) ? ; P = size(2375) ? ; P = mod_time(990826911) ? ; no

make_directory(+Dir)

Create a directory Dir. The name of the directory must be an atom.

rename_file(+OldFile,+NewFile)

Create file OldFile to NewFile. This predicate uses the C built-in function rename.

environ(?EnvVar,+EnvValue)

Unify environment variable EnvVar with its value EnvValue, if there is one. This predicate is backtrackable in Unix systems, but not currently in Win32 configurations.

?- environ('HOME',X). X = 'C:\\cygwin\\home\\administrator' ?

host_id(-Id)

Unify Id with an identifier of the current host. Yap uses the hostid function when available,

host_name(-Name)

Unify Name with a name for the current host. Yap uses the hostname function in Unix systems when available, and the GetComputerName function in WIN32 systems.

kill(Id,+SIGNAL)

Send signal SIGNAL to process Id. In Unix this predicate is a direct interface to kill so one can send signals to groups of processes. In WIN32 the predicate is an interface to TerminateProcess, so it kills Id indepent of SIGNAL.

mktemp(Spec,-File)

Direct interface to mktemp: given a Spec, that is a file name with six X to it, create a file name File. Use tmpnam/1 instead.

pid(-Id)

Unify Id with the process identifier for the current process. An interface to the getpid function.

tmpnam(-File)

Interface with tmpnam: create an unique file and unify its name with File.

exec(+Command,[+InputStream,+OutputStream,+ErrorStream], -Status) Execute command Command with its streams connected to InputStream, OutputStream, and ErrorStream. The result for the command is returned in Status. The command is executed by the default shell bin/sh -c in Unix.

The following example demonstrates the use of exec/3 to send a command and process its output:

exec(ls,[std,pipe(S),null],P),repeat, get0(S,C), (C = -1, close(S) ! ; put(C)).

The streams may be one of standard stream, std, null stream, null, or pipe(S), where S is a pipe stream. Note that it is up to the user to close the pipe.

working_directory(-CurDir,?NextDir)

Fetch the current directory at CurDir. If NextDir is bound to an atom, make its value the current working directory.

popen(+Command, +TYPE, -Stream)

Interface to the popen function. It opens a process by creating a pipe, forking and invoking Command on the current shell. Since a pipe is by definition unidirectional the Type argument may be read or write, not both. The stream should be closed using close/1, there is no need for a special pclose command.

The following example demonstrates the use of popen/3 to process the output of a command, as exec/3 would do:

?- popen(ls,read,X),repeat, get0(X,C), (C = -1, ! ; put(C)). X = 'C:\\cygwin\\home\\administrator' ?

The WIN32 implementation of popen/3 relies on exec/3.

shell

Start a new shell and leave Yap in background until the shell completes. Yap uses the shell given by the environment variable SHELL. In WIN32 environment YAP will use COMSPEC if SHELL is undefined.

shell(+Command)

Execute command Command under a new shell. Yap will be in background until the command completes. In Unix environments Yap uses the shell given by the environment variable SHELL with the option " -c ". In WIN32 environment YAP will use COMSPEC if SHELL is undefined, in this case with the option " /c ".

shell(+Command,-Status)

Execute command Command under a new shell and unify Status with the exit for the command. Yap will be in background until the command completes. In Unix environments Yap uses the shell given by the environment variable SHELL with the option " -c ". In WIN32 environment YAP will use COMSPEC if SHELL is undefined, in this case with the option " /c ".

sleep(+Time)

Block the current process for Time seconds. The number of seconds must be a positive number, and it may an integer or a float. The Unix implementation uses usleep if the number of seconds is below one, and sleep if it is over a second. The WIN32 implementation uses Sleep for both cases.

system

Start a new default shell and leave Yap in background until the shell completes. Yap uses /bin/sh in Unix systems and COMSPEC in WIN32.

system(+Command,-Res)

Interface to system: execute command Command and unify Res with the result.

wait(+PID,-Status)

Wait until process PID terminates, and return its exits Status.

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7.15 Utilities On Terms

The next routines provide a set of commonly used utilities to manipulate terms. Most of these utilities have been implemented in C for efficiency. They are available through the use_module(library(terms)) command.

acyclic_term(?Term)

Succeed if the argument Term is an acyclic term.

cyclic_term(?Term)

Succeed if the argument Term is a cyclic term.

term_hash(+Term, ?Hash)

If Term is ground unify Hash with a positive integer calculated from the structure of the term. Otherwise the argument Hash is left unbound. The range of the positive integer is from 0 to, but not including, 33554432.

term_hash(+Term, +Depth, +Range, ?Hash)

Unify Hash with a positive integer calculated from the structure of the term. The range of the positive integer is from 0 to, but not including, Range. If Depth is -1 the whole term is considered. Otherwise, the term is considered only up to depth 1, where the constants and the principal functor have depth 1, and an argument of a term with depth I has depth I+1.

term_variables(?Term, -Variables)

Unify Variables with a list of all variables in term Term.

variant(?Term1, ?Term2)

Succeed if Term1 and Term2 are variant terms.

subsumes(?Term1, ?Term2)

Succeed if Term1 subsumes Term2. Variables in term Term1 are bound so that the two terms become equal.

subsumes_chk(?Term1, ?Term2)

Succeed if Term1 subsumes Term2 but does not bind any variable in Term1.

variable_in_term(?Term,?Var)

Succeed if the second argument Var is a variable and occurs in term Term.

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7.16 Call Cleanup

call_cleanup/1 and call_cleanup/2 allow predicates to register code for execution after the call is finished. Predicates can be declared to be fragile to ensure that call_cleanup is called for any Goal which needs it. This library is loaded with the use_module(library(cleanup)) command.

:- fragile P,....,Pn

Declares the predicate P=[module:]name/arity as a fragile predicate, module is optional, default is the current typein_module. Whenever such a fragile predicate is used in a query it will be called through call_cleanup/1.

:- fragile foo/1,bar:baz/2.

call_cleanup(+Goal)

Execute goal Goal within a cleanup-context. Called predicates might register cleanup Goals which are called right after the end of the call to Goal. Cuts and exceptions inside Goal do not prevent the execution of the cleanup calls. call_cleanup might be nested.

call_cleanup(+Goal, +CleanUpGoal)

This is similar to call_cleanup/1 with an additional CleanUpGoal which gets called after Goal is finished.

on_cleanup(+CleanUpGoal)

Any Predicate might registers a CleanUpGoal. The CleanUpGoal is put onto the current cleanup context. All such CleanUpGoals are executed in reverse order of their registration when the surrounding cleanup-context ends. This call will throw an exception if a predicate tries to register a CleanUpGoal outside of any cleanup-context.

cleanup_all

Calls all pending CleanUpGoals and resets the cleanup-system to an initial state. Should only be used as one of the last calls in the main program.

There are some private predicates which could be used in special cases, such as manually setting up cleanup-contexts and registering CleanUpGoals for other than the current cleanup-context. Read the Source Luke.

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7.17 Calls With Timeout

The time_out/3 command relies on the alarm/3 built-in to implement a call with a maximum time of execution. The command is available with the use_module(library(timeout)) command.

time_out(+Goal, +Timeout, -Result)

Execute goal Goal with time limited Timeout, where Timeout is measured in milliseconds. If the goal succeeds, unify Result with success. If the timer expires before the goal terminates, unify Result with timeout.

This command is implemented by activating an alarm at procedure entry. If the timer expires before the goal completes, the alarm will through an exception timeout.

One should note that time_out/3 is not reentrant, that is, a goal called from time_out should never itself call time_out. Moreover, time_out/3 will deactivate any previous alarms set by alarm/3 and vice-versa, hence only one of these calls should be used in a program.

Last, even though the timer is set in milliseconds, the current implementation relies on alarm/3, and therefore can only offer precision on the scale of seconds.

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7.18 Updatable Binary Trees

The following queue manipulation routines are available once included with the use_module(library(trees)) command.

get_label(+Index, +Tree, ?Label)

Treats the tree as an array of N elements and returns the Index-th.

list_to_tree(+List, -Tree)

Takes a given List of N elements and constructs a binary Tree.

map_tree(+Pred, +OldTree, -NewTree)

Holds when OldTree and NewTree are binary trees of the same shape and Pred(Old,New) is true for corresponding elements of the two trees.

put_label(+Index, +OldTree, +Label, -NewTree)

constructs a new tree the same shape as the old which moreover has the same elements except that the Index-th one is Label.

tree_size(+Tree, -Size)

Calculates the number of elements in the Tree.

tree_to_list(+Tree, -List)

Is the converse operation to list_to_tree.

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7.19 Unweighted Graphs

The following graph manipulation routines are based in code originally written by Richard O'Keefe. The code was then extended to be compatible with the SICStus Prolog ugraphs library. The routines assume directed graphs, undirected graphs may be implemented by using two edges. Graphs are represented in one of two ways:

• The P-representation of a graph is a list of (from-to) vertex pairs, where the pairs can be in any old order. This form is convenient for input/output.
• The S-representation of a graph is a list of (vertex-neighbors) pairs, where the pairs are in standard order (as produced by keysort) and the neighbors of each vertex are also in standard order (as produced by sort). This form is convenient for many calculations.

These built-ins are available once included with the use_module(library(ugraphs)) command.

vertices_edges_to_ugraph(+Vertices, +Edges, -Graph)

Given a graph with a set of vertices Vertices and a set of edges Edges, Graph must unify with the corresponding s-representation. Note that the vertices without edges will appear in Vertices but not in Edges. Moreover, it is sufficient for a vertice to appear in Edges.

?- vertices_edges_to_ugraph([],[1-3,2-4,4-5,1-5],L). L = [1-[3,5],2-[4],3-[],4-[5],5-[]] ?

In this case all edges are defined implicitly. The next example shows three unconnected edges:

?- vertices_edges_to_ugraph([6,7,8],[1-3,2-4,4-5,1-5],L). L = [1-[3,5],2-[4],3-[],4-[5],5-[],6-[],7-[],8-[]] ?

vertices(+Graph, -Vertices)

Unify Vertices with all vertices appearing in graph Graph. In the next example:

?- vertices([1-[3,5],2-[4],3-[],4-[5],5-[]], V). L = [1,2,3,4,5]

edges(+Graph, -Edges)

Unify Edges with all edges appearing in graph Graph. In the next example:

?- vertices([1-[3,5],2-[4],3-[],4-[5],5-[]], V). L = [1,2,3,4,5]

add_vertices(+Graph, +Vertices, -NewGraph)

Unify NewGraph with a new graph obtained by adding the list of vertices Vertices to the graph Graph. In the next example:

?- add_vertices([1-[3,5],2-[4],3-[],4-[5], 5-[],6-[],7-[],8-[]], [0,2,9,10,11], NG). NG = [0-[],1-[3,5],2-[4],3-[],4-[5],5-[], 6-[],7-[],8-[],9-[],10-[],11-[]]

del_vertices(+Vertices, +Graph, -NewGraph)

Unify NewGraph with a new graph obtained by deleting the list of vertices Vertices and all the edges that start from or go to a vertex in Vertices to the graph Graph. In the next example:

?- del_vertices([2,1],[1-[3,5],2-[4],3-[], 4-[5],5-[],6-[],7-[2,6],8-[]],NL). NL = [3-[],4-[5],5-[],6-[],7-[6],8-[]]

add_edges(+Graph, +Edges, -NewGraph)

Unify NewGraph with a new graph obtained by adding the list of edges Edges to the graph Graph. In the next example:

?- add_edges([1-[3,5],2-[4],3-[],4-[5],5-[],6-[], 7-[],8-[]],[1-6,2-3,3-2,5-7,3-2,4-5],NL). NL = [1-[3,5,6],2-[3,4],3-[2],4-[5],5-[7],6-[],7-[],8-[]]

del_edges(+Graph, +Edges, -NewGraph)

Unify NewGraph with a new graph obtained by removing the list of edges Edges from the graph Graph. Notice that no vertices are deleted. In the next example:

?- del_edges([1-[3,5],2-[4],3-[],4-[5],5-[], 6-[],7-[],8-[]], [1-6,2-3,3-2,5-7,3-2,4-5,1-3],NL). NL = [1-[5],2-[4],3-[],4-[],5-[],6-[],7-[],8-[]]

transpose(+Graph, -NewGraph)

Unify NewGraph with a new graph obtained from Graph by replacing all edges of the form V1-V2 by edges of the form V2-V1. The cost is O(|V|^2). In the next example:

?- transpose([1-[3,5],2-[4],3-[], 4-[5],5-[],6-[],7-[],8-[]], NL). NL = [1-[],2-[],3-[1],4-[2],5-[1,4],6-[],7-[],8-[]]

Notice that an undirected graph is its own transpose.

neighbors(+Vertex, +Graph, -Vertices)

Unify Vertices with the list of neighbors of vertex Vertex in Graph. If the vertice is not in the graph fail. In the next example:

?- neighbors(4,[1-[3,5],2-[4],3-[], 4-[1,2,7,5],5-[],6-[],7-[],8-[]], NL). NL = [1,2,7,5]

neighbours(+Vertex, +Graph, -Vertices)

Unify Vertices with the list of neighbours of vertex Vertex in Graph. In the next example:

?- neighbours(4,[1-[3,5],2-[4],3-[], 4-[1,2,7,5],5-[],6-[],7-[],8-[]], NL). NL = [1,2,7,5]

complement(+Graph, -NewGraph)

Unify NewGraph with the graph complementary to Graph. In the next example:

?- complement([1-[3,5],2-[4],3-[], 4-[1,2,7,5],5-[],6-[],7-[],8-[]], NL). NL = [1-[2,4,6,7,8],2-[1,3,5,6,7,8],3-[1,2,4,5,6,7,8], 4-[3,5,6,8],5-[1,2,3,4,6,7,8],6-[1,2,3,4,5,7,8], 7-[1,2,3,4,5,6,8],8-[1,2,3,4,5,6,7]]

compose(+LeftGraph, +RightGraph, -NewGraph)

Compose the graphs LeftGraph and RightGraph to form NewGraph. In the next example:

?- compose([1-[2],2-[3]],[2-[4],3-[1,2,4]],L). L = [1-[4],2-[1,2,4],3-[]]

top_sort(+Graph, -Sort)

Generate the set of nodes Sort as a topological sorting of graph Graph, if one is possible. In the next example we show how topological sorting works for a linear graph:

?- top_sort([_138-[_219],_219-[_139], _139-[]],L). L = [_138,_219,_139]

top_sort(+Graph, -Sort0, -Sort)

Generate the difference list Sort-Sort0 as a topological sorting of graph Graph, if one is possible.

transitive_closure(+Graph, +Closure)

Generate the graph Closure as the transitive closure of graph Graph. In the next example:

?- transitive_closure([1-[2,3],2-[4,5],4-[6]],L). L = [1-[2,3,4,5,6],2-[4,5,6],4-[6]]

reachable(+Node, +Graph, -Vertices)

Unify Vertices with the set of all vertices in graph Graph that are reachable from Node. In the next example:

?- reachable(1,[1-[3,5],2-[4],3-[],4-[5],5-[]],V). V = [1,3,5]

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7.20 Directed Graphs

The following graph manipulation routines use the red-black tree library to try to avoid linear-time scans of the graph for all graph operations. Graphs are represented as a red-black tree, where the key is the vertex, and the associated value is a list of vertices reachable from that vertex through an edge (ie, a list of edges).

dgraph_new(+Graph)

Create a new directed graph. This operation must be performed before trying to use the graph.

dgraph_vertices(+Graph, -Vertices)

Unify Vertices with all vertices appearing in graph Graph.

dgraph_edges(+Graph, -Edges)

Unify Edges with all edges appearing in graph Graph.

dgraph_add_vertices(+Graph, +Vertices, -NewGraph)

Unify NewGraph with a new graph obtained by adding the list of vertices Vertices to the graph Graph.

dgraph_del_vertices(+Vertices, +Graph, -NewGraph)

Unify NewGraph with a new graph obtained by deleting the list of vertices Vertices and all the edges that start from or go to a vertex in Vertices to the graph Graph.

dgraph_add_edges(+Graph, +Edges, -NewGraph)

Unify NewGraph with a new graph obtained by adding the list of edges Edges to the graph Graph.

dgraph_del_edges(+Graph, +Edges, -NewGraph)

Unify NewGraph with a new graph obtained by removing the list of edges Edges from the graph Graph. Notice that no vertices are deleted.

dgraph_neighbors(+Vertex, +Graph, -Vertices)

Unify Vertices with the list of neighbors of vertex Vertex in Graph. If the vertice is not in the graph fail.

dgraph_neighbours(+Vertex, +Graph, -Vertices)

Unify Vertices with the list of neighbours of vertex Vertex in Graph.

dgraph_complement(+Graph, -NewGraph)

Unify NewGraph with the graph complementary to Graph.

dgraph_transpose(+Graph, -Transpose)

Unify NewGraph with a new graph obtained from Graph by replacing all edges of the form V1-V2 by edges of the form V2-V1.

dgraph_close(+Graph1, +Graph2, -ComposedGraph)

Unify ComposedGraph with a new graph obtained by composing Graph1 and Graph2, ie, ComposedGraph has an edge V1-V2 iff there is a V such that V1-V in Graph1 and V-V2 in Graph2.

dgraph_transitive_closure(+Graph, -Closure)

Unify Closure with the transitive closure of graph Graph.

dgraph_symmetric_closure(+Graph, -Closure)

Unify Closure with the symmetric closure of graph Graph, that is, if Closure contains an edge U-V it must also contain the edge V-U.

dgraph_top_sort(+Graph, -Vertices)

Unify Vertices with the topological sort of graph Graph.

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7.21 Undirected Graphs

The following graph manipulation routines use the red-black tree graph library to implement undirected graphs. Mostly, this is done by having two directed edges per undirected edge.

undgraph_new(+Graph)

Create a new directed graph. This operation must be performed before trying to use the graph.

undgraph_vertices(+Graph, -Vertices)

Unify Vertices with all vertices appearing in graph Graph.

undgraph_edges(+Graph, -Edges)

Unify Edges with all edges appearing in graph Graph.

undgraph_add_vertices(+Graph, +Vertices, -NewGraph)

Unify NewGraph with a new graph obtained by adding the list of vertices Vertices to the graph Graph.

undgraph_del_vertices(+Vertices, +Graph, -NewGraph)

Unify NewGraph with a new graph obtained by deleting the list of vertices Vertices and all the edges that start from or go to a vertex in Vertices to the graph Graph.

undgraph_add_edges(+Graph, +Edges, -NewGraph)

Unify NewGraph with a new graph obtained by adding the list of edges Edges to the graph Graph.

undgraph_del_edges(+Graph, +Edges, -NewGraph)

Unify NewGraph with a new graph obtained by removing the list of edges Edges from the graph Graph. Notice that no vertices are deleted.

undgraph_neighbors(+Vertex, +Graph, -Vertices)

Unify Vertices with the list of neighbors of vertex Vertex in Graph. If the vertice is not in the graph fail.

undgraph_neighbours(+Vertex, +Graph, -Vertices)

Unify Vertices with the list of neighbours of vertex Vertex in Graph.

undgraph_complement(+Graph, -NewGraph)

Unify NewGraph with the graph complementary to Graph.

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SWI-Prolog Emulation

Subnodes of SWI-Prolog 8.1 Invoking Predicates on all Members of a List  maplist and friends 8.2 Forall  forall built-in 8.3 hProlog and SWI-Prolog Attributed Variables  Emulating SWI-like attributed variables 8.4 SWI Global variables  Emulating SWI-like attributed variables

8. SWI-Prolog Emulation

This library provides a number of SWI-Prolog builtins that are not by default in YAP. This library is loaded with the use_module(library(swi)) command.

append(?List1,?List2,?List3)

Succeeds when List3 unifies with the concatenation of List1 and List2. The predicate can be used with any instantiation pattern (even three variables).

between(+Low,+High,?Value)

Low and High are integers, High less or equal than Low. If Value is an integer, Low less or equal than Value less or equal than High. When Value is a variable it is successively bound to all integers between Low and High. If High is inf, between/3 is true iff Value less or equal than Low, a feature that is particularly interesting for generating integers from a certain value.

chdir(+Dir)

Compatibility predicate. New code should use working_directory/2.

concat_atom(+List,-Atom)

List is a list of atoms, integers or floating point numbers. Succeeds if Atom can be unified with the concatenated elements of List. If List has exactly 2 elements it is equivalent to atom_concat/3, allowing for variables in the list.

concat_atom(?List,+Separator,?Atom)

Creates an atom just like concat_atom/2, but inserts Separator between each pair of atoms. For example: \

?- concat_atom([gnu, gnat], ', ', A). A = 'gnu, gnat'

(Unimplemented) This predicate can also be used to split atoms by instantiating Separator and Atom:

?- concat_atom(L, -, 'gnu-gnat'). L = [gnu, gnat]

nth1(+Index,?List,?Elem)

Succeeds when the Index-th element of List unifies with Elem. Counting starts at 1.

Set environment variable. Name and Value should be instantiated to atoms or integers. The environment variable will be passed to shell/[0-2] and can be requested using getenv/2. They also influence expand_file_name/2.

setenv(+Name,+Value)

Set environment variable. Name and Value should be instantiated to atoms or integers. The environment variable will be passed to shell/[0-2] and can be requested using getenv/2. They also influence expand_file_name/2.

term_to_atom(?Term,?Atom)

Succeeds if Atom describes a term that unifies with Term. When Atom is instantiated Atom is converted and then unified with Term. If Atom has no valid syntax, a syntax_error exception is raised. Otherwise Term is "written" on Atom using write/1.

working_directory(-Old,+New)

Unify Old with an absolute path to the current working directory and change working directory to New. Use the pattern working_directory(CWD, CWD) to get the current directory. See also absolute_file_name/2 and chdir/1.

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8.1 Invoking Predicates on all Members of a List

All the predicates in this section call a predicate on all members of a list or until the predicate called fails. The predicate is called via call/[2..], which implies common arguments can be put in front of the arguments obtained from the list(s). For example:

?- maplist(plus(1), [0, 1, 2], X). X = [1, 2, 3]

we will phrase this as "Predicate is applied on ..."

maplist(+Pred,+List)

Pred is applied successively on each element of List until the end of the list or Pred fails. In the latter case maplist/2 fails.

maplist(+Pred,+List1,+List2)

Apply Pred on all successive triples of elements from List1 and List2. Fails if Pred can not be applied to a pair. See the example above.

maplist(+Pred,+List1,+List2,+List4)

Apply Pred on all successive triples of elements from List1, List2 and List3. Fails if Pred can not be applied to a triple. See the example above.

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8.2 Forall

forall(+Cond,+Action)

For all alternative bindings of Cond Action can be proven. The next example verifies that all arithmetic statements in the list L are correct. It does not say which is wrong if one proves wrong.

?- forall(member(Result = Formula, [2 = 1 + 1, 4 = 2 * 2]), Result =:= Formula).

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8.3 hProlog and SWI-Prolog Attributed Variables

Attributed variables provide a technique for extending the Prolog unification algorithm by hooking the binding of attributed variables. There is little consensus in the Prolog community on the exact definition and interface to attributed variables. Yap Prolog traditionally implements a SICStus-like interface, but to enable SWI-compatibility we have implemented the SWI-Prolog interface, identical to the one realised by Bart Demoen for hProlog.

Binding an attributed variable schedules a goal to be executed at the first possible opportunity. In the current implementation the hooks are executed immediately after a successful unification of the clause-head or successful completion of a foreign language (builtin) predicate. Each attribute is associated to a module and the hook (attr_unify_hook/2) is executed in this module. The example below realises a very simple and incomplete finite domain reasoner.

:- module(domain, [ domain/2 % Var, ?Domain ]). :- use_module(library(oset)). domain(X, Dom) :- var(Dom), !, get_attr(X, domain, Dom). domain(X, List) :- sort(List, Domain), put_attr(Y, domain, Domain), X = Y. % An attributed variable with attribute value Domain has been % assigned the value Y attr_unify_hook(Domain, Y) :- ( get_attr(Y, domain, Dom2) -> oset_int(Domain, Dom2, NewDomain), ( NewDomain == [] -> fail ; NewDomain = [Value] -> Y = Value ; put_attr(Y, domain, NewDomain) ) ; var(Y) -> put_attr( Y, domain, Domain ) ; memberchk(Y, Domain) ).

Before explaining the code we give some example queries:

?- domain(X, [a,b]), X = c

no

?- domain(X, [a,b]), domain(X, [a,c]).

X = a

?- domain(X, [a,b,c]), domain(X, [a,c]).

X = _D0

The predicate domain/2 fetches (first clause) or assigns (second clause) the variable a domain, a set of values it can be unified with. In the second clause first associates the domain with a fresh variable and then unifies X to this variable to deal with the possibility that X already has a domain. The predicate attr_unify_hook/2 is a hook called after a variable with a domain is assigned a value. In the simple case where the variable is bound to a concrete value we simply check whether this value is in the domain. Otherwise we take the intersection of the domains and either fail if the intersection is empty (first example), simply assign the value if there is only one value in the intersection (second example) or assign the intersection as the new domain of the variable (third example).

put_attr(+Var,+Module,+Value)

If Var is a variable or attributed variable, set the value for the attribute named Module to Value. If an attribute with this name is already associated with Var, the old value is replaced. Backtracking will restore the old value (i.e. an attribute is a mutable term. See also setarg/3). This predicate raises a type error if Var is not a variable or Module is not an atom.

get_attr(+Var,+Module,+Value)

Request the current value for the attribute named Module. If Var is not an attributed variable or the named attribute is not associated to Var this predicate fails silently. If Module is not an atom, a type error is raised.

del_attr(+Var,+Module)

Delete the named attribute. If Var loses its last attribute it is transformed back into a traditional Prolog variable. If Module is not an atom, a type error is raised. In all other cases this predicate succeeds regarless whether or not the named attribute is present.

attr_unify_hook(+AttValue,+VarValue)

Hook that must be defined in the module an attributed variable refers to. Is is called after the attributed variable has been unified with a non-var term, possibly another attributed variable. AttValue is the attribute that was associated to the variable in this module and VarValue is the new value of the variable. Normally this predicate fails to veto binding the variable to VarValue, forcing backtracking to undo the binding. If VarValue is another attributed variable the hook often combines the two attribute and associates the combined attribute with VarValue using put_attr/3.

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8.3.1 Special Purpose SWI Predicates for Attributes

Normal user code should deal with put_attr/3, get_attr/3 and del_attr/2. The routines in this section fetch or set the entire attribute list of a variables. Use of these predicates is anticipated to be restricted to printing and other special purpose operations.

get_attrs(+Var,-Attributes)

Get all attributes of Var. Attributes is a term of the form att(Module, Value, MoreAttributes), where MoreAttributes is [] for the last attribute.

put_attrs(+Var,+Attributes)

Set all attributes of Var. See get_attrs/2 for a description of Attributes.

copy_term_nat(?TI,-TF)

As copy_term/2. Attributes however, are not copied but replaced by fresh variables.

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8.4 SWI Global variables

SWI-Prolog global variables are associations between names (atoms) and terms. They differ in various ways from storing information using assert/1 or recorda/3.

• The value lives on the Prolog (global) stack. This implies that lookup time is independent from the size of the term. This is particulary interesting for large data structures such as parsed XML documents or the CHR global constraint store.
• They support both global assignment using nb_setval/2 and backtrackable assignment using b_setval/2.
• Only one value (which can be an arbitrary complex Prolog term) can be associated to a variable at a time.
• Their value cannot be shared among threads. Each thread has its own namespace and values for global variables.
• Currently global variables are scoped globally. We may consider module scoping in future versions.

Both b_setval/2 and nb_setval/2 implicitely create a variable if the referenced name does not already refer to a variable.

Global variables may be initialised from directives to make them available during the program lifetime, but some considerations are necessary for saved-states and threads. Saved-states to not store global variables, which implies they have to be declared with initialization/1 to recreate them after loading the saved state. Each thread has its own set of global variables, starting with an empty set. Using thread_inititialization/1 to define a global variable it will be defined, restored after reloading a saved state and created in all threads that are created after the registration.

b_setval(+Name,+Value)

Associate the term Value with the atom Name or replaces the currently associated value with Value. If Name does not refer to an existing global variable a variable with initial value [] is created (the empty list). On backtracking the assignment is reversed.

b_getval(+Name,-Value)

Get the value associated with the global variable Name and unify it with Value. Note that this unification may further instantiate the value of the global variable. If this is undesirable the normal precautions (double negation or copy_term/2) must be taken. The b_getval/2 predicate generates errors if Name is not an atom or the requested variable does not exist.

nb_setval(+Name,+Value)

Associates a copy of Value created with duplicate_term/2 with the atom Name. Note that this can be used to set an initial value other than [] prior to backtrackable assignment.

nb_getval(+Name,-Value)

The nb_getval/2 predicate is a synonym for b_getval/2, introduced for compatibility and symetry. As most scenarios will use a particular global variable either using non-backtracable or backtrackable assignment, using nb_getval/2 can be used to document that the variable is used non-backtracable.

nb_current(?Name,?Value)

Enumerate all defined variables with their value. The order of enumeration is undefined.

nb_delete(?Name)

Delete the named global variable.

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8.4.1 Compatibility of SWI-Prolog Global Variables

Global variables have been introduced by various Prolog implementations recently. The implementation of them in SWI-Prolog is based on hProlog by Bart Demoen. In discussion with Bart it was decided that the semantics if hProlog nb_setval/2, which is equivalent to nb_linkval/2 is not acceptable for normal Prolog users as the behaviour is influenced by how builtin predicates constructing terms (read/1, =../2, etc.) are implemented.

GNU-Prolog provides a rich set of global variables, including arrays. Arrays can be implemented easily in SWI-Prolog using functor/3 and setarg/3 due to the unrestricted arity of compound terms.

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9. Extensions to Prolog

YAP includes several extensions that are not enabled by default, but that can be used to extend the functionality of the system. These options can be set at compilation time by enabling the related compilation flag, as explained in the Makefile

Extensions to Traditional Prolog

9.1 Rational Trees		Working with Rational Trees
9.2 Coroutining		Changing the Execution of Goals
10. Attributed Variables		Using attributed Variables
11. Constraint Logic Programming over Reals		The CLP(R) System
13. Logtalk		The Logtalk Object-Oriented system
14. Threads		Thread Library
15. Parallelism		Running in Or-Parallel
16. Tabling		Storing Intermediate Solutions of programs
18. Profiling the Abstract Machine		Profiling Abstract Machine Instructions
17. Tracing at Low Level		Tracing at Abstract Machine Level

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9.1 Rational Trees

Prolog unification is not a complete implementation. For efficiency considerations, Prolog systems do not perform occur checks while unifying terms. As an example, X = a(X) will not fail but instead will create an infinite term of the form a(a(a(a(a(...))))), or rational tree.

Rational trees are no supported by default in YAP. In previous versions, this was not the default and these terms could easily lead to infinite computation. For example, X = a(X), X = X would enter an infinite loop.

The RATIONAL_TREES flag improves support for these terms. Internal primitives are now aware that these terms can exist, and will not enter infinite loops. Hence, the previous unification will succeed. Another example, X = a(X), ground(X) will succeed instead of looping. Other affected built-ins include the term comparison primitives, numbervars/3, copy_term/2, and the internal data base routines. The support does not extend to Input/Output routines or to assert/1 YAP does not allow directly reading rational trees, and you need to use write_depth/2 to avoid entering an infinite cycle when trying to write an infinite term.

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9.2 Coroutining

Prolog uses a simple left-to-right flow of control. It is sometimes convenient to change this control so that goals will only be executed when conditions are fulfilled. This may result in a more "data-driven" execution, or may be necessary to correctly implement extensions such as negation by default.

The COROUTINING flag enables this option. Note that the support for coroutining will in general slow down execution.

The following declaration is supported:

block/1

The argument to block/1 is a condition on a goal or a conjunction of conditions, with each element separated by commas. Each condition is of the form predname(C1,...,CN), where N is the arity of the goal, and each CI is of the form -, if the argument must suspend until the variable is bound, or ?, otherwise.

wait/1

The argument to wait/1 is a predicate descriptor or a conjunction of these predicates. These predicates will suspend until their first argument is bound.

The following primitives are supported:

dif(X,Y)

Succeed if the two arguments do not unify. A call to dif/2 will suspend if unification may still succeed or fail, and will fail if they always unify.

freeze(?X,:G)

Delay execution of goal G until the variable X is bound.

frozen(X,G)

Unify G with a conjunction of goals suspended on variable X, or true if no goal has suspended.

when(+C,:G)

Delay execution of goal G until the conditions C are satisfied. The conditions are of the following form:

C1,C2

Delay until both conditions C1 and C2 are satisfied.

C1;C2

Delay until either condition C1 or condition C2 is satisfied.

?=(V1,C2)

Delay until terms V1 and V1 have been unified.

nonvar(V)

Delay until variable V is bound.

ground(V)

Delay until variable V is ground.

Note that when/2 will fail if the conditions fail.

call_residue(:G,L)

Call goal G. If subgoals of G are still blocked, return a list containing these goals and the variables they are blocked in. The goals are then considered as unblocked. The next example shows a case where dif/2 suspends twice, once outside call_residue/2, and the other inside:

?- dif(X,Y), call_residue((dif(X,Y),(X = f(Z) ; Y = f(Z))), L). X = f(Z), L = [[Y]-dif(f(Z),Y)], dif(f(Z),Y) ? ; Y = f(Z), L = [[X]-dif(X,f(Z))], dif(X,f(Z)) ? ; no

The system only reports one invocation of dif/2 as having suspended.

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10. Attributed Variables

10.1 Attribute Declarations		Declaring New Attributes
10.2 Attribute Manipulation		Setting and Reading Attributes
10.3 Attributed Unification		Tuning the Unification Algorithm
10.4 Displaying Attributes		Displaying Attributes in User-Readable Form
10.5 Projecting Attributes		Obtaining the Attributes of Interest
10.6 Attribute Examples		Two Simple Examples of how to use Attributes.

YAP now supports the attributed variables packaged developed at OFAI by Christian Holzbaur. Attributes are a means of declaring that an arbitrary term is a property for a variable. These properties can be updated during forward execution. Moreover, the unification algorithm is aware of attributed variables and will call user defined handlers when trying to unify these variables.

Attributed variables provide an elegant abstraction over which one can extend Prolog systems. Their main application so far has been in implementing constraint handlers, such as Holzbaur's CLPQR and Fruewirth and Holzbaur's CHR, but other applications have been proposed in the literature.

The command

| ?- use_module(library(atts)).

enables the use of attributed variables. The package provides the following functionality:

• Each attribute must be declared first. Attributes are described by a functor and are declared per module. Each Prolog module declares its own sets of attributes. Different modules may have different functors with the same module.
• The built-in put_atts/2 adds or deletes attributes to a variable. The variable may be unbound or may be an attributed variable. In the latter case, YAP discards previous values for the attributes.
• The built-in get_atts/2 can be used to check the values of an attribute associated with a variable.
• The unification algorithm calls the user-defined predicate verify_attributes/3 before trying to bind an attributed variable. Unification will resume after this call.
• The user-defined predicate attribute_goal/2 converts from an attribute to a goal.
• The user-defined predicate project_attributes/2 is used from a set of variables into a set of constraints or goals. One application of project_attributes/2 is in the top-level, where it is used to output the set of floundered constraints at the end of a query.

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10.1 Attribute Declarations

Attributes are compound terms associated with a variable. Each attribute has a name which is private to the module in which the attribute was defined. Variables may have at most one attribute with a name. Attribute names are defined with the following declaration:

:- attribute AttributeSpec, ..., AttributeSpec.

where each AttributeSpec has the form (Name/Arity). One single such declaration is allowed per module Module.

Although the YAP module system is predicate based, attributes are local to modules. This is implemented by rewriting all calls to the built-ins that manipulate attributes so that attribute names are preprocessed depending on the module. The user:goal_expansion/3 mechanism is used for this purpose.

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10.2 Attribute Manipulation

The attribute manipulation predicates always work as follows:

1. The first argument is the unbound variable associated with attributes,
2. The second argument is a list of attributes. Each attribute will be a Prolog term or a constant, prefixed with the + and - unary operators. The prefix + may be dropped for convenience.

The following three procedures are available to the user. Notice that these built-ins are rewritten by the system into internal built-ins, and that the rewriting process depends on the module on which the built-ins have been invoked.

Module:get_atts(-Var,?ListOfAttributes)

Unify the list ?ListOfAttributes with the attributes for the unbound variable Var. Each member of the list must be a bound term of the form +(Attribute), -(Attribute) (the kbd prefix may be dropped). The meaning of + and - is:

+(Attribute)

Unifies Attribute with a corresponding attribute associated with Var, fails otherwise.

-(Attribute)

Succeeds if a corresponding attribute is not associated with Var. The arguments of Attribute are ignored.

Module:put_atts(-Var,?ListOfAttributes)

Associate with or remove attributes from a variable Var. The attributes are given in ?ListOfAttributes, and the action depends on how they are prefixed:

+(Attribute)

Associate Var with Attribute. A previous value for the attribute is simply replace (like with set_mutable/2).

-(Attribute)

Remove the attribute with the same name. If no such attribute existed, simply succeed.

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10.3 Attributed Unification

The user-predicate predicate verify_attributes/3 is called when attempting to unify an attributed variable which might have attributes in some Module.

Module:verify_attributes(-Var, +Value, -Goals)

The predicate is called when trying to unify the attributed variable Var with the Prolog term Value. Note that Value may be itself an attributed variable, or may contain attributed variables. The goal verify_attributes/3 is actually called before Var is unified with Value.

It is up to the user to define which actions may be performed by verify_attributes/3 but the procedure is expected to return in Goals a list of goals to be called after Var is unified with Value. If verify_attributes/3 fails, the unification will fail.

Notice that the verify_attributes/3 may be called even if Var has no attributes in module Module. In this case the routine should simply succeed with Goals unified with the empty list.

attvar(-Var)

Succeed if Var is an attributed variable.

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10.4 Displaying Attributes

Attributes are usually presented as goals. The following routines are used by built-in predicates such as call_residue/2 and by the Prolog top-level to display attributes:

Module:attribute_goal(-Var, -Goal)

User-defined procedure, called to convert the attributes in Var to a Goal. Should fail when no interpretation is available.

Module:project_attributes(-QueryVars, +AttrVars)

User-defined procedure, called to project the attributes in the query, AttrVars, given that the set of variables in the query is QueryVars.

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10.5 Projecting Attributes

Constraint solvers must be able to project a set of constraints to a set of variables. This is useful when displaying the solution to a goal, but may also be used to manipulate computations. The user-defined project_attributes/2 is responsible for implementing this projection.

Module:project_attributes(+QueryVars, +AttrVars)

Given a list of variables QueryVars and list of attributed variables AttrVars, project all attributes in AttrVars to QueryVars. Although projection is constraint system dependent, typically this will involve expressing all constraints in terms of QueryVars and considering all remaining variables as existentially quantified.

Projection interacts with attribute_goal/2 at the prolog top level. When the query succeeds, the system first calls project_attributes/2. The system then calls attribute_goal/2 to get a user-level representation of the constraints. Typically, attribute_goal/2 will convert from the original constraints into a set of new constraints on the projection, and these constraints are the ones that will have an attribute_goal/2 handler.

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10.6 Attribute Examples

The following two examples example is taken from the SICStus Prolog manual. It sketches the implementation of a simple finite domain "solver". Note that an industrial strength solver would have to provide a wider range of functionality and that it quite likely would utilize a more efficient representation for the domains proper. The module exports a single predicate domain(-Var,?Domain) which associates Domain (a list of terms) with Var. A variable can be queried for its domain by leaving Domain unbound.

We do not present here a definition for project_attributes/2. Projecting finite domain constraints happens to be difficult.

:- module(domain, [domain/2]). :- use_module(library(atts)). :- use_module(library(ordsets), [ ord_intersection/3, ord_intersect/2, list_to_ord_set/2 ]). :- attribute dom/1. verify_attributes(Var, Other, Goals) :- get_atts(Var, dom(Da)), !, % are we involved? ( var(Other) -> % must be attributed then ( get_atts(Other, dom(Db)) -> % has a domain? ord_intersection(Da, Db, Dc), Dc = [El|Els], % at least one element ( Els = [] -> % exactly one element Goals = [Other=El] % implied binding ; Goals = [], put_atts(Other, dom(Dc))% rescue intersection ) ; Goals = [], put_atts(Other, dom(Da)) % rescue the domain ) ; Goals = [], ord_intersect([Other], Da) % value in domain? ). verify_attributes(_, _, []). % unification triggered % because of attributes % in other modules attribute_goal(Var, domain(Var,Dom)) :- % interpretation as goal get_atts(Var, dom(Dom)). domain(X, Dom) :- var(Dom), !, get_atts(X, dom(Dom)). domain(X, List) :- list_to_ord_set(List, Set), Set = [El|Els], % at least one element ( Els = [] -> % exactly one element X = El % implied binding ; put_atts(Fresh, dom(Set)), X = Fresh % may call % verify_attributes/3 ).

Note that the "implied binding" Other=El was deferred until after the completion of verify_attribute/3. Otherwise, there might be a danger of recursively invoking verify_attribute/3, which might bind Var, which is not allowed inside the scope of verify_attribute/3. Deferring unifications into the third argument of verify_attribute/3 effectively serializes the calls to verify_attribute/3.

Assuming that the code resides in the file `domain.yap', we can use it via:

| ?- use_module(domain).

Let's test it:

| ?- domain(X,[5,6,7,1]), domain(Y,[3,4,5,6]), domain(Z,[1,6,7,8]). domain(X,[1,5,6,7]), domain(Y,[3,4,5,6]), domain(Z,[1,6,7,8]) ? yes | ?- domain(X,[5,6,7,1]), domain(Y,[3,4,5,6]), domain(Z,[1,6,7,8]), X=Y. Y = X, domain(X,[5,6]), domain(Z,[1,6,7,8]) ? yes | ?- domain(X,[5,6,7,1]), domain(Y,[3,4,5,6]), domain(Z,[1,6,7,8]), X=Y, Y=Z. X = 6, Y = 6, Z = 6

To demonstrate the use of the Goals argument of verify_attributes/3, we give an implementation of freeze/2. We have to name it myfreeze/2 in order to avoid a name clash with the built-in predicate of the same name.

:- module(myfreeze, [myfreeze/2]). :- use_module(library(atts)). :- attribute frozen/1. verify_attributes(Var, Other, Goals) :- get_atts(Var, frozen(Fa)), !, % are we involved? ( var(Other) -> % must be attributed then ( get_atts(Other, frozen(Fb)) % has a pending goal? -> put_atts(Other, frozen((Fa,Fb))) % rescue conjunction ; put_atts(Other, frozen(Fa)) % rescue the pending goal ), Goals = [] ; Goals = [Fa] ). verify_attributes(_, _, []). attribute_goal(Var, Goal) :- % interpretation as goal get_atts(Var, frozen(Goal)). myfreeze(X, Goal) :- put_atts(Fresh, frozen(Goal)), Fresh = X.

Assuming that this code lives in file `myfreeze.yap', we would use it via:

| ?- use_module(myfreeze). | ?- myfreeze(X,print(bound(x,X))), X=2. bound(x,2) % side effect X = 2 % bindings

The two solvers even work together:

| ?- myfreeze(X,print(bound(x,X))), domain(X,[1,2,3]), domain(Y,[2,10]), X=Y. bound(x,2) % side effect X = 2, % bindings Y = 2

The two example solvers interact via bindings to shared attributed variables only. More complicated interactions are likely to be found in more sophisticated solvers. The corresponding verify_attributes/3 predicates would typically refer to the attributes from other known solvers/modules via the module prefix in Module:get_atts/2.

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11.1 Solver Predicates
11.2 Syntax of the predicate arguments
11.3 Use of unification
11.4 Non-Linear Constraints

11. Constraint Logic Programming over Reals

YAP now uses the CLP(R) package developed by Leslie De Koninck, K.U. Leuven as part of a thesis with supervisor Bart Demoen and daily advisor Tom Schrijvers, and distributed with SWI-Prolog.

This CLP(R) system is a port of the CLP(Q,R) system of Sicstus Prolog and YAP by Christian Holzbaur: Holzbaur C.: OFAI clp(q,r) Manual, Edition 1.3.3, Austrian Research Institute for Artificial Intelligence, Vienna, TR-95-09, 1995, http://www.ai.univie.ac.at/cgi-bin/tr-online?number+95-09 This port only contains the part concerning real arithmetics. This manual is roughly based on the manual of the above mentioned CLP(QR) implementation.

Please note that the `clpr' library is not an autoload library and therefore this library must be loaded explicitely before using it:

:- use_module(library(clpr)).

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11.1 Solver Predicates

The following predicates are provided to work with constraints:

{+Constraints}

Adds the constraints given by Constraints to the constraint store.

entailed(+Constraint)

Succeeds if Constraint is necessarily true within the current constraint store. This means that adding the negation of the constraint to the store results in failure.

inf(+Expression,-Inf)

Computes the infimum of Expression within the current state of the constraint store and returns that infimum in Inf. This predicate does not change the constraint store.

inf(+Expression,-Sup)

Computes the supremum of Expression within the current state of the constraint store and returns that supremum in Sup. This predicate does not change the constraint store.

min(+Expression)

Minimizes Expression within the current constraint store. This is the same as computing the infimum and equation the expression to that infimum.

max(+Expression)

Maximizes Expression within the current constraint store. This is the same as computing the supremum and equating the expression to that supremum.

bb_inf(+Ints,+Expression,-Inf,-Vertext,+Eps)

Computes the infimum of Expression within the current constraint store, with the additional constraint that in that infimum, all variables in Ints have integral values. Vertex will contain the values of Ints in the infimum. Eps denotes how much a value may differ from an integer to be considered an integer. E.g. when Eps = 0.001, then X = 4.999 will be considered as an integer (5 in this case). Eps should be between 0 and 0.5.

bb_inf(+Ints,+Expression,-Inf)

The same as bb_inf/5 but without returning the values of the integers and with an eps of 0.001.

bb_inf(+Target,+Newvars,-CodedAnswer)

Returns the constraints on Target in the list CodedAnswer where all variables of Target have veen replaced by NewVars. This operation does not change the constraint store. E.g. in

dump([X,Y,Z],[x,y,z],Cons)

Cons will contain the constraints on X, Y and Z where these variables have been replaced by atoms x, y and z.

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11.2 Syntax of the predicate arguments

The arguments of the predicates defined in the subsection above are defined in the following table. Failing to meet the syntax rules will result in an exception.

<Constraints> ---> <Constraint> \\ single constraint \\ | <Constraint> , <Constraints> \\ conjunction \\ | <Constraint> ; <Constraints> \\ disjunction \\ <Constraint> ---> <Expression> {<} <Expression> \\ less than \\ | <Expression> {>} <Expression> \\ greater than \\ | <Expression> {=<} <Expression> \\ less or equal \\ | {<=}(<Expression>, <Expression>) \\ less or equal \\ | <Expression> {>=} <Expression> \\ greater or equal \\ | <Expression> {=\=} <Expression> \\ not equal \\ | <Expression> =:= <Expression> \\ equal \\ | <Expression> = <Expression> \\ equal \\ <Expression> ---> <Variable> \\ Prolog variable \\ | <Number> \\ Prolog number (float, integer) \\ | +<Expression> \\ unary plus \\ | -<Expression> \\ unary minus \\ | <Expression> + <Expression> \\ addition \\ | <Expression> - <Expression> \\ substraction \\ | <Expression> * <Expression> \\ multiplication \\ | <Expression> / <Expression> \\ division \\ | abs(<Expression>) \\ absolute value \\ | sin(<Expression>) \\ sine \\ | cos(<Expression>) \\ cosine \\ | tan(<Expression>) \\ tangent \\ | exp(<Expression>) \\ exponent \\ | pow(<Expression>) \\ exponent \\ | <Expression> {^} <Expression> \\ exponent \\ | min(<Expression>, <Expression>) \\ minimum \\ | max(<Expression>, <Expression>) \\ maximum \\

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11.3 Use of unification

Instead of using the {}/1 predicate, you can also use the standard unification mechanism to store constraints. The following code samples are equivalent:

`Unification with a variable'

{X =:= Y} {X = Y} X = Y

`Unification with a number'

{X =:= 5.0} {X = 5.0} X = 5.0

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11.4 Non-Linear Constraints

In this version, non-linear constraints do not get solved until certain conditions are satisfied. We call these conditions the isolation axioms. They are given in the following table.

A = B * C when B or C is ground or // A = 5 * C or A = B * 4 \\ A and (B or C) are ground // 20 = 5 * C or 20 = B * 4 \\ A = B / C when C is ground or // A = B / 3 A and B are ground // 4 = 12 / C X = min(Y,Z) when Y and Z are ground or // X = min(4,3) X = max(Y,Z) Y and Z are ground // X = max(4,3) X = abs(Y) Y is ground // X = abs(-7) X = pow(Y,Z) when X and Y are ground or // 8 = 2 ^ Z X = exp(Y,Z) X and Z are ground // 8 = Y ^ 3 X = Y ^ Z Y and Z are ground // X = 2 ^ 3 X = sin(Y) when X is ground or // 1 = sin(Y) X = cos(Y) Y is ground // X = sin(1.5707) X = tan(Y)

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12.1 Introduction
12.2 Syntax and Semantics
12.3 CHR in YAP Programs
12.4 Debugging
12.5 Examples
12.6 Compatibility with SICStus CHR
12.7 Guidelines

12. CHR: Constraint Handling Rules

This chapter is written by Tom Schrijvers, K.U. Leuven for the hProlog system. Adjusted by Jan Wielemaker to fit the SWI-Prolog documentation infrastructure and remove hProlog specific references.

The CHR system of SWI-Prolog is the K.U.Leuven CHR system. The runtime environment is written by Christian Holzbaur and Tom Schrijvers while the compiler is written by Tom Schrijvers. Both are integrated with SWI-Prolog and licenced under compatible conditions with permission from the authors.

The main reference for SWI-Prolog's CHR system is:

• T. Schrijvers, and B. Demoen, The K.U.Leuven CHR System: Implementation and Application, First Workshop on Constraint Handling Rules: Selected Contributions (Fruwirth, T. and Meister, M., eds.), pp. 1-5, 2004.

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12.1 Introduction

Constraint Handling Rules (CHR) is a committed-choice bottom-up language embedded in Prolog. It is designed for writing constraint solvers and is particularily useful for providing application-specific constraints. It has been used in many kinds of applications, like scheduling, model checking, abduction, type checking among many others.

CHR has previously been implemented in other Prolog systems (SICStus, Eclipse, Yap), Haskell and Java. This CHR system is based on the compilation scheme and runtime environment of CHR in SICStus.

In this documentation we restrict ourselves to giving a short overview of CHR in general and mainly focus on elements specific to this implementation. For a more thorough review of CHR we refer the reader to [Freuhwirth:98]. More background on CHR can be found at the CHR web site.

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12.2 Syntax and Semantics

@c=============================

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12.2.1 Syntax

The syntax of CHR rules in hProlog is the following:

rules --> rule, rules. rules --> []. rule --> name, actual_rule, pragma, [atom('.')]. name --> atom, [atom(')']. name --> []. actual_rule --> simplification_rule. actual_rule --> propagation_rule. actual_rule --> simpagation_rule. simplification_rule --> constraints, [atom('<=>')], guard, body. propagation_rule --> constraints, [atom('==>')], guard, body. simpagation_rule --> constraints, [atom('\')], constraints, [atom('<=>')], guard, body. constraints --> constraint, constraint_id. constraints --> constraint, [atom(',')], constraints. constraint --> compound_term. constraint_id --> []. constraint_id --> [atom('#')], variable. guard --> []. guard --> goal, [atom('|')]. body --> goal. pragma --> []. pragma --> [atom('pragma')], actual_pragmas. actual_pragmas --> actual_pragma. actual_pragmas --> actual_pragma, [atom(',')], actual_pragmas. actual_pragma --> [atom('passive(')], variable, [atom(')')].

Additional syntax-related terminology:

• head: the constraints in an actual_rule before the arrow (either <=> or ==>)

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12.2.2 Semantics

In this subsection the operational semantics of CHR in Prolog are presented informally. They do not differ essentially from other CHR systems.

When a constraint is called, it is considered an active constraint and the system will try to apply the rules to it. Rules are tried and executed sequentially in the order they are written.

A rule is conceptually tried for an active constraint in the following way. The active constraint is matched with a constraint in the head of the rule. If more constraints appear in the head they are looked for among the suspended constraints, which are called passive constraints in this context. If the necessary passive constraints can be found and all match with the head of the rule and the guard of the rule succeeds, then the rule is committed and the body of the rule executed. If not all the necessary passive constraint can be found, the matching fails or the guard fails, then the body is not executed and the process of trying and executing simply continues with the following rules. If for a rule, there are multiple constraints in the head, the active constraint will try the rule sequentially multiple times, each time trying to match with another constraint.

This process ends either when the active constraint disappears, i.e. it is removed by some rule, or after the last rule has been processed. In the latter case the active constraint becomes suspended.

A suspended constraint is eligible as a passive constraint for an active constraint. The other way it may interact again with the rules, is when a variable appearing in the constraint becomes bound to either a nonvariable or another variable involved in one or more constraints. In that case the constraint is triggered, i.e. it becomes an active constraint and all the rules are tried.

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Rule Types

There are three different kinds of rules, each with their specific semantics:

simplification

The simplification rule removes the constraints in its head and calls its body.

propagation

The propagation rule calls its body exactly once for the constraints in its head.

simpagation

The simpagation rule removes the constraints in its head after the \ and then calls its body. It is an optimization of simplification rules of the form: \[constraints_1, constraints_2 <=> constraints_1, body \] Namely, in the simpagation form:

constraints1 \ constraints2 <=> body

constraints1 constraints are not called in the body.

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Rule Names

Naming a rule is optional and has no semantical meaning. It only functions as documentation for the programmer.

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Pragmas

The semantics of the pragmas are:

`passive(Identifier)'

The constraint in the head of a rule Identifier can only act as a passive constraint in that rule.

Additional pragmas may be released in the future.

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Options

It is possible to specify options that apply to all the CHR rules in the module. Options are specified with the option/2 declaration:

option(Option,Value).

Available options are:

check_guard_bindings

This option controls whether guards should be checked for illegal variable bindings or not. Possible values for this option are on, to enable the checks, and off, to disable the checks.

optimize

This is an experimental option controlling the degree of optimization. Possible values are full, to enable all available optimizations, and off (default), to disable all optimizations. The default is derived from the SWI-Prolog flag optimise, where true is mapped to full. Therefore the commandline option `-O' provides full CHR optimization. If optimization is enabled, debugging should be disabled.

debug

This options enables or disables the possibility to debug the CHR code. Possible values are on (default) and off. See `debugging' for more details on debugging. The default is derived from the prolog flag generate_debug_info, which is true by default. See `-nodebug'. If debugging is enabled, optimization should be disabled.

mode

This option specifies the mode for a particular constraint. The value is a term with functor and arity equal to that of a constraint. The arguments can be one of -, + or ?. The latter is the default. The meaning is the following:

-

The corresponding argument of every occurrence of the constraint is always unbound.

+

The corresponding argument of every occurrence of the constraint is always ground.

?

The corresponding argument of every occurrence of the constraint can have any instantiation, which may change over time. This is the default value.

The declaration is used by the compiler for various optimizations. Note that it is up to the user the ensure that the mode declaration is correct with respect to the use of the constraint. This option may occur once for each constraint.

type_declaration

This option specifies the argument types for a particular constraint. The value is a term with functor and arity equal to that of a constraint. The arguments can be a user-defined type or one of the built-in types:

int

The corresponding argument of every occurrence of the constraint is an integer number.

float

… a floating point number.

number

… a number.

natural

… a positive integer.

any

The corresponding argument of every occurrence of the constraint can have any type. This is the default value.

Currently, type declarations are only used to improve certain optimizations (guard simplification, occurrence subsumption, …).

type_definition

This option defines a new user-defined type which can be used in type declarations. The value is a term of the form type(name,list), where name is a term and list is a list of alternatives. Variables can be used to define generic types. Recursive definitions are allowed. Examples are

type(bool,[true,false]). type(complex_number,[float + float * i]). type(binary_tree(T),[ leaf(T) | node(binary_tree(T),binary_tree(T)) ]). type(list(T),[ [] | [T | list(T)]).

The mode, type_declaration and type_definition options are provided for backward compatibility. The new syntax is described below.

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12.3 CHR in YAP Programs

@c===========================

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12.3.1 Embedding in Prolog Programs

The CHR constraints defined in a particulary `chr' file are associated with a module. The default module is user. One should never load different `chr' files with the same CHR module name.

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12.3.2 Constraint declaration

Every constraint used in CHR rules has to be declared. There are two ways to do this. The old style is as follows:

option(type_definition,type(list(T),[ [] , [T|list(T)] ]). option(mode,foo(+,?)). option(type_declaration,foo(list(int),float)). :- constraints foo/2, bar/0.

The new style is as follows:

:- chr_type list(T) ---> [] ; [T|list(T)]. :- constraints foo(+list(int),?float), bar.

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12.3.3 Compilation

The SWI-Prolog CHR compiler exploits term_expansion/2 rules to translate the constraint handling rules to plain Prolog. These rules are loaded from the library `chr'. They are activated if the compiled file has the `chr' extension or after finding a declaration of the format below.

:- constraints ...

It is adviced to define CHR rules in a module file, where the module declaration is immediately followed by including the `chr' library as examplified below:

:- module(zebra, [ zebra/0 ]). :- use_module(library(chr)). :- constraints ...

Using this style CHR rules can be defined in ordinary Prolog `pl' files and the operator definitions required by CHR do not leak into modules where they might cause conflicts.

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12.4 Debugging

@c=================

The CHR debugging facilities are currently rather limited. Only tracing is currently available. To use the CHR debugging facilities for a CHR file it must be compiled for debugging. Generating debug info is controlled by the CHR option debug, whose default is derived from the SWI-Prolog flag generate_debug_info. Therefore debug info is provided unless the `-nodebug' is used.

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12.4.1 Ports

@c===============

For CHR constraints the four standard ports are defined:

call

A new constraint is called and becomes active.

exit

An active constraint exits: it has either been inserted in the store after trying all rules or has been removed from the constraint store.

fail

An active constraint fails.

redo

An active constraint starts looking for an alternative solution.

In addition to the above ports, CHR constraints have five additional ports:

wake

A suspended constraint is woken and becomes active.

insert

An active constraint has tried all rules and is suspended in the constraint store.

remove

An active or passive constraint is removed from the constraint store, if it had been inserted.

try

An active constraints tries a rule with possibly some passive constraints. The try port is entered just before committing to the rule.

apply

An active constraints commits to a rule with possibly some passive constraints. The apply port is entered just after committing to the rule.

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12.4.2 Tracing

@c=================

Tracing is enabled with the chr_trace/0 predicate and disabled with the chr_notrace/0 predicate.

When enabled the tracer will step through the call, exit, fail, wake and apply ports, accepting debug commands, and simply write out the other ports.

The following debug commans are currently supported:

CHR debug options: <cr> creep c creep s skip g ancestors n nodebug b break a abort f fail ? help h help

Their meaning is:

creep

Step to the next port.

skip

Skip to exit port of this call or wake port.

ancestors

Print list of ancestor call and wake ports.

nodebug

Disable the tracer.

break

Enter a recursive Prolog toplevel. See break/0.

abort

Exit to the toplevel. See abort/0.

fail

Insert failure in execution.

help

Print the above available debug options.

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12.4.3 CHR Debugging Predicates

@c====================================

The `chr' module contains several predicates that allow inspecting and printing the content of the constraint store.

chr_trace/0

Activate the CHR tracer. By default the CHR tracer is activated and deactivated automatically by the Prolog predicates trace/0 and notrace/0.

chr_notrace/0

De-activate the CHR tracer. By default the CHR tracer is activated and deactivated automatically by the Prolog predicates trace/0 and notrace/0.

chr_leash/0

Define the set of CHR ports on which the CHR tracer asks for user intervention (i.e. stops). Spec is either a list of ports or a predefined `alias'. Defined aliases are: full to stop at all ports, none or off to never stop, and default to stop at the call, exit, fail, wake and apply ports. See also leash/1.

chr_show_store(+Mod)

Prints all suspended constraints of module Mod to the standard output. This predicate is automatically called by the SWI-Prolog toplevel at the end of each query for every CHR module currently loaded. The prolog-flag chr_toplevel_show_store controls whether the toplevel shows the constraint stores. The value true enables it. Any other value disables it.

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12.5 Examples

@c================

Here are two example constraint solvers written in CHR.

• The program below defines a solver with one constraint, leq/2, which is a less-than-or-equal constraint.

  :- module(leq,[cycle/3, leq/2]). :- use_module(library(chr)). :- constraints leq/2. reflexivity  leq(X,X) <=> true. antisymmetry  leq(X,Y), leq(Y,X) <=> X = Y. idempotence  leq(X,Y) \ leq(X,Y) <=> true. transitivity  leq(X,Y), leq(Y,Z) ==> leq(X,Z). cycle(X,Y,Z):- leq(X,Y), leq(Y,Z), leq(Z,X).

• The program below implements a simple finite domain constraint solver.

  :- module(dom,[dom/2]). :- use_module(library(chr)). :- constraints dom/2. dom(X,[]) <=> fail. dom(X,[Y]) <=> X = Y. dom(X,L1), dom(X,L2) <=> intersection(L1,L2,L3), dom(X,L3). intersection([],_,[]). intersection([H|T],L2,[H|L3]) :- member(H,L2), !, intersection(T,L2,L3). intersection([_|T],L2,L3) :- intersection(T,L2,L3).

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12.6 Compatibility with SICStus CHR

@c==================

There are small differences between CHR in SWI-Prolog and newer YAPs and SICStus and older versions of YAP. Besides differences in available options and pragmas, the following differences should be noted:

[The handler/1 declaration]

In SICStus every CHR module requires a handler/1 declaration declaring a unique handler name. This declaration is valid syntax in SWI-Prolog, but will have no effect. A warning will be given during compilation.

[The rules/1 declaration]

In SICStus, for every CHR module it is possible to only enable a subset of the available rules through the rules/1 declaration. The declaration is valid syntax in SWI-Prolog, but has no effect. A warning is given during compilation.

[Sourcefile naming]

SICStus uses a two-step compiler, where `chr' files are first translated into `pl' files. For SWI-Prolog CHR rules may be defined in a file with any extension.

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12.7 Guidelines

In this section we cover several guidelines on how to use CHR to write constraint solvers and how to do so efficiently.

[Set semantics]

The CHR system allows the presence of identical constraints, i.e. multiple constraints with the same functor, arity and arguments. For most constraint solvers, this is not desirable: it affects efficiency and possibly termination. Hence appropriate simpagation rules should be added of the form:

{constraint \ constraint <=> true}.

[Multi-headed rules]

Multi-headed rules are executed more efficiently when the constraints share one or more variables.

[Mode and type declarations]

Provide mode and type declarations to get more efficient program execution. Make sure to disable debug (`-nodebug') and enable optimization (`-O').

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13. Logtalk

The Logtalk object-oriented extension is available once included with the use_module(library(logtalk)) command. Note that, although we load Logtalk using the use_module/1 built-in predicate, the system is not packaged as a module not does it use modules in its implementation.

Logtalk documentation is included in the Logtalk directory. Be sure to read the Logtalk/INSTALL file for additional instructions on how to customize your Logtalk installation to match your working environment.

For the latest Llogtalk news, please see the URL http://www.logtalk.org/.

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14. Threads

YAP implements a SWI-Prolog compatible multithreading library. Like in SWI-Prolog, Prolog threads have their own stacks and only share the Prolog heap: predicates, records, flags and other global non-backtrackable data. The package is based on the POSIX thread standard (Butenhof:1997:PPT) used on most popular systems except for MS-Windows.

Subnodes of Threads 14.1 Creating and Destroying Prolog Threads 

14.2 Monitoring Threads
14.3 Thread communication
14.4 Thread Synchronisation
Subnodes of Thread Communication
14.3.1 Message Queues
14.3.2 Signalling Threads
14.3.3 Threads and Dynamic Predicates

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14.1 Creating and Destroying Prolog Threads

thread_create(:Goal, -Id, +Options)

Create a new Prolog thread (and underlying C-thread) and start it by executing Goal. If the thread is created succesfully, the thread-identifier of the created thread is unified to Id. Options is a list of options. Currently defined options are:

stack

Set the limit in K-Bytes to which the Prolog stacks of this thread may grow. If omited, the limit of the calling thread is used. See also the commandline -S option.

trail

Set the limit in K-Bytes to which the trail stack of this thread may grow. If omited, the limit of the calling thread is used. See also the commandline option -T.

alias

Associate an alias-name with the thread. This named may be used to refer to the thread and remains valid until the thread is joined (see thread_join/2).

detached

If false (default), the thread can be waited for using thread_join/2. thread_join/2 must be called on this thread to reclaim the all resources associated to the thread. If true, the system will reclaim all associated resources automatically after the thread finishes. Please note that thread identifiers are freed for reuse after a detached thread finishes or a normal thread has been joined. See also thread_join/2 and thread_detach/1.

The Goal argument is copied to the new Prolog engine. This implies further instantiation of this term in either thread does not have consequences for the other thread: Prolog threads do not share data from their stacks.

thread_self(-Id)

Get the Prolog thread identifier of the running thread. If the thread has an alias, the alias-name is returned.

thread_join(+Id, -Status)

Wait for the termination of thread with given Id. Then unify the result-status of the thread with Status. After this call, Id becomes invalid and all resources associated with the thread are reclaimed. Note that threads with the attribute detached true cannot be joined. See also current_thread/2.

A thread that has been completed without thread_join/2 being called on it is partly reclaimed: the Prolog stacks are released and the C-thread is destroyed. A small data-structure representing the exit-status of the thread is retained until thread_join/2 is called on the thread. Defined values for Status are:

true

The goal has been proven successfully.

false

The goal has failed.

exception(Term)

The thread is terminated on an exception. See print_message/2 to turn system exceptions into readable messages.

exited(Term)

The thread is terminated on thread_exit/1 using the argument Term.

thread_detach(+Id)

Switch thread into detached-state (see detached option at thread_create/3 at runtime. Id is the identifier of the thread placed in detached state.

One of the possible applications is to simplify debugging. Threads that are created as detached leave no traces if they crash. For not-detached threads the status can be inspected using current_thread/2. Threads nobody is waiting for may be created normally and detach themselves just before completion. This way they leave no traces on normal completion and their reason for failure can be inspected.

thread_exit(+Term)

Terminates the thread immediately, leaving exited(Term) as result-state for thread_join/2. If the thread has the attribute detached true it terminates, but its exit status cannot be retrieved using thread_join/2 making the value of Term irrelevant. The Prolog stacks and C-thread are reclaimed.

thread_at_exit(:Term)

Run Goal just before releasing the thread resources. This is to be compared to at_halt/1, but only for the current thread. These hooks are ran regardless of why the execution of the thread has been completed. As these hooks are run, the return-code is already available through current_thread/2 using the result of thread_self/1 as thread-identifier.

thread_setconcurrency(+Old, -New)

Determine the concurrency of the process, which is defined as the maximum number of concurrently active threads. `Active' here means they are using CPU time. This option is provided if the thread-implementation provides pthread_setconcurrency(). Solaris is a typical example of this family. On other systems this predicate unifies Old to 0 (zero) and succeeds silently.

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14.2 Monitoring Threads

Normal multi-threaded applications should not need these the predicates from this section because almost any usage of these predicates is unsafe. For example checking the existence of a thread before signalling it is of no use as it may vanish between the two calls. Catching exceptions using catch/3 is the only safe way to deal with thread-existence errors.

These predicates are provided for diagnosis and monitoring tasks.

current_thread(+Id, -Status)

Enumerates identifiers and status of all currently known threads. Calling current_thread/2 does not influence any thread. See also thread_join/2. For threads that have an alias-name, this name is returned in Id instead of the numerical thread identifier. Status is one of:

running

The thread is running. This is the initial status of a thread. Please note that threads waiting for something are considered running too.

false

The Goal of the thread has been completed and failed.

true

The Goal of the thread has been completed and succeeded.

exited(Term)

The Goal of the thread has been terminated using thread_exit/1 with Term as argument. If the underlying native thread has exited (using pthread_exit()) Term is unbound.

exception(Term)

The Goal of the thread has been terminated due to an uncaught exception (see throw/1 and catch/3).

thread_statistics(+Id, +Key, -Value)

Obtains statistical information on thread Id as statistics/2 does in single-threaded applications. This call returns all keys of statistics/2, although only information statistics about the stacks and CPU time yield different values for each thread.

mutex_statistics

Print usage statistics on internal mutexes and mutexes associated with dynamic predicates. For each mutex two numbers are printed: the number of times the mutex was acquired and the number of collisions: the number times the calling thread has to wait for the mutex. The collistion-count is not available on Windows as this would break portability to Windows-95/98/ME or significantly harm performance. Generally collision count is close to zero on single-CPU hardware.

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14.3 Thread communication

Subnodes of Thread Communication 14.3.1 Message Queues 

14.3.2 Signalling Threads
14.3.3 Threads and Dynamic Predicates

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14.3.1 Message Queues

Prolog threads can exchange data using dynamic predicates, database records, and other globally shared data. These provide no suitable means to wait for data or a condition as they can only be checked in an expensive polling loop. Message queues provide a means for threads to wait for data or conditions without using the CPU.

Each thread has a message-queue attached to it that is identified by the thread. Additional queues are created using message_queue_create/2.

thread_send_message(+QueueOrThreadId, +Term)

Place Term in the given queue or default queue of the indicated thread (which can even be the message queue of itself (see thread_self/1). Any term can be placed in a message queue, but note that the term is copied to the receiving thread and variable-bindings are thus lost. This call returns immediately.

If more than one thread is waiting for messages on the given queue and at least one of these is waiting with a partially instantiated Term, the waiting threads are all sent a wakeup signal, starting a rush for the available messages in the queue. This behaviour can seriously harm performance with many threads waiting on the same queue as all-but-the-winner perform a useless scan of the queue. If there is only one waiting thread or all waiting threads wait with an unbound variable an arbitrary thread is restarted to scan the queue.%

thread_get_message(?Term)

Examines the thread message-queue and if necessary blocks execution until a term that unifies to Term arrives in the queue. After a term from the queue has been unified unified to Term, the term is deleted from the queue and this predicate returns.

Please note that not-unifying messages remain in the queue. After the following has been executed, thread 1 has the term gnu in its queue and continues execution using A is gnat.

<thread 1> thread_get_message(a(A)), <thread 2> thread_send_message(b(gnu)), thread_send_message(a(gnat)),

See also thread_peek_message/1.

thread_peek_message(?Term)

Examines the thread message-queue and compares the queued terms with Term until one unifies or the end of the queue has been reached. In the first case the call succeeds (possibly instantiating Term. If no term from the queue unifies this call fails.

thread_message_queue_create(?Queue)

If Queue is an atom, create a named queue. To avoid ambiguity on thread_send_message/2, the name of a queue may not be in use as a thread-name. If Queue is unbound an anonymous queue is created and Queue is unified to its identifier.

thread_message_queue_destroy(+Queue)

Destroy a message queue created with message_queue_create/1. It is not allows to destroy the queue of a thread. Neither is it allowed to destroy a queue other threads are waiting for or, for anynymous message queues, may try to wait for later.%

thread_get_message(+Queue, +Term)

As thread_get_message/1, operating on a given queue. It is allowed to peek into another thread's message queue, an operation that can be used to check whether a thread has swallowed a message sent to it.

Explicit message queues are designed with the worker-pool model in mind, where multiple threads wait on a single queue and pick up the first goal to execute. Below is a simple implementation where the workers execute arbitrary Prolog goals. Note that this example provides no means to tell when all work is done. This must be realised using additional synchronisation.

% create_workers(+Id, +N) % % Create a pool with given Id and number of workers. create_workers(Id, N) :- message_queue_create(Id), forall(between(1, N, _), thread_create(do_work(Id), _, [])). do_work(Id) :- repeat, thread_get_message(Id, Goal), ( catch(Goal, E, print_message(error, E)) -> true ; print_message(error, goal_failed(Goal, worker(Id))) ), fail. % work(+Id, +Goal) % % Post work to be done by the pool work(Id, Goal) :- thread_send_message(Id, Goal).

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14.3.2 Signalling Threads

These predicates provide a mechanism to make another thread execute some goal as an interrupt. Signalling threads is safe as these interrupts are only checked at safe points in the virtual machine. Nevertheless, signalling in multi-threaded environments should be handled with care as the receiving thread may hold a mutex (see with_mutex). Signalling probably only makes sense to start debugging threads and to cancel no-longer-needed threads with throw/1, where the receiving thread should be designed carefully do handle exceptions at any point.

thread_signal(+ThreadId, :Goal)

Make thread ThreadId execute Goal at the first opportunity. In the current implementation, this implies at the first pass through the Call-port. The predicate thread_signal/2 itself places Goal into the signalled-thread's signal queue and returns immediately.

Signals (interrupts) do not cooperate well with the world of multi-threading, mainly because the status of mutexes cannot be guaranteed easily. At the call-port, the Prolog virtual machine holds no locks and therefore the asynchronous execution is safe.

Goal can be any valid Prolog goal, including throw/1 to make the receiving thread generate an exception and trace/0 to start tracing the receiving thread.

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14.3.3 Threads and Dynamic Predicates

Besides queues threads can share and exchange data using dynamic predicates. The multi-threaded version knows about two types of dynamic predicates. By default, a predicate declared dynamic (see dynamic/1) is shared by all threads. Each thread may assert, retract and run the dynamic predicate. Synchronisation inside Prolog guarantees the consistency of the predicate. Updates are logical: visible clauses are not affected by assert/retract after a query started on the predicate. In many cases primitive from thread synchronysation should be used to ensure application invariants on the predicate are maintained.

Besides shared predicates, dynamic predicates can be declared with the thread_local/1 directive. Such predicates share their attributes, but the clause-list is different in each thread.

thread_local(+Functor/Arity)

related to the dynamic/1 directive. It tells the system that the predicate may be modified using assert/1, retract/1, etc, during execution of the program. Unlike normal shared dynamic data however each thread has its own clause-list for the predicate. As a thread starts, this clause list is empty. If there are still clauses as the thread terminates these are automatically reclaimed by the system. The thread_local property implies the property dynamic.

Thread-local dynamic predicates are intended for maintaining thread-specific state or intermediate results of a computation.

It is not recommended to put clauses for a thread-local predicate into a file as in the example below as the clause is only visible from the thread that loaded the source-file. All other threads start with an empty clause-list.

:- thread_local foo/1. foo(gnat).

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14.4 Thread Synchronisation

All internal Prolog operations are thread-safe. This implies two Prolog threads can operate on the same dynamic predicate without corrupting the consistency of the predicate. This section deals with user-level mutexes (called monitors in ADA or critical-sections by Microsoft). A mutex is a MUTual EXclusive device, which implies at most one thread can hold a mutex.

Mutexes are used to realise related updates to the Prolog database. With `related', we refer to the situation where a `transaction' implies two or more changes to the Prolog database. For example, we have a predicate address/2, representing the address of a person and we want to change the address by retracting the old and asserting the new address. Between these two operations the database is invalid: this person has either no address or two addresses, depending on the assert/retract order.

Here is how to realise a correct update:

:- initialization mutex_create(addressbook). change_address(Id, Address) :- mutex_lock(addressbook), retractall(address(Id, _)), asserta(address(Id, Address)), mutex_unlock(addressbook).

mutex_create(?MutexId)

Create a mutex. if MutexId is an atom, a named mutex is created. If it is a variable, an anonymous mutex reference is returned. There is no limit to the number of mutexes that can be created.

mutex_destroy(+MutexId)

Destroy a mutex. After this call, MutexId becomes invalid and further references yield an existence_error exception.

with_mutex(+MutexId, :Goal)

Execute Goal while holding MutexId. If Goal leaves choicepoints, these are destroyed (as in once/1). The mutex is unlocked regardless of whether Goal succeeds, fails or raises an exception. An exception thrown by Goal is re-thrown after the mutex has been successfully unlocked. See also mutex_create/2.

Although described in the thread-section, this predicate is also available in the single-threaded version, where it behaves simply as once/1.

mutex_lock(+MutexId)

Lock the mutex. Prolog mutexes are recursive mutexes: they can be locked multiple times by the same thread. Only after unlocking it as many times as it is locked, the mutex becomes available for locking by other threads. If another thread has locked the mutex the calling thread is suspended until to mutex is unlocked.

If MutexId is an atom, and there is no current mutex with that name, the mutex is created automatically using mutex_create/1. This implies named mutexes need not be declared explicitly.

Please note that locking and unlocking mutexes should be paired carefully. Especially make sure to unlock mutexes even if the protected code fails or raises an exception. For most common cases use with_mutex/2, wich provides a safer way for handling prolog-level mutexes.

mutex_trylock(+MutexId)

As mutex_lock/1, but if the mutex is held by another thread, this predicates fails immediately.

mutex_unlock(+MutexId)

Unlock the mutex. This can only be called if the mutex is held by the calling thread. If this is not the case, a permission_error exception is raised.

mutex_unlock_all

Unlock all mutexes held by the current thread. This call is especially useful to handle thread-termination using abort/0 or exceptions. See also thread_signal/2.

current_mutex(?MutexId, ?ThreadId, ?Count)

Enumerates all existing mutexes. If the mutex is held by some thread, ThreadId is unified with the identifier of te holding thread and Count with the recursive count of the mutex. Otherwise, ThreadId is [] and Count is 0.

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15. Parallelism

There has been a sizeable amount of work on an or-parallel implementation for YAP, called YapOr. Most of this work has been performed by Ricardo Rocha. In this system parallelism is exploited implicitly by running several alternatives in or-parallel. This option can be enabled from the configure script or by checking the system's Makefile.

YapOr is still a very experimental system, going through rapid development. The following restrictions are of note:

• YapOr currently only supports the Linux/X86 and SPARC/Solaris platforms. Porting to other Unix-like platforms should be straightforward.
• YapOr does not support parallel updates to the data-base.
• YapOr does not support opening or closing of streams during parallel execution.
• Garbage collection and stack shifting are not supported in YapOr.
• Built-ins that cause side-effects can only be executed when left-most in the search-tree. There are no primitives to provide asynchronous or cavalier execution of these built-ins, as in Aurora or Muse.
• YAP does not support voluntary suspension of work.

We expect that some of these restrictions will be removed in future releases.

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16. Tabling

An initial cut for an implementation of tabling in the style of XSB-Prolog is now available. Tabling was implemented by Ricardo Rocha. To experiment with tabling use -DTABLING to YAP_EXTRAS in the system's Makefile.

You can use the directive table to force calls for the argument predicate to be tabled. Tabling information is stored in a trie, as for XSB-Prolog.

The following predicates may be useful to control tabled execution:

is_tabled(+PredIndicator)

Succeeds if the predicate PredIndicator, of the form Name/Arity, is a tabled predicate.

tabling_mode(+PredIndicator,+Options)

Sets tabling mode options for the list or predicate given by PredIndicator. The list of Options includes:

batched: use batched scheduling for this predicate (default).

local: use local scheduling for this predicate.

exec_answers: use complete tries as code (default).

load_answers: use complete tries as a consumer, somewhat less

efficient but creates less choice-points.

abolish_table(+PredIndicator)

Remove tables for PredIndicator

show_table(+PredIndicator)

Print out the contents of the table generated for PredIndicator.

table_statistics(+PredIndicator)

Print out some information on the current tables for PredIndicator.

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17. Tracing at Low Level

It is possible to follow the flow at abstract machine level if YAP is compiled with the flag LOW_LEVEL_TRACER. Note that this option is of most interest to implementers, as it quickly generates an huge amount of information.

Low level tracing can be toggled from an interrupt handler by using the option T. There are also two built-ins that activate and deactivate low level tracing:

start_low_level_trace

Begin display of messages at procedure entry and retry.

stop_low_level_trace

Stop display of messages at procedure entry and retry.

Note that this compile-time option will slow down execution.

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18. Profiling the Abstract Machine

Implementors may be interested in detecting on which abstract machine instructions are executed by a program. The ANALYST flag can give WAM level information. Note that this option slows down execution very substantially, and is only of interest to developers of the system internals, or to system debuggers.

reset_op_counters

Reinitialize all counters.

show_op_counters(+A)

Display the current value for the counters, using label A. The label must be an atom.

show_ops_by_group(+A)

Display the current value for the counters, organized by groups, using label A. The label must be an atom.

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19. Debugging

19.1 Debugging Predicates
19.2 Interacting with the debugger

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19.1 Debugging Predicates

The following predicates are available to control the debugging of programs:

debug

Switches the debugger on.

debugging

Outputs status information about the debugger which includes the leash mode and the existing spy-points, when the debugger is on.

nodebug

Switches the debugger off.

spy +P

Sets spy-points on all the predicates represented by P. P can either be a single specification or a list of specifications. Each one must be of the form Name/Arity or Name. In the last case all predicates with the name Name will be spied. As in C-Prolog, system predicates and predicates written in C, cannot be spied.

nospy +P

Removes spy-points from all predicates specified by P. The possible forms for P are the same as in spy P.

nospyall

Removes all existing spy-points.

notrace

Switches off the debugger and stops tracing.

leash(+M)

Sets leashing mode to M. The mode can be specified as:

full

prompt on Call, Exit, Redo and Fail

tight

prompt on Call, Redo and Fail

half

prompt on Call and Redo

loose

prompt on Call

off

never prompt

none

never prompt, same as off

The initial leashing mode is full.

The user may also specify directly the debugger ports where he wants to be prompted. If the argument for leash is a number N, each of lower four bits of the number is used to control prompting at one the ports of the box model. The debugger will prompt according to the following conditions:

• if N/\ 1 =\= 0 prompt on fail
• if N/\ 2 =\= 0 prompt on redo
• if N/\ 4 =\= 0 prompt on exit
• if N/\ 8 =\= 0 prompt on call

Therefore, leash(15) is equivalent to leash(full) and leash(0) is equivalent to leash(off).

Another way of using leash is to give it a list with the names of the ports where the debugger should stop. For example, leash([call,exit,redo,fail]) is the same as leash(full) or leash(15) and leash([fail]) might be used instead of leash(1).

spy_write(+Stream,Term)

If defined by the user, this predicate will be used to print goals by the debugger instead of write/2.

trace

Switches on the debugger and starts tracing.

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19.2 Interacting with the debugger

Debugging with YAP is similar to debugging with C-Prolog. Both systems include a procedural debugger, based in the four port model. In this model, execution is seen at the procedure level: each activation of a procedure is seen as a box with control flowing into and out of that box.

In the four port model control is caught at four key points: before entering the procedure, after exiting the procedure (meaning successful evaluation of all queries activated by the procedure), after backtracking but before trying new alternative to the procedure and after failing the procedure. Each one of these points is named a port:

*--------------------------------------* Call | | Exit ---------> + descendant(X,Y) :- offspring(X,Y). + ---------> | | | descendant(X,Z) :- | <--------- + offspring(X,Y), descendant(Y,Z). + <--------- Fail | | Redo *--------------------------------------*

Call

The call port is activated before initial invocation of procedure. Afterwards, execution will try to match the goal with the head of existing clauses for the procedure.

Exit

This port is activated if the procedure succeeds. Control will now leave the procedure and return to its ancestor.

Redo

if the goal, or goals, activated after the call port fail then backtracking will eventually return control to this procedure through the redo port.

Fail

If all clauses for this predicate fail, then the invocation fails, and control will try to redo the ancestor of this invocation.

To start debugging, the user will usually spy the relevant procedures, entering debug mode, and start execution of the program. When finding the first spy-point, YAP's debugger will take control and show a message like:

* (1) call: quicksort([1,2,3],_38) ?

The debugger message will be shown while creeping, or at spy-points, and it includes four or five fields:

• The first three characters are used to point out special states of the debugger. If the port is exit and the first character is '?', the current call still has alternatives to be tried. If the second character is a *, execution is at a spy-point. If the third character is a >, execution has returned either from a skip, a fail or a redo command.
• The second field is the activation number, and uniquely identifies the activation. The number will start from 1 and will be incremented for each activation found by the debugger.
• In the third field, the debugger shows the active port.
• The fourth field is the goal. The goal is written by write/1.

If the active port is leashed, the debugger will prompt the user with a ?, and wait for a command. A debugger command is just a character, followed by a return. By default, only the call and redo entries are leashed, but the leash/1 predicate can be used in order to make the debugger stop where needed.

There are several commands available, but the user only needs to remember the help command, which is h. This command shows all the available options, which are:

c - creep

this command makes YAP continue execution and stop at the next leashed port.

return - creep

the same as c

l - leap

YAP will continue execution until a port of a spied predicate is found;

k - quasi-leap

similar to leap but faster since the computation history is not kept; useful when leap becomes too slow.

s - skip

YAP will continue execution without showing any messages until returning to the current activation. Spy-points will be ignored in this mode. This command is meaningless, and therefore illegal, in the fail and exit ports.

t - fast-skip

similar to skip but faster since the computation history is not kept; useful when skip becomes too slow.

q - quasi-leap

YAP will continue execution until a port of a spied predicate is found or until returning to the current activation.

f - fail

forces YAP to fail the goal proceeding directly to the fail port. The command is not available in the fail port.

r - retry

after this command, YAP will retry the present goal, and so go back to the call port. Note that any side effects of the goal will not be undone. This command is not available at the call port.

a - abort

execution will be aborted, and the interpreter will return to the top-level.

n - nodebug

stop debugging but continue execution. The command will clear all active spy-points, leave debugging mode and continue execution.

e - exit

leave YAP.

h - help

show the debugger commands.

! Query

execute a query. YAP will not show the result of the query.

b - break

break active execution and launch a break level. This is the same as ! break.

+ - spy this goal

start spying the active goal. The same as ! spy G where G is the active goal.

- - nospy this goal

stop spying the active goal. The same as ! nospy G where G is the active goal.

p - print

shows the active goal using print/1

d - display

shows the active goal using display/1

<Depth - debugger write depth

sets the maximum write depth, both for composite terms and lists, that will be used by the debugger. For more information about write_depth/2 (see section Controlling Input/Output).

< - full term

resets to the default of ten the debugger's maximum write depth. For more information about write_depth/2 (see section Controlling Input/Output).

A - alternatives

show the list of backtrack points in the current execution.

The debugging information, when fast-skip quasi-leap is used, will be lost.

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20. Indexing

The indexation mechanism restricts the set of clauses to be tried in a procedure by using information about the status of a selected argument of the goal (in YAP, as in most compilers, the first argument). This argument is then used as a key, selecting a restricted set of a clauses from all the clauses forming the procedure.

As an example, the two clauses for concatenate:

concatenate([],L,L). concatenate([H|T],A,[H|NT]) :- concatenate(T,A,NT).

If the first argument for the goal is a list, then only the second clause is of interest. If the first argument is the nil atom, the system needs to look only for the first clause. The indexation generates instructions that test the value of the first argument, and then proceed to a selected clause, or group of clauses.

Note that if the first argument was a free variable, then both clauses should be tried. In general, indexation will not be useful if the first argument is a free variable.

When activating a predicate, a Prolog system needs to store state information. This information, stored in a structure known as choice point or fail point, is necessary when backtracking to other clauses for the predicate. The operations of creating and using a choice point are very expensive, both in the terms of space used and time spent. Creating a choice point is not necessary if there is only a clause for the predicate as there are no clauses to backtrack to. With indexation, this situation is extended: in the example, if the first argument was the atom nil, then only one clause would really be of interest, and it is pointless to create a choice point. This feature is even more useful if the first argument is a list: without indexation, execution would try the first clause, creating a choice point. The clause would fail, the choice point would then be used to restore the previous state of the computation and the second clause would be tried. The code generated by the indexation mechanism would behave much more efficiently: it would test the first argument and see whether it is a list, and then proceed directly to the second clause.

An important side effect concerns the use of "cut". In the above example, some programmers would use a "cut" in the first clause just to inform the system that the predicate is not backtrackable and force the removal the choice point just created. As a result, less space is needed but with a great loss in expressive power: the "cut" would prevent some uses of the procedure, like generating lists through backtracking. Of course, with indexation the "cut" becomes useless: the choice point is not even created.

Indexation is also very important for predicates with a large number of clauses that are used like tables:

logician(aristhoteles,greek). logician(frege,german). logician(russel,english). logician(godel,german). logician(whitehead,english).

An interpreter like C-Prolog, trying to answer the query:

?- logician(godel,X).

would blindly follow the standard Prolog strategy, trying first the first clause, then the second, the third and finally finding the relevant clause. Also, as there are some more clauses after the important one, a choice point has to be created, even if we know the next clauses will certainly fail. A "cut" would be needed to prevent some possible uses for the procedure, like generating all logicians. In this situation, the indexing mechanism generates instructions that implement a search table. In this table, the value of the first argument would be used as a key for fast search of possibly matching clauses. For the query of the last example, the result of the search would be just the fourth clause, and again there would be no need for a choice point.

If the first argument is a complex term, indexation will select clauses just by testing its main functor. However, there is an important exception: if the first argument of a clause is a list, the algorithm also uses the list's head if not a variable. For instance, with the following clauses,

rules([],B,B). rules([n(N)|T],I,O) :- rules_for_noun(N,I,N), rules(T,N,O). rules([v(V)|T],I,O) :- rules_for_verb(V,I,N), rules(T,N,O). rules([q(Q)|T],I,O) :- rules_for_qualifier(Q,I,N), rules(T,N,O).

if the first argument of the goal is a list, its head will be tested, and only the clauses matching it will be tried during execution.

Some advice on how to take a good advantage of this mechanism:

• Try to make the first argument an input argument.
• Try to keep together all clauses whose first argument is not a variable, that will decrease the number of tests since the other clauses are always tried.
• Try to avoid predicates having a lot of clauses with the same key. For instance, the procedure:

type(n(mary),person). type(n(john), person). type(n(chair),object). type(v(eat),active). type(v(rest),passive).

becomes more efficient with:

type(n(N),T) :- type_of_noun(N,T). type(v(V),T) :- type_of_verb(V,T). type_of_noun(mary,person). type_of_noun(john,person). type_of_noun(chair,object). type_of_verb(eat,active). type_of_verb(rest,passive).

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21. C Language interface to YAP

YAP provides the user with the necessary facilities for writing predicates in a language other than prolog. Since, under Unix systems, most language implementations are link-able to C, we will describe here only the YAP interface to the C language.

Before describing in full detail how to interface to C code, we will examine a brief example.

Assume the user requires a predicate my_process_id(Id) which succeeds when Id unifies with the number of the process under which YAP is running.

In this case we will create a my_process.c file containing the C-code described below.

#include "Yap/YapInterface.h" static int my_process_id(void) { YAP_Term pid = YAP_MkIntTerm(getpid()); YAP_Term out = YAP_ARG1; return(YAP_Unify(out,pid)); } void init_my_predicates() { YAP_UserCPredicate("my_process_id",my_process_id,1); }

The commands to compile the above file depend on the operating system. Under Linux (i386 and Alpha) you should use:

gcc -c -shared -fPIC my_process.c ld -shared -o my_process.so my_process.o

Under Solaris2 it is sufficient to use:

gcc -fPIC -c my_process.c

Under SunOS it is sufficient to use:

gcc -c my_process.c

Under Digital Unix you need to create a so file. Use:

gcc tst.c -c -fpic ld my_process.o -o my_process.so -shared -expect_unresolved '*'

and replace my process.so for my process.o in the remainder of the example. And could be loaded, under YAP, by executing the following prolog goal

load_foreign_files(['my_process'],[],init_my_predicates).

Note that since Yap4.3.3 you should not give the suffix for object files. YAP will deduce the correct suffix from the operating system it is running under.

Yap4.3.3 now supports loading WIN/NT DLLs. Currently you must compile YAP under cygwin to create a library yap.dll first. You can then use this dll to create your own dlls. Have a look at the code in library/regex to see how to create a dll under the cygwin/mingw32 environment.

After loading that file the following prolog goal

my_process_id(N)

would unify N with the number of the process under which Yap is running.

Having presented a full example, we will now examine in more detail the contents of the C source code file presented above.

The include statement is used to make available to the C source code the macros for the handling of prolog terms and also some Yap public definitions.

The function my_process_id is the implementation, in C, of the desired predicate. Note that it returns an integer denoting the success of failure of the goal and also that it has no arguments even though the predicate being defined has one. In fact the arguments of a prolog predicate written in C are accessed through macros, defined in the include file, with names YAP_ARG1, YAP_ARG2, ..., YAP_ARG16 or with YAP_A(N) where N is the argument number (starting with 1). In the present case the function uses just one local variable of type YAP_Term, the type used for holding Yap terms, where the integer returned by the standard unix function getpid() is stored as an integer term (the conversion is done by YAP_MkIntTerm(Int)). Then it calls the pre-defined routine YAP_Unify(YAP_Term, YAP_Term) which in turn returns an integer denoting success or failure of the unification.

The role of the procedure init_my_predicates is to make known to YAP, by calling YAP_UserCPredicate, the predicates being defined in the file. This is in fact why, in the example above, init_my_predicates was passed as the third argument to load_foreign_files.

The rest of this appendix describes exhaustively how to interface C to YAP.

21.1 Terms		Primitives available to the C programmer
21.2 Unification		How to Unify Two Prolog Terms
21.3 Strings		From character arrays to Lists of codes and back
21.4 Memory Allocation		Stealing Memory From Yap
21.5 Controlling Yap Streams from C		Control How Yap sees Streams
21.6 From C back to Prolog		From C to Yap to C to Yap
21.7 Writing predicates in C		Writing Predicates in C
21.8 Loading Object Files
21.9 Saving and Restoring
21.10 Changes to the C-Interface in Yap4		Changes in Foreign Predicates Interface

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21.1 Terms

This section provides information about the primitives available to the C programmer for manipulating prolog terms.

Several C typedefs are included in the header file yap/YapInterface.h to describe, in a portable way, the C representation of prolog terms. The user should write is programs using this macros to ensure portability of code across different versions of YAP.

The more important typedef is YAP_Term which is used to denote the type of a prolog term.

Terms, from a point of view of the C-programmer, can be classified as follows

uninstantiated variables

instantiated variables

integers

floating-point numbers

database references

atoms

pairs (lists)

compound terms

The primitive

YAP_Bool YAP_IsVarTerm(YAP_Term t)

returns true iff its argument is an uninstantiated variable. Conversely the primitive

YAP_Bool YAP_NonVarTerm(YAP_Term t)

returns true iff its argument is not a variable.

The user can create a new uninstantiated variable using the primitive

YAP_Term YAP_MkVarTerm()

The following primitives can be used to discriminate among the different types of non-variable terms:

YAP_Bool YAP_IsIntTerm(YAP_Term t) YAP_Bool YAP_IsFloatTerm(YAP_Term t) YAP_Bool YAP_IsDbRefTerm(YAP_Term t) YAP_Bool YAP_IsAtomTerm(YAP_Term t) YAP_Bool YAP_IsPairTerm(YAP_Term t) YAP_Bool YAP_IsApplTerm(YAP_Term t)

Next, we mention the primitives that allow one to destruct and construct terms. All the above primitives ensure that their result is dereferenced, i.e. that it is not a pointer to another term.

The following primitives are provided for creating an integer term from an integer and to access the value of an integer term.

YAP_Term YAP_MkIntTerm(YAP_Int i) YAP_Int YAP_IntOfTerm(YAP_Term t)

where YAP_Int is a typedef for the C integer type appropriate for the machine or compiler in question (normally a long integer). The size of the allowed integers is implementation dependent but is always greater or equal to 24 bits: usually 32 bits on 32 bit machines, and 64 on 64 bit machines.

The two following primitives play a similar role for floating-point terms

YAP_Term YAP_MkFloatTerm(YAP_flt double) YAP_flt YAP_FloatOfTerm(YAP_Term t)

where flt is a typedef for the appropriate C floating point type, nowadays a double

The following primitives are provided for verifying whether a term is a big int, creating a term from a big integer and to access the value of a big int from a term.

YAP_Bool YAP_IsBigNumTerm(YAP_Term t) YAP_Term YAP_MkBigNumTerm(void *b) void *YAP_BigNumOfTerm(YAP_Term t, void *b)

YAP must support bignum for the configuration you are using (check the YAP configuration and setup). For now, Yap only supports the GNU GMP library, and void * will be a cast for mpz_t. Notice that YAP_BigNumOfTerm requires the number to be already initialised. As an example, we show how to print a bignum:

static int p_print_bignum(void) { mpz_t mz; if (!YAP_IsBigNumTerm(YAP_ARG1)) return FALSE; mpz_init(mz); YAP_BigNumOfTerm(YAP_ARG1, mz); gmp_printf("Shows up as %Zd\n", mz); mpz_clear(mz); return TRUE; }

Currently, no primitives are supplied to users for manipulating data base references.

A special typedef YAP_Atom is provided to describe prolog atoms (symbolic constants). The two following primitives can be used to manipulate atom terms

YAP_Term YAP_MkAtomTerm(YAP_Atom at) YAP_Atom YAP_AtomOfTerm(YAP_Term t)

The following primitives are available for associating atoms with their names

YAP_Atom YAP_LookupAtom(char * s) YAP_Atom YAP_FullLookupAtom(char * s) char *YAP_AtomName(YAP_Atom t)

The function YAP_LookupAtom looks up an atom in the standard hash table. The function YAP_FullLookupAtom will also search if the atom had been "hidden": this is useful for system maintenance from C code. The functor YAP_AtomName returns a pointer to the string for the atom.

A pair is a Prolog term which consists of a tuple of two prolog terms designated as the head and the tail of the term. Pairs are most often used to build lists. The following primitives can be used to manipulate pairs:

YAP_Term YAP_MkPairTerm(YAP_Term Head, YAP_Term Tail) YAP_Term YAP_MkNewPairTerm(void) YAP_Term YAP_HeadOfTerm(YAP_Term t) YAP_Term YAP_TailOfTerm(YAP_Term t)

One can construct a new pair from two terms, or one can just build a pair whose head and tail are new unbound variables. Finally, one can fetch the head or the tail.

A compound term consists of a functor and a sequence of terms with length equal to the arity of the functor. A functor, described in C by the typedef Functor, consists of an atom and of an integer. The following primitives were designed to manipulate compound terms and functors

YAP_Term YAP_MkApplTerm(YAP_Functor f, unsigned long int n, YAP_Term[] args) YAP_Term YAP_MkNewApplTerm(YAP_Functor f, int n) YAP_Term YAP_ArgOfTerm(int argno,YAP_Term ts) YAP_Functor YAP_FunctorOfTerm(YAP_Term ts)

The YAP_MkApplTerm function constructs a new term, with functor f (of arity n), and using an array args of n terms with n equal to the arity of the functor. YAP_MkNewApplTerm builds up a compound term whose arguments are unbound variables. YAP_ArgOfTerm gives an argument to a compound term. argno should be greater or equal to 1 and less or equal to the arity of the functor.

YAP allows one to manipulate the functors of compound term. The function YAP_FunctorOfTerm allows one to obtain a variable of type YAP_Functor with the functor to a term. The following functions then allow one to construct functors, and to obtain their name and arity.

YAP_Functor YAP_MkFunctor(YAP_Atom a,unsigned long int arity) YAP_Atom YAP_NameOfFunctor(YAP_Functor f) YAP_Int YAP_ArityOfFunctor(YAP_Functor f)

Note that the functor is essentially a pair formed by an atom, and arity.

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21.2 Unification

YAP provides a single routine to attempt the unification of two prolog terms. The routine may succeed or fail:

Int YAP_Unify(YAP_Term a, YAP_Term b)

The routine attempts to unify the terms a and b returning TRUE if the unification succeeds and FALSE otherwise.

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21.3 Strings

The YAP C-interface now includes an utility routine to copy a string represented as a list of a character codes to a previously allocated buffer

int YAP_StringToBuffer(YAP_Term String, char *buf, unsigned int bufsize)

The routine copies the list of character codes String to a previously allocated buffer buf. The string including a terminating null character must fit in bufsize characters, otherwise the routine will simply fail. The StringToBuffer routine fails and generates an exception if String is not a valid string.

The C-interface also includes utility routines to do the reverse, that is, to copy a from a buffer to a list of character codes or to a list of character atoms

YAP_Term YAP_BufferToString(char *buf) YAP_Term YAP_BufferToAtomList(char *buf)

The user-provided string must include a terminating null character.

The C-interface function calls the parser on a sequence of characters stored at buf and returns the resulting term.

YAP_Term YAP_ReadBuffer(char *buf,YAP_Term *error)

The user-provided string must include a terminating null character. Syntax errors will cause returning FALSE and binding error to a Prolog term.

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21.4 Memory Allocation

The next routine can be used to ask space from the Prolog data-base:

void *YAP_AllocSpaceFromYap(int size)

The routine returns a pointer to a buffer allocated from the code area, or NULL if sufficient space was not available.

The space allocated with YAP_AllocSpaceFromYap can be released back to Yap by using:

void YAP_FreeSpaceFromYap(void *buf)

The routine releases a buffer allocated from the code area. The system may crash if buf is not a valid pointer to a buffer in the code area.

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21.5 Controlling Yap Streams from C

The C-Interface also provides the C-application with a measure of control over the Yap Input/Output system. The first routine allows one to find a file number given a current stream:

int YAP_StreamToFileNo(YAP_Term stream)

This function gives the file descriptor for a currently available stream. Note that null streams and in memory streams do not have corresponding open streams, so the routine will return a negative. Moreover, Yap will not be aware of any direct operations on this stream, so information on, say, current stream position, may become stale.

A second routine that is sometimes useful is:

void YAP_CloseAllOpenStreams(void)

This routine closes the Yap Input/Output system except for the first three streams, that are always associated with the three standard Unix streams. It is most useful if you are doing fork().

The next routine allows a currently open file to become a stream. The routine receives as arguments a file descriptor, the true file name as a string, an atom with the user name, and a set of flags:

void YAP_OpenStream(void *FD, char *name, YAP_Term t, int flags)

The available flags are YAP_INPUT_STREAM, YAP_OUTPUT_STREAM, YAP_APPEND_STREAM, YAP_PIPE_STREAM, YAP_TTY_STREAM, YAP_POPEN_STREAM, YAP_BINARY_STREAM, and YAP_SEEKABLE_STREAM. By default, the stream is supposed to be at position 0. The argument name gives the name by which YAP should know the new stream.

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21.6 From C back to Prolog

Newer versions of YAP allow for calling the Prolog interpreter from C. One must first construct a goal G, and then it is sufficient to perform:

YAP_Bool YapCallProlog(YAP_Term G)

the result will be FALSE, if the goal failed, or TRUE, if the goal succeeded. In this case, the variables in G will store the values they have been unified with. Execution only proceeds until finding the first solution to the goal, but you can call findall/3 or friends if you need all the solutions.

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21.7 Writing predicates in C

We will distinguish two kinds of predicates:

deterministic predicates which either fail or succeed but are not

backtrackable, like the one in the introduction;

backtrackable

predicates which can succeed more than once.

The first kind of predicates should be implemented as a C function with no arguments which should return zero if the predicate fails and a non-zero value otherwise. The predicate should be declared to YAP, in the initialization routine, with a call to

void YAP_UserCPredicate(char *name, YAP_Bool *fn(), unsigned long int arity);

where name is the name of the predicate, fn is the C function implementing the predicate and arity is its arity.

For the second kind of predicates we need two C functions. The first one which is called when the predicate is first activated, and the second one to be called on backtracking to provide (possibly) other solutions. Note also that we normally also need to preserve some information to find out the next solution.

In fact the role of the two functions can be better understood from the following prolog definition

p :- start. p :- repeat, continue.

where start and continue correspond to the two C functions described above.

As an example we will consider implementing in C a predicate n100(N) which, when called with an instantiated argument should succeed if that argument is a numeral less or equal to 100, and, when called with an uninstantiated argument, should provide, by backtracking, all the positive integers less or equal to 100.

To do that we first declare a structure, which can only consist of prolog terms, containing the information to be preserved on backtracking and a pointer variable to a structure of that type.

#include "YapInterface.h" static int start_n100(void); static int continue_n100(void); typedef struct { YAP_Term next_solution; /* the next solution */ } n100_data_type; n100_data_type *n100_data;

We now write the C function to handle the first call:

static int start_n100(void) { YAP_Term t = YAP_ARG1; YAP_PRESERVE_DATA(n100_data,n100_data_type); if(YAP_IsVarTerm(t)) { n100_data->next_solution = YAP_MkIntTerm(0); return continue_n100(); } if(!YAP_IsIntTerm(t) || YAP_IntOfTerm(t)<0 || YAP_IntOfTerm(t)>100) { YAP_cut_fail(); } else { YAP_cut_succeed(); } }

The routine starts by getting the dereference value of the argument. The call to YAP_PRESERVE_DATA is used to initialize the memory which will hold the information to be preserved across backtracking. The first argument is the variable we shall use, and the second its type. Note that we can only use YAP_PRESERVE_DATA once, so often we will want the variable to be a structure.

If the argument of the predicate is a variable, the routine initializes the structure to be preserved across backtracking with the information required to provide the next solution, and exits by calling continue_n100 to provide that solution.

If the argument was not a variable, the routine then checks if it was an integer, and if so, if its value is positive and less than 100. In that case it exits, denoting success, with YAP_cut_succeed, or otherwise exits with YAP_cut_fail denoting failure.

The reason for using for using the functions YAP_cut_succeed and YAP_cut_fail instead of just returning a non-zero value in the first case, and zero in the second case, is that otherwise, if backtracking occurred later, the routine continue_n100 would be called to provide additional solutions.

The code required for the second function is

static int continue_n100(void) { int n; YAP_Term t; YAP_Term sol = YAP_ARG1; YAP_PRESERVED_DATA(n100_data,n100_data_type); n = YAP_IntOfTerm(n100_data->next_solution); if( n == 100) { t = YAP_MkIntTerm(n); YAP_Unify(sol,t); YAP_cut_succeed(); } else { YAP_Unify(sol,n100_data->next_solution); n100_data->next_solution = YAP_MkIntTerm(n+1); return(TRUE); } }

Note that again the macro YAP_PRESERVED_DATA is used at the beginning of the function to access the data preserved from the previous solution. Then it checks if the last solution was found and in that case exits with YAP_cut_succeed in order to cut any further backtracking. If this is not the last solution then we save the value for the next solution in the data structure and exit normally with 1 denoting success. Note also that in any of the two cases we use the function YAP_unify to bind the argument of the call to the value saved in n100_state->next_solution.

Note also that the only correct way to signal failure in a backtrackable predicate is to use the YAP_cut_fail macro.

Backtrackable predicates should be declared to YAP, in a way similar to what happened with deterministic ones, but using instead a call to

void YAP_UserBackCPredicate(char *name, int *init(), int *cont(), unsigned long int arity, unsigned int sizeof);

where name is a string with the name of the predicate, init and cont are the C functions used to start and continue the execution of the predicate, arity is the predicate arity, and sizeof is the size of the data to be preserved in the stack. In this example, we would have something like

void init_n100(void) { YAP_UserBackCPredicate("n100", start_n100, continue_n100, 1, 1); }

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21.8 Loading Object Files

The primitive predicate

load_foreign_files(Files,Libs,InitRoutine)

should be used, from inside YAP, to load object files produced by the C compiler. The argument ObjectFiles should be a list of atoms specifying the object files to load, Libs is a list (possibly empty) of libraries to be passed to the unix loader (ld) and InitRoutine is the name of the C routine (to be called after the files are loaded) to perform the necessary declarations to YAP of the predicates defined in the files.

YAP will search for ObjectFiles in the current directory first. If it cannot find them it will search for the files using the environment variable YAPLIBDIR, if defined, or in the default library.

In a.out systems YAP by default only reserves a fixed amount of memory for object code (64 Kbytes in the current version). Should this size prove inadequate the flag -c n can be passed to YAP (in the command line invoking YAP) to force the allocation of n Kbytes.

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21.9 Saving and Restoring

Yap4 currently does not support save and restore for object code loaded with load_foreign_files. We plan to support save and restore in future releases of Yap.

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21.10 Changes to the C-Interface in Yap4

Yap4 includes several changes over the previous load_foreign_files interface. These changes were required to support the new binary code formats, such as ELF used in Solaris2 and Linux.

• All Names of YAP objects now start with YAP_. This is designed to avoid clashes with other code. Use YapInterface.h to take advantage of the new interface. c_interface.h is still available if you cannot port the code to the new interface.
• Access to elements in the new interface always goes through functions. This includes access to the argument registers, YAP_ARG1 to YAP_ARG16. This change breaks code such as unify(&ARG1,&t), which is nowadays:

  { YAP_Unify(ARG1, t); }

• cut_fail() and cut_succeed() are now functions.
• The use of Deref is deprecated. All functions that return Prolog terms, including the ones that access arguments, already dereferenciate their arguments.
• Space allocated with PRESERVE_DATA is ignored by garbage collection and stack shifting. As a result, any pointers to a Prolog stack object, including some terms, may be corrupted after garbage collection or stack shifting. Prolog terms should instead be stored as arguments to the backtrackable procedure.

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22. Using YAP as a Library

YAP can be used as a library to be called from other programs. To do so, you must first create the YAP library:

make library make install_library

This will install a file libyap.a in LIBDIR and the Prolog headers in INCLUDEDIR. The library contains all the functionality available in YAP, except the foreign function loader and for Yap's startup routines.

To actually use this library you must follow a five step process:

1. You must initialize the YAP environment. A single function, YAP_FastInit asks for a contiguous chunk in your memory space, fills it in with the data-base, and sets up YAP's stacks and execution registers. You can use a saved space from a standard system by calling save_program/1.
2. You then have to prepare a query to give to YAP. A query is a Prolog term, and you just have to use the same functions that are available in the C-interface.
3. You can then use YAP_RunGoal(query) to actually evaluate your query. The argument is the query term query, and the result is 1 if the query succeeded, and 0 if it failed.
4. You can use the term destructor functions to check how arguments were instantiated.
5. If you want extra solutions, you can use YAP_RestartGoal() to obtain the next solution.

The next program shows how to use this system. We assume the saved program contains two facts for the procedure b:

#include <stdio.h> #include "Yap/YapInterface.h" int main(int argc, char *argv[]) { if (YAP_FastInit("saved_state") == YAP_BOOT_ERROR) exit(1); if (YAP_RunGoal(YAP_MkAtomTerm(YAP_LookupAtom("do")))) { printf("Success\n"); while (YAP_RestartGoal()) printf("Success\n"); } printf("NO\n"); }

The program first initializes YAP, calls the query for the first time and succeeds, and then backtracks twice. The first time backtracking succeeds, the second it fails and exits.

To compile this program it should be sufficient to do:

cc -o exem -I../Yap4.3.0 test.c -lYap -lreadline -lm

You may need to adjust the libraries and library paths depending on the Operating System and your installation of Yap.

Note that Yap4.3.0 provides the first version of the interface. The interface may change and improve in the future.

The following C-functions are available from Yap:

• YAP_CompileClause(YAP_Term Clause) Compile the Prolog term Clause and assert it as the last clause for the corresponding procedure.
• int YAP_ContinueGoal(void) Continue execution from the point where it stopped.
• void YAP_Error(int ID,YAP_Term Cause,char * error_description) Generate an YAP System Error with description given by the string error_description. ID is the error ID, if known, or 0. Cause is the term that caused the crash.
• void YAP_Exit(int exit_code) Exit YAP immediately. The argument exit_code gives the error code and is supposed to be 0 after successful execution in Unix and Unix-like systems.
• YAP_Term YAP_GetValue(Atom at) Return the term value associated with the atom at. If no such term exists the function will return the empty list.
• YAP_FastInit(char * SavedState) Initialize a copy of YAP from SavedState. The copy is monolithic and currently must be loaded at the same address where it was saved. YAP_FastInit is a simpler version of YAP_Init.
• YAP_Init(InitInfo) Initialize YAP. The arguments are in a C structure of type YAP_init_args.

  The fields of InitInfo are char * SavedState, int HeapSize, int StackSize, int TrailSize, int NumberofWorkers, int SchedulerLoop, int DelayedReleaseLoad, int argc, char ** argv, int ErrorNo, and char * ErrorCause. The function returns an integer, which indicates the current status. If the result is YAP_BOOT_ERROR booting failed.

  If SavedState is not NULL, try to open and restore the file SavedState. Initially YAP will search in the current directory. If the saved state does not exist in the current directory YAP will use either the default library directory or the directory given by the environment variable YAPLIBDIR. Note that currently the saved state must be loaded at the same address where it was saved.

  If HeapSize is different from 0 use HeapSize as the minimum size of the Heap (or code space). If StackSize is different from 0 use HeapSize as the minimum size for the Stacks. If TrailSize is different from 0 use TrailSize as the minimum size for the Trails.

  The NumberofWorkers, NumberofWorkers, and DelayedReleaseLoad are only of interest to the or-parallel system.

  The argument count argc and string of arguments argv arguments are to be passed to user programs as the arguments used to call YAP.

  If booting failed you may consult ErrorNo and ErrorCause for the cause of the error, or call YAP_Error(ErrorNo,0L,ErrorCause) to do default processing.

• void YAP_PutValue(Atom at, YAP_Term value) Associate the term value with the atom at. The term value must be a constant. This functionality is used by YAP as a simple way for controlling and communicating with the Prolog run-time.
• YAP_Term YAP_Read(int (*)(void) GetC) Parse a Term using the function GetC to input characters.
• YAP_Term YAP_RunGoal(YAP_Term Goal) Execute query Goal and return 1 if the query succeeds, and 0 otherwise. The predicate returns 0 if failure, otherwise it will return YAP_Term. Note that YAP_Term may change due to garbage collection, so you should use something like:

  t = YAP_RunGoal(t); if (t == 0) return FALSE;

  If the execution fails, garbage collection might still have changed the term, so you should not use the input argument again.

  An alternative is to use slots, as shown next:

  long sl = YAP_InitSlot(scoreTerm); out = YAP_RunGoal(t); t = YAP_GetFromSlot(sl); YAP_RecoverSlots(1); if (out == 0) return FALSE;

  Slots are safe houses in the stack, preserved by the garbage collector and the stack shifter. In this case, we use a slot to preserve t during the execution of YAP_RunGoal. When the execution of t is over we read the (possibly changed) value of t back from the slot sl and tell YAP that the slot sl is not needed and can be given back to the system.

• int YAP_RestartGoal(void) Look for the next solution to the current query by forcing YAP to backtrack.
• int YAP_Reset(void) Reset execution environment (similar to the abort/0 built-in). This is useful when you want to start a new query before asking all solutions to the previous query.
• YAP_Bool YAP_GoalHasException(YAP_Term *tp) Check if the last goal generated an exception, and if so copy it to the space pointed to by tp
• void YAP_ClearExceptions(void) Reset any exceptions left over by the system.
• void YAP_Write(YAP_Term t, void (*)(int) PutC, int flags) Write a Term t using the function PutC to output characters. The term is written according to a mask of the following flags in the flag argument: YAP_WRITE_QUOTED, YAP_WRITE_HANDLE_VARS, and YAP_WRITE_IGNORE_OPS.
• void YAP_WriteBuffer(YAP_Term t, char * buff, unsigned int size, int flags) Write a YAP_Term t to buffer buff with size size. The term is written according to a mask of the following flags in the flag argument: YAP_WRITE_QUOTED, YAP_WRITE_HANDLE_VARS, and YAP_WRITE_IGNORE_OPS.
• void YAP_InitConsult(int mode, char * filename) Enter consult mode on file filename. This mode maintains a few data-structures internally, for instance to know whether a predicate before or not. It is still possible to execute goals in consult mode.

  If mode is TRUE the file will be reconsulted, otherwise just consulted. In practice, this function is most useful for bootstrapping Prolog, as otherwise one may call the Prolog predicate compile/1 or consult/1 to do compilation.

  Note that it is up to the user to open the file filename. The YAP_InitConsult function only uses the file name for internal bookkeeping.

• void YAP_EndConsult(void) Finish consult mode.

Some observations:

• The system will core dump if you try to load the saved state in a different address from where it was made. This may be a problem if your program uses mmap. This problem will be addressed in future versions of YAP.
• Currently, the YAP library will pollute the name space for your program.
• The initial library includes the complete YAP system. In the future we plan to split this library into several smaller libraries (e.g. if you do not want to perform I/O).
• You can generate your own saved states. Look at the boot.yap and init.yap files.

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23. Compatibility with Other Prolog systems

YAP has been designed to be as compatible as possible with other Prolog systems, and initially with C-Prolog. More recent work on YAP has included features initially proposed for the Quintus and SICStus Prolog systems.

Developments since Yap4.1.6 we have striven at making YAP compatible with the ISO-Prolog standard.

23.1 Compatibility with the C-Prolog interpreter
23.2 Compatibility with the Quintus and SICStus Prolog systems		Compatibility with the SICStus Prolog system
23.3 Compatibility with the ISO Prolog standard

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23.1 Compatibility with the C-Prolog interpreter

C-Prolog Compatibility 23.1.1 Major Differences between YAP and C-Prolog.  Major Differences between YAP and C-Prolog 23.1.2 Yap predicates fully compatible with C-Prolog  Yap predicates fully compatible with C-Prolog 23.1.3 Yap predicates not strictly compatible with C-Prolog  Yap predicates not strictly as C-Prolog 23.1.4 Yap predicates not available in C-Prolog  Yap predicates not available in C-Prolog 23.1.5 Yap predicates not available in C-Prolog  C-Prolog predicates not available in YAP

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23.1.1 Major Differences between YAP and C-Prolog.

YAP includes several extensions over the original C-Prolog system. Even so, most C-Prolog programs should run under YAP without changes.

The most important difference between YAP and C-Prolog is that, being YAP a compiler, some changes should be made if predicates such as assert, clause and retract are used. First predicates which will change during execution should be declared as dynamic by using commands like:

:- dynamic f/n.

where f is the predicate name and n is the arity of the predicate. Note that several such predicates can be declared in a single command:

:- dynamic f/2, ..., g/1.

Primitive predicates such as retract apply only to dynamic predicates. Finally note that not all the C-Prolog primitive predicates are implemented in YAP. They can easily be detected using the unknown system predicate provided by YAP.

Last, by default YAP enables character escapes in strings. You can disable the special interpretation for the escape character by using:

:- yap_flag(character_escapes,off).

or by using:

:- yap_flag(language,cprolog).

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23.1.2 Yap predicates fully compatible with C-Prolog

These are the Prolog built-ins that are fully compatible in both C-Prolog and YAP:

Jump to:	!   ,   ;   <   =   >   @   [   \   A   B   C   D   E   F   G   H   I   K   L   N   O   P   R   S   T   V   W

	Index Entry	Section
!
	!/0	6.1 Control Predicates
,
	,/2	6.1 Control Predicates
;
	;/2	6.1 Control Predicates
<
	</2	6.5 Arithmetic
=
	=../2	6.3 Predicates on terms
	=:=/2	6.5 Arithmetic
	=</2	6.5 Arithmetic
	==/2	6.4 Comparing Terms
	=\=/2	6.5 Arithmetic
>
	>/2	6.5 Arithmetic
	>=/2	6.5 Arithmetic
@
	@</2	6.4 Comparing Terms
	@</2	6.4 Comparing Terms
	@>/2	6.4 Comparing Terms
	@>=/2	6.4 Comparing Terms
[
	[-]/1	4.1 Program loading and updating
	[]/1	4.1 Program loading and updating
\
	\+/1	6.1 Control Predicates
	\==/2	6.4 Comparing Terms
A
	abort/0	6.1 Control Predicates
	atom/1	6.3 Predicates on terms
	atomic/1	6.3 Predicates on terms
B
	bagof/3	6.10 Collecting Solutions to a Goal
	break/0	6.1 Control Predicates
C
	call/1	6.1 Control Predicates
	close/1	6.6.1 Handling Streams and Files
	compare/3	6.4 Comparing Terms
	consult/1	4.1 Program loading and updating
	current_atom/1	6.7.2 Looking at the Data Base
	current_predicate/1	6.7.2 Looking at the Data Base
D
	db_reference/1	6.3 Predicates on terms
	debug/0	19.1 Debugging Predicates
	debugging/0	19.1 Debugging Predicates
	display/1	6.6.3 Handling Input/Output of Terms
E
	erase/1	6.8 Internal Data Base
	erased/1	6.8 Internal Data Base
	exists/1	6.6.7 Controlling Input/Output
	expand_exprs/2	4.2 Changing the Compiler's Behavior
	expand_term/2	6.11 Grammar Rules
F
	fail/0	6.1 Control Predicates
	fileerrors/0	6.6.7 Controlling Input/Output
	findall/3	6.10 Collecting Solutions to a Goal
	functor/3	6.3 Predicates on terms
G
	get/1	6.6.4 Handling Input/Output of Characters
	get0/1	6.6.4 Handling Input/Output of Characters
H
	halt/0	6.1 Control Predicates
I
	instance/2	6.8 Internal Data Base
	integer/1	6.3 Predicates on terms
K
	keysort/2	6.4 Comparing Terms
L
	leash/1	19.1 Debugging Predicates
	length/2	6.4 Comparing Terms
N
	name/2	6.3 Predicates on terms
	nl/0	6.6.4 Handling Input/Output of Characters
	nodebug/0	19.1 Debugging Predicates
	nofileerrors/0	6.6.7 Controlling Input/Output
	nonvar/1	6.3 Predicates on terms
	nospy/1	19.1 Debugging Predicates
	not/1	6.1 Control Predicates
	number/1	6.3 Predicates on terms
O
	op/3	6.18 Miscellaneous
P
	primitive/1	6.3 Predicates on terms
	print/1	6.6.3 Handling Input/Output of Terms
	prompt/2	6.18 Miscellaneous
	put/1	6.6.4 Handling Input/Output of Characters
R
	read/1	6.6.3 Handling Input/Output of Terms
	reconsult/1	4.1 Program loading and updating
	recorda/3	6.8 Internal Data Base
	recorded/3	6.8 Internal Data Base
	recordz/3	6.8 Internal Data Base
	rename/2	6.12 Access to Operating System Functionality
	repeat/0	6.1 Control Predicates
S
	save/1	4.3 Saving and Loading Prolog States
	see/1	6.6.2 Handling Streams and Files
	seeing/1	6.6.2 Handling Streams and Files
	seen/0	6.6.2 Handling Streams and Files
	setof/3	6.10 Collecting Solutions to a Goal
	sh/0	6.12 Access to Operating System Functionality
	skip/1	6.6.4 Handling Input/Output of Characters
	sort/2	6.4 Comparing Terms
	spy/1	19.1 Debugging Predicates
	statistics/0	6.18 Miscellaneous
	system/1	6.12 Access to Operating System Functionality
T
	tab/1	6.6.4 Handling Input/Output of Characters
	tell/1	6.6.2 Handling Streams and Files
	telling/1	6.6.2 Handling Streams and Files
	term_expansion/2	6.11 Grammar Rules
	told/0	6.6.2 Handling Streams and Files
	true/0	6.1 Control Predicates
V
	var/1	6.3 Predicates on terms
W
	write/1	6.6.3 Handling Input/Output of Terms
	writeq/1	6.6.3 Handling Input/Output of Terms

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23.1.3 Yap predicates not strictly compatible with C-Prolog

These are YAP built-ins that are also available in C-Prolog, but that are not fully compatible:

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	Index Entry	Section
A
	abolish/1	6.7.1 Modification of the Data Base
	abolish/2	6.7.1 Modification of the Data Base
	assert/1	6.7.1 Modification of the Data Base
	assert/2	6.7.3 Using Data Base References
	asserta/1	6.7.1 Modification of the Data Base
	asserta/2	6.7.3 Using Data Base References
	assertz/1	6.7.1 Modification of the Data Base
	assertz/2	6.7.3 Using Data Base References
C
	clause/2	6.7.1 Modification of the Data Base
	clause/3	6.7.1 Modification of the Data Base
I
	is/2	6.5 Arithmetic
L
	listing/0	6.7.2 Looking at the Data Base
	listing/1	6.7.2 Looking at the Data Base
N
	nth_clause/3	6.7.1 Modification of the Data Base
R
	retract/2	6.7.3 Using Data Base References

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23.1.4 Yap predicates not available in C-Prolog

These are YAP built-ins not available in C-Prolog.

Jump to:	-   =   \   A   B   C   D   E   F   G   H   I   J   K   L   M   N   O   P   Q   R   S   T   U   V   W   Y

	Index Entry	Section
-
	->/2	6.1 Control Predicates
=
	=/2	6.3 Predicates on terms
\
	\=/2	6.3 Predicates on terms
A
	abolish_table/1	16. Tabling
	absolute_file_name/2	6.6.1 Handling Streams and Files
	add_edges/3	7.19 Unweighted Graphs
	add_to_array_element/4	6.16 Arrays
	add_to_heap/4	7.4 Heaps
	add_vertices/3	7.19 Unweighted Graphs
	alarm/3	6.12 Access to Operating System Functionality
	all/3	6.10 Collecting Solutions to a Goal
	always_prompt_user/0	6.6.7 Controlling Input/Output
	append/3	7.5 List Manipulation
	append/3	8. SWI-Prolog Emulation
	arg/3	6.3 Predicates on terms
	array/2	6.16 Arrays
	array_element/3	6.16 Arrays
	assert_static/1	6.7.1 Modification of the Data Base
	asserta_static/1	6.7.1 Modification of the Data Base
	assertz_static/1	6.7.1 Modification of the Data Base
	assoc_to_list/2	7.2 Association Lists
	at_end_of_stream/0	6.6.1 Handling Streams and Files
	at_end_of_stream/1	6.6.1 Handling Streams and Files
	atom_chars/2	6.3 Predicates on terms
	atom_codes/2	6.3 Predicates on terms
	atom_concat/2	6.3 Predicates on terms
	atom_concat/3	6.3 Predicates on terms
	atom_concat/3	6.3 Predicates on terms
	atom_length/2	6.3 Predicates on terms
	atom_to_chars/2	7.13 Reading From and Writing To Strings
	atom_to_chars/3	7.13 Reading From and Writing To Strings
	atomic_concat/2	6.3 Predicates on terms
	attr_unify_hook/2	8.3 hProlog and SWI-Prolog Attributed Variables
	attribute_goal/2	10.4 Displaying Attributes
	attvar/1	10.3 Attributed Unification
	avl_insert/4	7.3 AVL Trees
	avl_lookup/3	7.3 AVL Trees
B
	b_getval/2	8.4 SWI Global variables
	b_setval/2	8.4 SWI Global variables
	bb_delete/2	6.9 The Blackboard
	bb_get/2	6.9 The Blackboard
	bb_put/2	6.9 The Blackboard
	bb_update/3	6.9 The Blackboard
	between/3	8. SWI-Prolog Emulation
C
	C/3	6.11 Grammar Rules
	call_cleanup/1	7.16 Call Cleanup
	call_cleanup/2	7.16 Call Cleanup
	call_count_data/0	6.15 Counting Calls
	call_count_data/3	6.15 Counting Calls
	call_count_data/3	6.15 Counting Calls
	call_residue/2	9.2 Coroutining
	call_with_args/n	6.1 Control Predicates
	callable/1	6.3 Predicates on terms
	catch/3	6.1 Control Predicates
	cd/1	6.12 Access to Operating System Functionality
	char_code/2	6.3 Predicates on terms
	char_conversion/2	6.6.3 Handling Input/Output of Terms
	chdir/1	8. SWI-Prolog Emulation
	checklist/2	7.1 Apply Macros
	checknodes/3	7.1 Apply Macros
	cleanup_all/0	7.16 Call Cleanup
	close/2	6.6.1 Handling Streams and Files
	close_static_array/1	6.16 Arrays
	compile/1	4.1 Program loading and updating
	compile/1	4.1 Program loading and updating
	compile/1 (directive)	4.1 Program loading and updating
	compile_expressions/0	4.2 Changing the Compiler's Behavior
	complement/2	7.19 Unweighted Graphs
	compose/3	7.19 Unweighted Graphs
	compound/1	6.3 Predicates on terms
	concat_atom/2	8. SWI-Prolog Emulation
	concat_atom/3	8. SWI-Prolog Emulation
	convlist/3	7.1 Apply Macros
	copy_term/2	6.3 Predicates on terms
	copy_term_nat/2	8.3.1 Special Purpose SWI Predicates for Attributes
	create_mutable/2	6.13 Term Modification
	current_char_conversion/2	6.6.3 Handling Input/Output of Terms
	current_input/1	6.6.1 Handling Streams and Files
	current_key/2	6.8 Internal Data Base
	current_module/1	6.17 Predicate Information
	current_module/2	6.17 Predicate Information
	current_mutex/3	14.4 Thread Synchronisation
	current_op/3	6.18 Miscellaneous
	current_output/1	6.6.1 Handling Streams and Files
	current_predicate/2	6.7.2 Looking at the Data Base
	current_prolog_flag/2	6.18 Miscellaneous
	current_stream/3	6.6.1 Handling Streams and Files
	current_thread/2	14.2 Monitoring Threads
	cyclic_term/1	7.15 Utilities On Terms
	cyclic_term/1	7.15 Utilities On Terms
D
	datime/1	7.14 Calling The Operating System from YAP
	del_assoc/4	7.2 Association Lists
	del_attr/2	8.3 hProlog and SWI-Prolog Attributed Variables
	del_edges/3	7.19 Unweighted Graphs
	del_max_assoc/4	7.2 Association Lists
	del_min_assoc/4	7.2 Association Lists
	del_vertices/3	7.19 Unweighted Graphs
	delete/3	7.5 List Manipulation
	delete_file/1	7.14 Calling The Operating System from YAP
	delete_file/2	7.14 Calling The Operating System from YAP
	dgraph_add_edges/3	7.20 Directed Graphs
	dgraph_add_vertices/3	7.20 Directed Graphs
	dgraph_complement/2	7.20 Directed Graphs
	dgraph_compose/3	7.20 Directed Graphs
	dgraph_del_edges/3	7.20 Directed Graphs
	dgraph_del_vertices/3	7.20 Directed Graphs
	dgraph_edges/2	7.20 Directed Graphs
	dgraph_neighbors/3	7.20 Directed Graphs
	dgraph_neighbours/3	7.20 Directed Graphs
	dgraph_new/1	7.20 Directed Graphs
	dgraph_symmetric_closure/2	7.20 Directed Graphs
	dgraph_top_sort/2	7.20 Directed Graphs
	dgraph_transitive_closure/2	7.20 Directed Graphs
	dgraph_transpose/2	7.20 Directed Graphs
	dgraph_vertices/2	7.20 Directed Graphs
	dif/2	9.2 Coroutining
	directory_files/2	7.14 Calling The Operating System from YAP
	discontiguous/1 (directive)	4.2 Changing the Compiler's Behavior
	display/1	6.6.3 Handling Input/Output of Terms
	display/1	6.6.5 Input/Output Predicates applied to Streams
	display/2	6.6.5 Input/Output Predicates applied to Streams
	do_not_compile_expressions/0	4.2 Changing the Compiler's Behavior
	dynamic/1	6.7 Using the Clausal Data Base
	dynamic_predicate/2	6.7 Using the Clausal Data Base
E
	edges/2	7.19 Unweighted Graphs
	empty_assoc/1	7.2 Association Lists
	empty_heap/1	7.4 Heaps
	empty_queue/1	7.8 Queues
	environ/2	6.12 Access to Operating System Functionality
	environ/2	7.14 Calling The Operating System from YAP
	eraseall/1	6.8 Internal Data Base
	exec/3	7.14 Calling The Operating System from YAP
F
	false/0	6.1 Control Predicates
	file_exists/1	7.14 Calling The Operating System from YAP
	file_exists/2	7.14 Calling The Operating System from YAP
	file_property/2	7.14 Calling The Operating System from YAP
	file_search_path/2	4.2 Changing the Compiler's Behavior
	findall/4	6.10 Collecting Solutions to a Goal
	flatten/2	7.5 List Manipulation
	float/1	6.3 Predicates on terms
	flush_output/0	6.6.1 Handling Streams and Files
	flush_output/1	6.6.1 Handling Streams and Files
	forall/2	8.2 Forall
	format/2	6.6.3 Handling Input/Output of Terms
	format/3	6.6.3 Handling Input/Output of Terms
	format_to_chars/3	7.13 Reading From and Writing To Strings
	format_to_chars/4	7.13 Reading From and Writing To Strings
	fragile	7.16 Call Cleanup
	freeze/2	9.2 Coroutining
	frozen/2	9.2 Coroutining
G
	garbage_collect/0	6.1 Control Predicates
	garbage_collect_atoms/0	6.1 Control Predicates
	gc/0	6.1 Control Predicates
	gen_assoc/3	7.2 Association Lists
	get/2	6.6.5 Input/Output Predicates applied to Streams
	get0/2	6.6.5 Input/Output Predicates applied to Streams
	get_assoc/3	7.2 Association Lists
	get_assoc/5	7.2 Association Lists
	get_attr/3	8.3 hProlog and SWI-Prolog Attributed Variables
	get_attrs/2	8.3.1 Special Purpose SWI Predicates for Attributes
	get_atts/2	10.2 Attribute Manipulation
	get_byte/1	6.6.4 Handling Input/Output of Characters
	get_byte/2	6.6.5 Input/Output Predicates applied to Streams
	get_char/1	6.6.4 Handling Input/Output of Characters
	get_char/2	6.6.5 Input/Output Predicates applied to Streams
	get_code/1	6.6.4 Handling Input/Output of Characters
	get_code/2	6.6.5 Input/Output Predicates applied to Streams
	get_from_heap/4	7.4 Heaps
	get_label/3	7.18 Updatable Binary Trees
	get_mutable/2	6.13 Term Modification
	get_mutable/2	6.13 Term Modification
	get_next_assoc/4	7.2 Association Lists
	get_prev_assoc/4	7.2 Association Lists
	get_value/2	6.8 Internal Data Base
	getcwd/1	6.12 Access to Operating System Functionality
	getrand/1	7.9 Random Number Generator
	goal_expansion/3	6.11 Grammar Rules
	ground/1	6.3 Predicates on terms
	grow_heap/1	6.1 Control Predicates
	grow_stack/1	6.1 Control Predicates
H
	halt/1	6.1 Control Predicates
	head_queue/2	7.8 Queues
	heap_size/2	7.4 Heaps
	heap_to_list/2	7.4 Heaps
	hide/1	4.2 Changing the Compiler's Behavior
	hide_predicate/1	4.2 Changing the Compiler's Behavior
	host_id/1	7.14 Calling The Operating System from YAP
	host_name/1	7.14 Calling The Operating System from YAP
I
	if/3	6.1 Control Predicates
	incore/1	6.1 Control Predicates
	initialization/0	6.18 Miscellaneous
	initialization/1 (directive)	4.2 Changing the Compiler's Behavior
	is_assoc/1	7.2 Association Lists
	is_list/1	7.5 List Manipulation
	is_mutable/1	6.13 Term Modification
	is_rbtree/1	7.10 Red-Black Trees
	is_tabled/1	16. Tabling
	is_tabled/1	16. Tabling
J
	join_queue/3	7.8 Queues
	jump_queue/3	7.8 Queues
K
	key_statistics/3	6.8 Internal Data Base
	key_statistics/4	6.8 Internal Data Base
	kill/2	7.14 Calling The Operating System from YAP
L
	last/2	7.5 List Manipulation
	length_queue/2	7.8 Queues
	library_directory/1	4.2 Changing the Compiler's Behavior
	library_directory/1	4.2 Changing the Compiler's Behavior
	list_concat/2	7.5 List Manipulation
	list_join_queue/3	7.8 Queues
	list_jump_queue/3	7.8 Queues
	list_to_assoc/2	7.2 Association Lists
	list_to_heap/2	7.4 Heaps
	list_to_ord_set/2	7.6 Ordered Sets
	list_to_queue/2	7.8 Queues
	list_to_rbtree/2	7.10 Red-Black Trees
	list_to_rbtree/2	7.10 Red-Black Trees
	list_to_tree/2	7.18 Updatable Binary Trees
M
	make_directory/2	7.14 Calling The Operating System from YAP
	make_queue/1	7.8 Queues
	map_assoc/2	7.2 Association Lists
	map_assoc/3	7.2 Association Lists
	map_tree/3	7.18 Updatable Binary Trees
	mapargs/3	7.1 Apply Macros
	maplist/2	8.1 Invoking Predicates on all Members of a List
	maplist/3	7.1 Apply Macros
	maplist/3	8.1 Invoking Predicates on all Members of a List
	maplist/4	8.1 Invoking Predicates on all Members of a List
	mapnodes/3	7.1 Apply Macros
	max_assoc/3	7.2 Association Lists
	max_list/2	7.5 List Manipulation
	member/2	7.5 List Manipulation
	memberchk/2	7.5 List Manipulation
	merge/3	7.6 Ordered Sets
	meta_predicate/1 (directive)	5.4 Meta-Predicates in Modules
	min_assoc/3	7.2 Association Lists
	min_list/2	7.5 List Manipulation
	min_of_heap/3	7.4 Heaps
	min_of_heap/5	7.4 Heaps
	mktemp/2	7.14 Calling The Operating System from YAP
	mktime/2	7.14 Calling The Operating System from YAP
	module/1	5.2 Defining a New Module
	module/2 (directive)	5.2 Defining a New Module
	module/3 (directive)	5.2 Defining a New Module
	multifile/1 (directive)	4.2 Changing the Compiler's Behavior
	mutex_create/1	14.4 Thread Synchronisation
	mutex_destroy/1	14.4 Thread Synchronisation
	mutex_lock/1	14.4 Thread Synchronisation
	mutex_statistics/0	14.2 Monitoring Threads
	mutex_trylock/1	14.4 Thread Synchronisation
	mutex_unlock/1	14.4 Thread Synchronisation
	mutex_unlock_all/0	14.4 Thread Synchronisation
N
	nb_current/2	8.4 SWI Global variables
	nb_delete/1	8.4 SWI Global variables
	nb_getval/2	8.4 SWI Global variables
	nb_setval/2	8.4 SWI Global variables
	neighbors/3	7.19 Unweighted Graphs
	neighbours/3	7.19 Unweighted Graphs
	nl/1	6.6.5 Input/Output Predicates applied to Streams
	no_source/0	4.2 Changing the Compiler's Behavior
	nogc/0	6.1 Control Predicates
	nospyall/0	19.1 Debugging Predicates
	nth/2	7.5 List Manipulation
	nth/4	7.5 List Manipulation
	nth0/2	7.5 List Manipulation
	nth0/4	7.5 List Manipulation
	nth1/3	8. SWI-Prolog Emulation
	nth_recorded/3	6.8 Internal Data Base
	number_atom/2	6.3 Predicates on terms
	number_chars/2	6.3 Predicates on terms
	number_codes/2	6.3 Predicates on terms
	number_to_chars/2	7.13 Reading From and Writing To Strings
	number_to_chars/3	7.13 Reading From and Writing To Strings
	numbervars/3	6.3 Predicates on terms
O
	on_cleanup/1	7.16 Call Cleanup
	on_signal/3	6.12 Access to Operating System Functionality
	once/1	6.1 Control Predicates
	open/3	6.6.1 Handling Streams and Files
	open/4	6.6.1 Handling Streams and Files
	open_chars_stream/2	7.13 Reading From and Writing To Strings
	ord_add_element/3	7.6 Ordered Sets
	ord_del_element/3	7.6 Ordered Sets
	ord_disjoint/2	7.6 Ordered Sets
	ord_insert/3	7.6 Ordered Sets
	ord_intersect/2	7.6 Ordered Sets
	ord_intersect/3	7.6 Ordered Sets
	ord_intersect/4	7.6 Ordered Sets
	ord_list_to_assoc/2	7.2 Association Lists
	ord_member/2	7.6 Ordered Sets
	ord_seteq/2	7.6 Ordered Sets
	ord_setproduct/3	7.6 Ordered Sets
	ord_subtract/3	7.6 Ordered Sets
	ord_symdiff/3	7.6 Ordered Sets
	ord_union/2	7.6 Ordered Sets
	ord_union/3	7.6 Ordered Sets
	ord_union/4	7.6 Ordered Sets
	ordsubset/2	7.6 Ordered Sets
P
	path/1	4.2 Changing the Compiler's Behavior
	path/1	4.2 Changing the Compiler's Behavior
	path/1	4.2 Changing the Compiler's Behavior
	peek_byte/1	6.6.4 Handling Input/Output of Characters
	peek_byte/2	6.6.5 Input/Output Predicates applied to Streams
	peek_char/1	6.6.4 Handling Input/Output of Characters
	peek_char/2	6.6.5 Input/Output Predicates applied to Streams
	peek_code/1	6.6.4 Handling Input/Output of Characters
	peek_code/2	6.6.5 Input/Output Predicates applied to Streams
	permutation/2	7.5 List Manipulation
	phrase/2	6.11 Grammar Rules
	phrase/3	6.11 Grammar Rules
	pid/1	7.14 Calling The Operating System from YAP
	popen/3	7.14 Calling The Operating System from YAP
	portray_clause/1	6.7.2 Looking at the Data Base
	portray_clause/2	6.7.2 Looking at the Data Base
	predicate_property/2	6.7.2 Looking at the Data Base
	print/2	6.6.5 Input/Output Predicates applied to Streams
	profile_data/3	6.14 Profiling Prolog Programs
	profiled_reset/0	6.14 Profiling Prolog Programs
	project_attributes/2	10.4 Displaying Attributes
	project_attributes/2	10.5 Projecting Attributes
	prolog_file_name/2	4.2 Changing the Compiler's Behavior
	prolog_flag/3	6.18 Miscellaneous
	prolog_initialization/1	6.18 Miscellaneous
	prolog_load_context/2	6.18 Miscellaneous
	public/1 (directive)	4.2 Changing the Compiler's Behavior
	put/2	6.6.5 Input/Output Predicates applied to Streams
	put_assoc/4	7.2 Association Lists
	put_attr/3	8.3 hProlog and SWI-Prolog Attributed Variables
	put_attrs/2	8.3.1 Special Purpose SWI Predicates for Attributes
	put_atts/2	10.2 Attribute Manipulation
	put_byte/1	6.6.4 Handling Input/Output of Characters
	put_byte/2	6.6.5 Input/Output Predicates applied to Streams
	put_char/1	6.6.4 Handling Input/Output of Characters
	put_char/2	6.6.5 Input/Output Predicates applied to Streams
	put_code/1	6.6.4 Handling Input/Output of Characters
	put_code/2	6.6.5 Input/Output Predicates applied to Streams
	put_label/4	7.18 Updatable Binary Trees
	putenv/2	6.12 Access to Operating System Functionality
Q
	queue_to_list/2	7.8 Queues
R
	random/1	7.9 Random Number Generator
	random/3	7.9 Random Number Generator
	randseq/3	7.9 Random Number Generator
	randset/3	7.9 Random Number Generator
	rannum/1	7.7 Pseudo Random Number Integer Generator
	ranstart/0	7.7 Pseudo Random Number Integer Generator
	ranstart/1	7.7 Pseudo Random Number Integer Generator
	ranunif/2	7.7 Pseudo Random Number Integer Generator
	rb_apply/4	7.10 Red-Black Trees
	rb_clone/3	7.10 Red-Black Trees
	rb_del_max/4	7.10 Red-Black Trees
	rb_del_min/4	7.10 Red-Black Trees
	rb_delete/3	7.10 Red-Black Trees
	rb_delete/4	7.10 Red-Black Trees
	rb_empty/1	7.10 Red-Black Trees
	rb_insert/4	7.10 Red-Black Trees
	rb_keys/2	7.10 Red-Black Trees
	rb_lookup/3	7.10 Red-Black Trees
	rb_lookupall/3	7.10 Red-Black Trees
	rb_map/3	7.10 Red-Black Trees
	rb_max/3	7.10 Red-Black Trees
	rb_min/3	7.10 Red-Black Trees
	rb_new/1	7.10 Red-Black Trees
	rb_next/4	7.10 Red-Black Trees
	rb_partial_map/4	7.10 Red-Black Trees
	rb_previous/4	7.10 Red-Black Trees
	rb_size/2	7.10 Red-Black Trees
	rb_update/4	7.10 Red-Black Trees
	rb_visit/2	7.10 Red-Black Trees
	reachable/3	7.19 Unweighted Graphs
	read/2	6.6.5 Input/Output Predicates applied to Streams
	read_from_chars/2	7.13 Reading From and Writing To Strings
	read_term/2	6.6.3 Handling Input/Output of Terms
	read_term/3	6.6.5 Input/Output Predicates applied to Streams
	recorda/3	6.8 Internal Data Base
	recorda_at/3	6.8 Internal Data Base
	recordaifnot/3	6.8 Internal Data Base
	recordaifnot/3	6.8 Internal Data Base
	recordz_at/3	6.8 Internal Data Base
	recordzifnot/3	6.8 Internal Data Base
	regexp/3	7.11 Regular Expressions
	regexp/4	7.11 Regular Expressions
	remove_duplicates/2	7.5 List Manipulation
	remove_from_path/1	4.2 Changing the Compiler's Behavior
	rename_file/2	7.14 Calling The Operating System from YAP
	reset_op_counters/0	18. Profiling the Abstract Machine
	resize_static_array/3	6.16 Arrays
	restore/1	4.3 Saving and Loading Prolog States
	retract/1	6.7.1 Modification of the Data Base
	retractall/1	6.7.1 Modification of the Data Base
	reverse/2	7.5 List Manipulation
S
	same_length/2	7.5 List Manipulation
	save/2	4.3 Saving and Loading Prolog States
	save_program/1	4.3 Saving and Loading Prolog States
	save_program/2	4.3 Saving and Loading Prolog States
	select/3	7.5 List Manipulation
	selectlist/3	7.1 Apply Macros
	serve_queue/3	7.8 Queues
	set_input/1	6.6.1 Handling Streams and Files
	set_output/1	6.6.1 Handling Streams and Files
	set_prolog_flag/2	6.18 Miscellaneous
	set_stream_position/2	6.6.1 Handling Streams and Files
	set_value/2	6.8 Internal Data Base
	setarg/3n	6.13 Term Modification
	setenv/2	8. SWI-Prolog Emulation
	setrand/1	7.9 Random Number Generator
	shell/0	7.14 Calling The Operating System from YAP
	shell/1	7.14 Calling The Operating System from YAP
	shell/1	7.14 Calling The Operating System from YAP
	show_op_counters/1	18. Profiling the Abstract Machine
	show_ops_by_group/1	18. Profiling the Abstract Machine
	show_table/1	16. Tabling
	simple/1	6.3 Predicates on terms
	skip/2	6.6.5 Input/Output Predicates applied to Streams
	sleep/1	7.14 Calling The Operating System from YAP
	socket/2	6.6.8 Using Sockets From Yap
	socket/4	6.6.8 Using Sockets From Yap
	socket_accept/2	6.6.8 Using Sockets From Yap
	socket_accept/2	6.6.8 Using Sockets From Yap
	socket_accept/3	6.6.8 Using Sockets From Yap
	socket_bind/2	6.6.8 Using Sockets From Yap
	socket_buffering/4	6.6.8 Using Sockets From Yap
	socket_close/1	6.6.8 Using Sockets From Yap
	socket_connect/3	6.6.8 Using Sockets From Yap
	socket_listen/2	6.6.8 Using Sockets From Yap
	socket_select/5	6.6.8 Using Sockets From Yap
	source/0	4.2 Changing the Compiler's Behavior
	source_mode/2	4.2 Changing the Compiler's Behavior
	splay_access/5	7.12 Splay Trees
	splay_delete/4	7.12 Splay Trees
	splay_init/3	7.12 Splay Trees
	splay_insert/4	7.12 Splay Trees
	splay_join/3	7.12 Splay Trees
	splay_split/5	7.12 Splay Trees
	spy_write/2	19.1 Debugging Predicates
	srandom/1	6.5 Arithmetic
	start_low_level_trace/0	17. Tracing at Low Level
	start_low_level_trace/0	17. Tracing at Low Level
	static_array/3	6.16 Arrays
	static_array/3	6.16 Arrays
	static_array_location/4	6.16 Arrays
	static_array_properties/3	6.16 Arrays
	static_array_to_term/3	6.16 Arrays
	statistics/2	6.18 Miscellaneous
	stream_property/2	6.6.1 Handling Streams and Files
	stream_select/3	6.6.1 Handling Streams and Files
	style_check/1	4.2 Changing the Compiler's Behavior
	style_check/1	4.2 Changing the Compiler's Behavior
	sub_atom/5	6.3 Predicates on terms
	sublist/2	7.5 List Manipulation
	subsumes/2	7.15 Utilities On Terms
	subsumes_chk/2	7.15 Utilities On Terms
	suffix/2	7.5 List Manipulation
	sum_list/2	7.5 List Manipulation
	sumargs/4	7.1 Apply Macros
	sumlist/2	7.5 List Manipulation
	sumlist/4	7.1 Apply Macros
	sumnodes/4	7.1 Apply Macros
	system/0	7.14 Calling The Operating System from YAP
	system/2	7.14 Calling The Operating System from YAP
	system_predicate/2	6.7.2 Looking at the Data Base
T
	tab/2	6.6.5 Input/Output Predicates applied to Streams
	table_statistics/1	16. Tabling
	term_hash/2	7.15 Utilities On Terms
	term_hash/4	7.15 Utilities On Terms
	term_to_atom/2	8. SWI-Prolog Emulation
	term_variables/2	7.15 Utilities On Terms
	thread_at_exit/1	14.1 Creating and Destroying Prolog Threads
	thread_create/3	14.1 Creating and Destroying Prolog Threads
	thread_detach/1	14.1 Creating and Destroying Prolog Threads
	thread_exit/1	14.1 Creating and Destroying Prolog Threads
	thread_get_message/1	14.3.1 Message Queues
	thread_get_message/2	14.3.1 Message Queues
	thread_join/2	14.1 Creating and Destroying Prolog Threads
	thread_local/1 (directive)	14.3.3 Threads and Dynamic Predicates
	thread_message_queue_create/1	14.3.1 Message Queues
	thread_message_queue_destroy/1	14.3.1 Message Queues
	thread_peek_message/1	14.3.1 Message Queues
	thread_self/1	14.1 Creating and Destroying Prolog Threads
	thread_send_message/2	14.3.1 Message Queues
	thread_setconcurrency/2	14.1 Creating and Destroying Prolog Threads
	thread_signal/2	14.3.2 Signalling Threads
	thread_statistics/3	14.2 Monitoring Threads
	throw/1	6.1 Control Predicates
	time_out/3	7.17 Calls With Timeout
	tmpnam/1	7.14 Calling The Operating System from YAP
	top_sort/2	7.19 Unweighted Graphs
	top_sort/3	7.19 Unweighted Graphs
	transitive_closure/2	7.19 Unweighted Graphs
	transpose/3	7.19 Unweighted Graphs
	tree_size/2	7.18 Updatable Binary Trees
	tree_to_list/2	7.18 Updatable Binary Trees
	ttyget/1	6.6.6 Compatible C-Prolog predicates for Terminal I/O
	ttyget0/1	6.6.6 Compatible C-Prolog predicates for Terminal I/O
	ttynl/0	6.6.6 Compatible C-Prolog predicates for Terminal I/O
	ttyput/1	6.6.6 Compatible C-Prolog predicates for Terminal I/O
	ttyskip/1	6.6.6 Compatible C-Prolog predicates for Terminal I/O
	ttytab/1	6.6.6 Compatible C-Prolog predicates for Terminal I/O
U
	undgraph_add_edges/3	7.21 Undirected Graphs
	undgraph_add_vertices/3	7.21 Undirected Graphs
	undgraph_complement/2	7.21 Undirected Graphs
	undgraph_del_edges/3	7.21 Undirected Graphs
	undgraph_del_vertices/3	7.21 Undirected Graphs
	undgraph_edges/2	7.21 Undirected Graphs
	undgraph_neighbors/3	7.21 Undirected Graphs
	undgraph_neighbours/3	7.21 Undirected Graphs
	undgraph_new/1	7.21 Undirected Graphs
	undgraph_vertices/2	7.21 Undirected Graphs
	unhide/1	4.2 Changing the Compiler's Behavior
	unify_with_occurs_check/2	6.3 Predicates on terms
	unix/1	6.12 Access to Operating System Functionality
	unknown/2	6.2 Handling Undefined Procedures
	unknown_predicate_handler/3	6.2 Handling Undefined Procedures
	update_array/3	6.16 Arrays
	update_mutable/2	6.13 Term Modification
	use_module/1	5.3 Using Modules
	use_module/2	5.3 Using Modules
	use_module/3	5.3 Using Modules
V
	variable_in_term/2	7.15 Utilities On Terms
	variant/2	7.15 Utilities On Terms
	verify_attributes/3	10.3 Attributed Unification
	version/0	6.18 Miscellaneous
	version/1	6.18 Miscellaneous
	vertices/2	7.19 Unweighted Graphs
	vertices_edges_to_ugraph/3	7.19 Unweighted Graphs
W
	wait/2	7.14 Calling The Operating System from YAP
	when/2	9.2 Coroutining
	with_mutex/2	14.4 Thread Synchronisation
	with_output_to_chars/2	7.13 Reading From and Writing To Strings
	with_output_to_chars/3	7.13 Reading From and Writing To Strings
	with_output_to_chars/4	7.13 Reading From and Writing To Strings
	working_directory/2	7.14 Calling The Operating System from YAP
	working_directory/2	8. SWI-Prolog Emulation
	write/2	6.6.5 Input/Output Predicates applied to Streams
	write_depth/3	6.6.7 Controlling Input/Output
	write_term/2	6.6.3 Handling Input/Output of Terms
	write_term/3	6.6.5 Input/Output Predicates applied to Streams
	write_to_chars/2	7.13 Reading From and Writing To Strings
	write_to_chars/3	7.13 Reading From and Writing To Strings
	writeq/2	6.6.5 Input/Output Predicates applied to Streams
Y
	yap_flag/2	6.18 Miscellaneous

Jump to:	-   =   \   A   B   C   D   E   F   G   H   I   J   K   L   M   N   O   P   Q   R   S   T   U   V   W   Y

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23.1.5 Yap predicates not available in C-Prolog

These are C-Prolog built-ins not available in YAP:

'LC'

The following Prolog text uses lower case letters.

'NOLC'

The following Prolog text uses upper case letters only.

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23.2 Compatibility with the Quintus and SICStus Prolog systems

The Quintus Prolog system was the first Prolog compiler to use Warren's Abstract Machine. This system was very influential in the Prolog community. Quintus Prolog implemented compilation into an abstract machine code, which was then emulated. Quintus Prolog also included several new built-ins, an extensive library, and in later releases a garbage collector. The SICStus Prolog system, developed at SICS (Swedish Institute of Computer Science), is an emulator based Prolog system largely compatible with Quintus Prolog. SICStus Prolog has evolved through several versions. The current version includes several extensions, such as an object implementation, co-routining, and constraints.

Recent work in YAP has been influenced by work in Quintus and SICStus Prolog. Wherever possible, we have tried to make YAP compatible with recent versions of these systems, and specifically of SICStus Prolog. You should use

:- yap_flag(language, sicstus).

for maximum compatibility with SICStus Prolog.

SICStus Compatibility 23.2.1 Major Differences between YAP and SICStus Prolog.  Major Differences between YAP and SICStus Prolog 23.2.2 Yap predicates fully compatible with SICStus Prolog  Yap predicates fully compatible with SICStus Prolog 23.2.3 Yap predicates not strictly compatible with SICStus Prolog  Yap predicates not strictly as SICStus Prolog 23.2.4 Yap predicates not available in SICStus Prolog  Yap predicates not available in SICStus Prolog

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23.2.1 Major Differences between YAP and SICStus Prolog.

Both YAP and SICStus Prolog obey the Edinburgh Syntax and are based on the WAM. Even so, there are quite a few important differences:

• Differently from SICStus Prolog, YAP does not have a notion of interpreted code. All code in YAP is compiled.
• YAP does not support an intermediate byte-code representation, so the fcompile/1 and load/1 built-ins are not available in YAP.
• YAP implements escape sequences as in the ISO standard. SICStus Prolog implements Unix-like escape sequences.
• YAP implements initialization/1 as per the ISO standard. Use prolog_initialization/1 for the SICStus Prolog compatible built-in.
• Prolog flags are different in SICStus Prolog and in YAP.
• The SICStus Prolog on_exception/3 and raise_exception built-ins correspond to the ISO built-ins catch/3 and throw/1.
• The following SICStus Prolog v3 built-ins are not (currently) implemented in YAP (note that this is only a partial list): call_cleanup/1, file_search_path/2, stream_interrupt/3, reinitialize/0, help/0, help/1, trimcore/0, load_files/1, load_files/2, and require/1.

  The previous list is incomplete. We also cannot guarantee full compatibility for other built-ins (although we will try to address any such incompatibilities). Last, SICStus Prolog is an evolving system, so one can be expect new incompatibilities to be introduced in future releases of SICStus Prolog.

• YAP allows asserting and abolishing static code during execution through the assert_static/1 and abolish/1 built-ins. This is not allowed in Quintus Prolog or SICStus Prolog.
• YAP implements rational trees and co-routining but they are not included by default in the system. You must enable these extensions when compiling the system.
• YAP does not currently implement constraints.
• The socket predicates, although designed to be compatible with SICStus Prolog, are built-ins, not library predicates, in YAP.
• This list is incomplete.

The following differences only exist if the language flag is set to yap (the default):

• The consult/1 predicate in YAP follows C-Prolog semantics. That is, it adds clauses to the data base, even for preexisting procedures. This is different from consult/1 in SICStus Prolog.
• By default, the data-base in YAP follows "immediate update semantics", instead of "logical update semantics", as Quintus Prolog or SICStus Prolog do. The difference is depicted in the next example:

  :- dynamic a/1. ?- assert(a(1)). ?- retract(a(X)), X1 is X +1, assertz(a(X)).

  With immediate semantics, new clauses or entries to the data base are visible in backtracking. In this example, the first call to retract/1 will succeed. The call to assertz/1 will then succeed. On backtracking, the system will retry retract/1. Because the newly asserted goal is visible to retract/1, it can be retracted from the data base, and retract(a(X)) will succeed again. The process will continue generating integers for ever. Immediate semantics were used in C-Prolog.

  With logical update semantics, any additions or deletions of clauses for a goal will not affect previous activations of the goal. In the example, the call to assertz/1 will not see the update performed by the assertz/1, and the query will have a single solution.

  Calling yap_flag(update_semantics,logical) will switch YAP to use logical update semantics.

• dynamic/1 is a built-in, not a directive, in YAP.
• By default, YAP fails on undefined predicates. To follow default SICStus Prolog use:

  :- yap_flag(unknown,error).

• By default, directives in YAP can be called from the top level.

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23.2.2 Yap predicates fully compatible with SICStus Prolog

These are the Prolog built-ins that are fully compatible in both SICStus Prolog and YAP:

Jump to:	!   ,   -   ;   <   =   >   @   \   A   B   C   D   E   F   G   H   I   J   K   L   M   N   O   P   Q   R   S   T   U   V   W

	Index Entry	Section
!
	!/0	6.1 Control Predicates
,
	,/2	6.1 Control Predicates
-
	->/2	6.1 Control Predicates
;
	;/2	6.1 Control Predicates
<
	</2	6.5 Arithmetic
=
	=../2	6.3 Predicates on terms
	=/2	6.3 Predicates on terms
	=:=/2	6.5 Arithmetic
	=</2	6.5 Arithmetic
	==/2	6.4 Comparing Terms
	=\=/2	6.5 Arithmetic
>
	>/2	6.5 Arithmetic
	>=/2	6.5 Arithmetic
@
	@</2	6.4 Comparing Terms
	@</2	6.4 Comparing Terms
	@>/2	6.4 Comparing Terms
	@>=/2	6.4 Comparing Terms
\
	\+/1	6.1 Control Predicates
	\==/2	6.4 Comparing Terms
A
	abort/0	6.1 Control Predicates
	absolute_file_name/2	6.6.1 Handling Streams and Files
	add_edges/3	7.19 Unweighted Graphs
	add_to_heap/4	7.4 Heaps
	add_vertices/3	7.19 Unweighted Graphs
	append/3	7.5 List Manipulation
	arg/3	6.3 Predicates on terms
	assoc_to_list/2	7.2 Association Lists
	at_end_of_stream/0	6.6.1 Handling Streams and Files
	at_end_of_stream/1	6.6.1 Handling Streams and Files
	atom/1	6.3 Predicates on terms
	atom_codes/2	6.3 Predicates on terms
	atom_concat/2	6.3 Predicates on terms
	atom_concat/3	6.3 Predicates on terms
	atom_to_chars/2	7.13 Reading From and Writing To Strings
	atom_to_chars/3	7.13 Reading From and Writing To Strings
	atomic/1	6.3 Predicates on terms
	attribute_goal/2	10.4 Displaying Attributes
B
	bb_delete/2	6.9 The Blackboard
	bb_get/2	6.9 The Blackboard
	bb_put/2	6.9 The Blackboard
	bb_update/3	6.9 The Blackboard
	break/0	6.1 Control Predicates
C
	C/3	6.11 Grammar Rules
	call/1	6.1 Control Predicates
	call_cleanup/1	7.16 Call Cleanup
	call_cleanup/2	7.16 Call Cleanup
	call_residue/2	9.2 Coroutining
	callable/1	6.3 Predicates on terms
	char_code/2	6.3 Predicates on terms
	char_conversion/2	6.6.3 Handling Input/Output of Terms
	cleanup_all/0	7.16 Call Cleanup
	close/1	6.6.1 Handling Streams and Files
	compare/3	6.4 Comparing Terms
	compile/1	4.1 Program loading and updating
	compile/1	4.1 Program loading and updating
	complement/2	7.19 Unweighted Graphs
	compose/3	7.19 Unweighted Graphs
	compound/1	6.3 Predicates on terms
	copy_term/2	6.3 Predicates on terms
	create_mutable/2	6.13 Term Modification
	current_atom/1	6.7.2 Looking at the Data Base
	current_char_conversion/2	6.6.3 Handling Input/Output of Terms
	current_input/1	6.6.1 Handling Streams and Files
	current_key/2	6.8 Internal Data Base
	current_module/1	6.17 Predicate Information
	current_module/2	6.17 Predicate Information
	current_op/3	6.18 Miscellaneous
	current_output/1	6.6.1 Handling Streams and Files
	current_predicate/1	6.7.2 Looking at the Data Base
	current_predicate/2	6.7.2 Looking at the Data Base
	current_stream/3	6.6.1 Handling Streams and Files
	cyclic_term/1	7.15 Utilities On Terms
	cyclic_term/1	7.15 Utilities On Terms
D
	datime/1	7.14 Calling The Operating System from YAP
	db_reference/1	6.3 Predicates on terms
	debugging/0	19.1 Debugging Predicates
	del_assoc/4	7.2 Association Lists
	del_edges/3	7.19 Unweighted Graphs
	del_max_assoc/4	7.2 Association Lists
	del_min_assoc/4	7.2 Association Lists
	del_vertices/3	7.19 Unweighted Graphs
	delete/3	7.5 List Manipulation
	delete_file/1	7.14 Calling The Operating System from YAP
	delete_file/2	7.14 Calling The Operating System from YAP
	dgraph_del_vertices/3	7.20 Directed Graphs
	dif/2	9.2 Coroutining
	directory_files/2	7.14 Calling The Operating System from YAP
	discontiguous/1 (directive)	4.2 Changing the Compiler's Behavior
	display/1	6.6.3 Handling Input/Output of Terms
	display/1	6.6.3 Handling Input/Output of Terms
	display/1	6.6.5 Input/Output Predicates applied to Streams
	display/2	6.6.5 Input/Output Predicates applied to Streams
E
	edges/2	7.19 Unweighted Graphs
	empty_assoc/1	7.2 Association Lists
	empty_heap/1	7.4 Heaps
	empty_queue/1	7.8 Queues
	environ/2	6.12 Access to Operating System Functionality
	environ/2	7.14 Calling The Operating System from YAP
	exec/3	7.14 Calling The Operating System from YAP
	expand_term/2	6.11 Grammar Rules
F
	fail/0	6.1 Control Predicates
	false/0	6.1 Control Predicates
	file_exists/1	7.14 Calling The Operating System from YAP
	file_exists/2	7.14 Calling The Operating System from YAP
	file_property/2	7.14 Calling The Operating System from YAP
	file_search_path/2	4.2 Changing the Compiler's Behavior
	fileerrors/0	6.6.7 Controlling Input/Output
	findall/3	6.10 Collecting Solutions to a Goal
	findall/4	6.10 Collecting Solutions to a Goal
	flatten/2	7.5 List Manipulation
	float/1	6.3 Predicates on terms
	flush_output/0	6.6.1 Handling Streams and Files
	flush_output/1	6.6.1 Handling Streams and Files
	format_to_chars/3	7.13 Reading From and Writing To Strings
	format_to_chars/4	7.13 Reading From and Writing To Strings
	fragile	7.16 Call Cleanup
	freeze/2	9.2 Coroutining
	frozen/2	9.2 Coroutining
	functor/3	6.3 Predicates on terms
G
	garbage_collect/0	6.1 Control Predicates
	garbage_collect_atoms/0	6.1 Control Predicates
	gc/0	6.1 Control Predicates
	gen_assoc/3	7.2 Association Lists
	get/1	6.6.4 Handling Input/Output of Characters
	get/2	6.6.5 Input/Output Predicates applied to Streams
	get0/1	6.6.4 Handling Input/Output of Characters
	get0/2	6.6.5 Input/Output Predicates applied to Streams
	get_assoc/3	7.2 Association Lists
	get_assoc/5	7.2 Association Lists
	get_atts/2	10.2 Attribute Manipulation
	get_from_heap/4	7.4 Heaps
	get_label/3	7.18 Updatable Binary Trees
	get_mutable/2	6.13 Term Modification
	get_mutable/2	6.13 Term Modification
	get_next_assoc/4	7.2 Association Lists
	get_prev_assoc/4	7.2 Association Lists
	getrand/1	7.9 Random Number Generator
	ground/1	6.3 Predicates on terms
H
	halt/0	6.1 Control Predicates
	halt/1	6.1 Control Predicates
	head_queue/2	7.8 Queues
	heap_size/2	7.4 Heaps
	heap_to_list/2	7.4 Heaps
	host_id/1	7.14 Calling The Operating System from YAP
	host_name/1	7.14 Calling The Operating System from YAP
I
	if/3	6.1 Control Predicates
	incore/1	6.1 Control Predicates
	initialization/0	6.18 Miscellaneous
	integer/1	6.3 Predicates on terms
	is/2	6.5 Arithmetic
	is_assoc/1	7.2 Association Lists
	is_list/1	7.5 List Manipulation
	is_mutable/1	6.13 Term Modification
J
	join_queue/3	7.8 Queues
	jump_queue/3	7.8 Queues
K
	keysort/2	6.4 Comparing Terms
	kill/2	7.14 Calling The Operating System from YAP
L
	last/2	7.5 List Manipulation
	leash/1	19.1 Debugging Predicates
	length/2	6.4 Comparing Terms
	length_queue/2	7.8 Queues
	list_join_queue/3	7.8 Queues
	list_jump_queue/3	7.8 Queues
	list_to_assoc/2	7.2 Association Lists
	list_to_heap/2	7.4 Heaps
	list_to_ord_set/2	7.6 Ordered Sets
	list_to_queue/2	7.8 Queues
	list_to_tree/2	7.18 Updatable Binary Trees
	listing/1	6.7.2 Looking at the Data Base
M
	make_directory/2	7.14 Calling The Operating System from YAP
	make_queue/1	7.8 Queues
	map_assoc/2	7.2 Association Lists
	map_assoc/3	7.2 Association Lists
	map_tree/3	7.18 Updatable Binary Trees
	max_assoc/3	7.2 Association Lists
	max_list/2	7.5 List Manipulation
	member/2	7.5 List Manipulation
	memberchk/2	7.5 List Manipulation
	merge/3	7.6 Ordered Sets
	meta_predicate/1 (directive)	5.4 Meta-Predicates in Modules
	min_assoc/3	7.2 Association Lists
	min_list/2	7.5 List Manipulation
	min_of_heap/3	7.4 Heaps
	min_of_heap/5	7.4 Heaps
	mktemp/2	7.14 Calling The Operating System from YAP
	module/1	5.2 Defining a New Module
	module/2 (directive)	5.2 Defining a New Module
	module/3 (directive)	5.2 Defining a New Module
	multifile/1 (directive)	4.2 Changing the Compiler's Behavior
N
	name/2	6.3 Predicates on terms
	neighbors/3	7.19 Unweighted Graphs
	neighbours/3	7.19 Unweighted Graphs
	nl/0	6.6.4 Handling Input/Output of Characters
	nl/1	6.6.5 Input/Output Predicates applied to Streams
	nodebug/0	19.1 Debugging Predicates
	nofileerrors/0	6.6.7 Controlling Input/Output
	nogc/0	6.1 Control Predicates
	nonvar/1	6.3 Predicates on terms
	nospy/1	19.1 Debugging Predicates
	nospyall/0	19.1 Debugging Predicates
	nth/2	7.5 List Manipulation
	nth/4	7.5 List Manipulation
	nth0/2	7.5 List Manipulation
	nth0/4	7.5 List Manipulation
	number/1	6.3 Predicates on terms
	number_codes/2	6.3 Predicates on terms
	number_to_chars/2	7.13 Reading From and Writing To Strings
	number_to_chars/3	7.13 Reading From and Writing To Strings
	numbervars/3	6.3 Predicates on terms
O
	on_cleanup/1	7.16 Call Cleanup
	op/3	6.18 Miscellaneous
	open/3	6.6.1 Handling Streams and Files
	open_chars_stream/2	7.13 Reading From and Writing To Strings
	ord_add_element/3	7.6 Ordered Sets
	ord_del_element/3	7.6 Ordered Sets
	ord_disjoint/2	7.6 Ordered Sets
	ord_insert/3	7.6 Ordered Sets
	ord_intersect/2	7.6 Ordered Sets
	ord_intersect/3	7.6 Ordered Sets
	ord_intersect/4	7.6 Ordered Sets
	ord_list_to_assoc/2	7.2 Association Lists
	ord_member/2	7.6 Ordered Sets
	ord_seteq/2	7.6 Ordered Sets
	ord_setproduct/3	7.6 Ordered Sets
	ord_subtract/3	7.6 Ordered Sets
	ord_symdiff/3	7.6 Ordered Sets
	ord_union/2	7.6 Ordered Sets
	ord_union/3	7.6 Ordered Sets
	ord_union/4	7.6 Ordered Sets
	ordsubset/2	7.6 Ordered Sets
P
	peek_char/1	6.6.4 Handling Input/Output of Characters
	permutation/2	7.5 List Manipulation
	phrase/2	6.11 Grammar Rules
	phrase/3	6.11 Grammar Rules
	pid/1	7.14 Calling The Operating System from YAP
	popen/3	7.14 Calling The Operating System from YAP
	portray_clause/1	6.7.2 Looking at the Data Base
	portray_clause/2	6.7.2 Looking at the Data Base
	primitive/1	6.3 Predicates on terms
	print/1	6.6.3 Handling Input/Output of Terms
	print/2	6.6.5 Input/Output Predicates applied to Streams
	project_attributes/2	10.4 Displaying Attributes
	project_attributes/2	10.5 Projecting Attributes
	prolog_file_name/1	4.2 Changing the Compiler's Behavior
	prolog_flag/3	6.18 Miscellaneous
	prolog_load_context/2	6.18 Miscellaneous
	prompt/2	6.18 Miscellaneous
	put/1	6.6.4 Handling Input/Output of Characters
	put/2	6.6.5 Input/Output Predicates applied to Streams
	put_assoc/4	7.2 Association Lists
	put_atts/2	10.2 Attribute Manipulation
	put_label/4	7.18 Updatable Binary Trees
Q
	queue_to_list/2	7.8 Queues
R
	random/1	7.9 Random Number Generator
	random/3	7.9 Random Number Generator
	randseq/3	7.9 Random Number Generator
	randset/3	7.9 Random Number Generator
	reachable/3	7.19 Unweighted Graphs
	read/1	6.6.3 Handling Input/Output of Terms
	read/2	6.6.5 Input/Output Predicates applied to Streams
	read_from_chars/2	7.13 Reading From and Writing To Strings
	remove_duplicates/2	7.5 List Manipulation
	rename_file/2	7.14 Calling The Operating System from YAP
	repeat/0	6.1 Control Predicates
	restore/1	4.3 Saving and Loading Prolog States
	reverse/2	7.5 List Manipulation
S
	same_length/2	7.5 List Manipulation
	save_program/1	4.3 Saving and Loading Prolog States
	save_program/2	4.3 Saving and Loading Prolog States
	see/1	6.6.2 Handling Streams and Files
	seeing/1	6.6.2 Handling Streams and Files
	seen/0	6.6.2 Handling Streams and Files
	select/3	7.5 List Manipulation
	serve_queue/3	7.8 Queues
	set_input/1	6.6.1 Handling Streams and Files
	set_output/1	6.6.1 Handling Streams and Files
	set_stream_position/2	6.6.1 Handling Streams and Files
	setrand/1	7.9 Random Number Generator
	shell/0	7.14 Calling The Operating System from YAP
	shell/1	7.14 Calling The Operating System from YAP
	shell/1	7.14 Calling The Operating System from YAP
	simple/1	6.3 Predicates on terms
	skip/1	6.6.4 Handling Input/Output of Characters
	skip/2	6.6.5 Input/Output Predicates applied to Streams
	sleep/1	7.14 Calling The Operating System from YAP
	socket/2	6.6.8 Using Sockets From Yap
	socket/4	6.6.8 Using Sockets From Yap
	socket_accept/2	6.6.8 Using Sockets From Yap
	socket_accept/2	6.6.8 Using Sockets From Yap
	socket_accept/3	6.6.8 Using Sockets From Yap
	socket_bind/2	6.6.8 Using Sockets From Yap
	socket_buffering/4	6.6.8 Using Sockets From Yap
	socket_close/1	6.6.8 Using Sockets From Yap
	socket_connect/3	6.6.8 Using Sockets From Yap
	socket_listen/2	6.6.8 Using Sockets From Yap
	socket_select/5	6.6.8 Using Sockets From Yap
	sort/2	6.4 Comparing Terms
	spy/1	19.1 Debugging Predicates
	stream_select/3	6.6.1 Handling Streams and Files
	sublist/2	7.5 List Manipulation
	subsumes/2	7.15 Utilities On Terms
	subsumes_chk/2	7.15 Utilities On Terms
	suffix/2	7.5 List Manipulation
	sum_list/2	7.5 List Manipulation
	sumlist/2	7.5 List Manipulation
	system/0	7.14 Calling The Operating System from YAP
	system/2	7.14 Calling The Operating System from YAP
	system_predicate/2	6.7.2 Looking at the Data Base
T
	tab/1	6.6.4 Handling Input/Output of Characters
	tab/2	6.6.5 Input/Output Predicates applied to Streams
	tell/1	6.6.2 Handling Streams and Files
	telling/1	6.6.2 Handling Streams and Files
	term_expansion/2	6.11 Grammar Rules
	term_hash/2	7.15 Utilities On Terms
	term_hash/4	7.15 Utilities On Terms
	term_variables/2	7.15 Utilities On Terms
	time_out/3	7.17 Calls With Timeout
	tmpnam/1	7.14 Calling The Operating System from YAP
	told/0	6.6.2 Handling Streams and Files
	top_sort/2	7.19 Unweighted Graphs
	top_sort/3	7.19 Unweighted Graphs
	transitive_closure/2	7.19 Unweighted Graphs
	transpose/3	7.19 Unweighted Graphs
	tree_size/2	7.18 Updatable Binary Trees
	tree_to_list/2	7.18 Updatable Binary Trees
	true/0	6.1 Control Predicates
	ttyget/1	6.6.6 Compatible C-Prolog predicates for Terminal I/O
	ttyget0/1	6.6.6 Compatible C-Prolog predicates for Terminal I/O
	ttynl/0	6.6.6 Compatible C-Prolog predicates for Terminal I/O
	ttyput/1	6.6.6 Compatible C-Prolog predicates for Terminal I/O
	ttyskip/1	6.6.6 Compatible C-Prolog predicates for Terminal I/O
	ttytab/1	6.6.6 Compatible C-Prolog predicates for Terminal I/O
U
	undgraph_del_vertices/3	7.21 Undirected Graphs
	unify_with_occurs_check/2	6.3 Predicates on terms
	unknown_predicate_handler/3	6.2 Handling Undefined Procedures
	update_mutable/2	6.13 Term Modification
	use_module/1	5.3 Using Modules
	use_module/2	5.3 Using Modules
	use_module/3	5.3 Using Modules
V
	var/1	6.3 Predicates on terms
	variant/2	7.15 Utilities On Terms
	verify_attributes/3	10.3 Attributed Unification
	version/1	6.18 Miscellaneous
	vertices/2	7.19 Unweighted Graphs
	vertices_edges_to_ugraph/3	7.19 Unweighted Graphs
W
	wait/2	7.14 Calling The Operating System from YAP
	when/2	9.2 Coroutining
	with_output_to_chars/2	7.13 Reading From and Writing To Strings
	with_output_to_chars/3	7.13 Reading From and Writing To Strings
	with_output_to_chars/4	7.13 Reading From and Writing To Strings
	working_directory/2	7.14 Calling The Operating System from YAP
	write/1	6.6.3 Handling Input/Output of Terms
	write/2	6.6.5 Input/Output Predicates applied to Streams
	write_term/2	6.6.3 Handling Input/Output of Terms
	write_term/3	6.6.5 Input/Output Predicates applied to Streams
	write_to_chars/2	7.13 Reading From and Writing To Strings
	write_to_chars/3	7.13 Reading From and Writing To Strings
	writeq/1	6.6.3 Handling Input/Output of Terms
	writeq/2	6.6.5 Input/Output Predicates applied to Streams

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23.2.3 Yap predicates not strictly compatible with SICStus Prolog

These are YAP built-ins that are also available in SICStus Prolog, but that are not fully compatible:

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	Index Entry	Section
[
	[-]/1	4.1 Program loading and updating
	[]/1	4.1 Program loading and updating
A
	abolish/1	6.7.1 Modification of the Data Base
	abolish/2	6.7.1 Modification of the Data Base
	assert/1	6.7.1 Modification of the Data Base
	assert/2	6.7.3 Using Data Base References
	asserta/1	6.7.1 Modification of the Data Base
	asserta/2	6.7.3 Using Data Base References
	assertz/1	6.7.1 Modification of the Data Base
	assertz/2	6.7.3 Using Data Base References
	atom_chars/2	6.3 Predicates on terms
B
	bagof/3	6.10 Collecting Solutions to a Goal
C
	clause/2	6.7.1 Modification of the Data Base
	clause/3	6.7.1 Modification of the Data Base
	close/2	6.6.1 Handling Streams and Files
D
	debug/0	19.1 Debugging Predicates
	dynamic/1	6.7 Using the Clausal Data Base
E
	erase/1	6.8 Internal Data Base
	erased/1	6.8 Internal Data Base
F
	format/2	6.6.3 Handling Input/Output of Terms
	format/3	6.6.3 Handling Input/Output of Terms
I
	instance/2	6.8 Internal Data Base
L
	listing/0	6.7.2 Looking at the Data Base
N
	nth_clause/3	6.7.1 Modification of the Data Base
	nth_recorded/3	6.8 Internal Data Base
	number_chars/2	6.3 Predicates on terms
O
	open/4	6.6.1 Handling Streams and Files
P
	predicate_property/2	6.7.2 Looking at the Data Base
	prolog_initialization/1	6.18 Miscellaneous
R
	read_term/2	6.6.3 Handling Input/Output of Terms
	read_term/3	6.6.5 Input/Output Predicates applied to Streams
	recorda/3	6.8 Internal Data Base
	recordaifnot/3	6.8 Internal Data Base
	recorded/3	6.8 Internal Data Base
	recordz/3	6.8 Internal Data Base
	retract/1	6.7.1 Modification of the Data Base
	retract/2	6.7.3 Using Data Base References
	retractall/1	6.7.1 Modification of the Data Base
S
	setof/3	6.10 Collecting Solutions to a Goal
	statistics/0	6.18 Miscellaneous
	statistics/2	6.18 Miscellaneous
U
	unknown/2	6.2 Handling Undefined Procedures
V
	version/0	6.18 Miscellaneous

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23.2.4 Yap predicates not available in SICStus Prolog

These are YAP built-ins not available in SICStus Prolog.

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	Index Entry	Section
\
	\=/2	6.3 Predicates on terms
A
	abolish_table/1	16. Tabling
	add_to_array_element/4	6.16 Arrays
	alarm/3	6.12 Access to Operating System Functionality
	all/3	6.10 Collecting Solutions to a Goal
	always_prompt_user/0	6.6.7 Controlling Input/Output
	append/3	8. SWI-Prolog Emulation
	array/2	6.16 Arrays
	array_element/3	6.16 Arrays
	assert_static/1	6.7.1 Modification of the Data Base
	asserta_static/1	6.7.1 Modification of the Data Base
	assertz_static/1	6.7.1 Modification of the Data Base
	atom_concat/3	6.3 Predicates on terms
	atom_length/2	6.3 Predicates on terms
	atomic_concat/2	6.3 Predicates on terms
	attr_unify_hook/2	8.3 hProlog and SWI-Prolog Attributed Variables
	attvar/1	10.3 Attributed Unification
	avl_insert/4	7.3 AVL Trees
	avl_lookup/3	7.3 AVL Trees
B
	b_getval/2	8.4 SWI Global variables
	b_setval/2	8.4 SWI Global variables
	between/3	8. SWI-Prolog Emulation
C
	call_count_data/0	6.15 Counting Calls
	call_count_data/3	6.15 Counting Calls
	call_count_data/3	6.15 Counting Calls
	call_with_args/n	6.1 Control Predicates
	catch/3	6.1 Control Predicates
	cd/1	6.12 Access to Operating System Functionality
	chdir/1	8. SWI-Prolog Emulation
	checklist/2	7.1 Apply Macros
	checknodes/3	7.1 Apply Macros
	close_static_array/1	6.16 Arrays
	compile/1 (directive)	4.1 Program loading and updating
	compile_expressions/0	4.2 Changing the Compiler's Behavior
	concat_atom/2	8. SWI-Prolog Emulation
	concat_atom/3	8. SWI-Prolog Emulation
	consult/1	4.1 Program loading and updating
	convlist/3	7.1 Apply Macros
	copy_term_nat/2	8.3.1 Special Purpose SWI Predicates for Attributes
	current_mutex/3	14.4 Thread Synchronisation
	current_prolog_flag/2	6.18 Miscellaneous
	current_thread/2	14.2 Monitoring Threads
D
	del_attr/2	8.3 hProlog and SWI-Prolog Attributed Variables
	dgraph_add_edges/3	7.20 Directed Graphs
	dgraph_add_vertices/3	7.20 Directed Graphs
	dgraph_complement/2	7.20 Directed Graphs
	dgraph_compose/3	7.20 Directed Graphs
	dgraph_del_edges/3	7.20 Directed Graphs
	dgraph_edges/2	7.20 Directed Graphs
	dgraph_neighbors/3	7.20 Directed Graphs
	dgraph_neighbours/3	7.20 Directed Graphs
	dgraph_new/1	7.20 Directed Graphs
	dgraph_symmetric_closure/2	7.20 Directed Graphs
	dgraph_top_sort/2	7.20 Directed Graphs
	dgraph_transitive_closure/2	7.20 Directed Graphs
	dgraph_transpose/2	7.20 Directed Graphs
	dgraph_vertices/2	7.20 Directed Graphs
	do_not_compile_expressions/0	4.2 Changing the Compiler's Behavior
	dynamic_predicate/2	6.7 Using the Clausal Data Base
E
	eraseall/1	6.8 Internal Data Base
	exists/1	6.6.7 Controlling Input/Output
	expand_exprs/2	4.2 Changing the Compiler's Behavior
F
	forall/2	8.2 Forall
G
	get_attr/3	8.3 hProlog and SWI-Prolog Attributed Variables
	get_attrs/2	8.3.1 Special Purpose SWI Predicates for Attributes
	get_byte/1	6.6.4 Handling Input/Output of Characters
	get_byte/2	6.6.5 Input/Output Predicates applied to Streams
	get_char/1	6.6.4 Handling Input/Output of Characters
	get_char/2	6.6.5 Input/Output Predicates applied to Streams
	get_code/1	6.6.4 Handling Input/Output of Characters
	get_code/2	6.6.5 Input/Output Predicates applied to Streams
	get_value/2	6.8 Internal Data Base
	getcwd/1	6.12 Access to Operating System Functionality
	goal_expansion/3	6.11 Grammar Rules
	grow_heap/1	6.1 Control Predicates
	grow_stack/1	6.1 Control Predicates
H
	hide/1	4.2 Changing the Compiler's Behavior
	hide_predicate/1	4.2 Changing the Compiler's Behavior
I
	initialization/1 (directive)	4.2 Changing the Compiler's Behavior
	is_rbtree/1	7.10 Red-Black Trees
	is_tabled/1	16. Tabling
	is_tabled/1	16. Tabling
K
	key_statistics/3	6.8 Internal Data Base
	key_statistics/4	6.8 Internal Data Base
L
	library_directory/1	4.2 Changing the Compiler's Behavior
	library_directory/1	4.2 Changing the Compiler's Behavior
	list_concat/2	7.5 List Manipulation
	list_to_rbtree/2	7.10 Red-Black Trees
	list_to_rbtree/2	7.10 Red-Black Trees
M
	mapargs/3	7.1 Apply Macros
	maplist/2	8.1 Invoking Predicates on all Members of a List
	maplist/3	7.1 Apply Macros
	maplist/3	8.1 Invoking Predicates on all Members of a List
	maplist/4	8.1 Invoking Predicates on all Members of a List
	mapnodes/3	7.1 Apply Macros
	mktime/2	7.14 Calling The Operating System from YAP
	mutex_create/1	14.4 Thread Synchronisation
	mutex_destroy/1	14.4 Thread Synchronisation
	mutex_lock/1	14.4 Thread Synchronisation
	mutex_statistics/0	14.2 Monitoring Threads
	mutex_trylock/1	14.4 Thread Synchronisation
	mutex_unlock/1	14.4 Thread Synchronisation
	mutex_unlock_all/0	14.4 Thread Synchronisation
N
	nb_current/2	8.4 SWI Global variables
	nb_delete/1	8.4 SWI Global variables
	nb_getval/2	8.4 SWI Global variables
	nb_setval/2	8.4 SWI Global variables
	no_source/0	4.2 Changing the Compiler's Behavior
	not/1	6.1 Control Predicates
	nth1/3	8. SWI-Prolog Emulation
	number_atom/2	6.3 Predicates on terms
O
	on_signal/3	6.12 Access to Operating System Functionality
	once/1	6.1 Control Predicates
P
	path/1	4.2 Changing the Compiler's Behavior
	path/1	4.2 Changing the Compiler's Behavior
	path/1	4.2 Changing the Compiler's Behavior
	peek_byte/1	6.6.4 Handling Input/Output of Characters
	peek_byte/2	6.6.5 Input/Output Predicates applied to Streams
	peek_char/2	6.6.5 Input/Output Predicates applied to Streams
	peek_code/1	6.6.4 Handling Input/Output of Characters
	peek_code/2	6.6.5 Input/Output Predicates applied to Streams
	profile_data/3	6.14 Profiling Prolog Programs
	profiled_reset/0	6.14 Profiling Prolog Programs
	public/1 (directive)	4.2 Changing the Compiler's Behavior
	put_attr/3	8.3 hProlog and SWI-Prolog Attributed Variables
	put_attrs/2	8.3.1 Special Purpose SWI Predicates for Attributes
	put_byte/1	6.6.4 Handling Input/Output of Characters
	put_byte/2	6.6.5 Input/Output Predicates applied to Streams
	put_char/1	6.6.4 Handling Input/Output of Characters
	put_char/2	6.6.5 Input/Output Predicates applied to Streams
	put_code/1	6.6.4 Handling Input/Output of Characters
	put_code/2	6.6.5 Input/Output Predicates applied to Streams
	putenv/2	6.12 Access to Operating System Functionality
R
	rannum/1	7.7 Pseudo Random Number Integer Generator
	ranstart/0	7.7 Pseudo Random Number Integer Generator
	ranstart/1	7.7 Pseudo Random Number Integer Generator
	ranunif/2	7.7 Pseudo Random Number Integer Generator
	rb_apply/4	7.10 Red-Black Trees
	rb_clone/3	7.10 Red-Black Trees
	rb_del_max/4	7.10 Red-Black Trees
	rb_del_min/4	7.10 Red-Black Trees
	rb_delete/3	7.10 Red-Black Trees
	rb_delete/4	7.10 Red-Black Trees
	rb_empty/1	7.10 Red-Black Trees
	rb_insert/4	7.10 Red-Black Trees
	rb_keys/2	7.10 Red-Black Trees
	rb_lookup/3	7.10 Red-Black Trees
	rb_lookupall/3	7.10 Red-Black Trees
	rb_map/3	7.10 Red-Black Trees
	rb_max/3	7.10 Red-Black Trees
	rb_min/3	7.10 Red-Black Trees
	rb_new/1	7.10 Red-Black Trees
	rb_next/4	7.10 Red-Black Trees
	rb_partial_map/4	7.10 Red-Black Trees
	rb_previous/4	7.10 Red-Black Trees
	rb_size/2	7.10 Red-Black Trees
	rb_update/4	7.10 Red-Black Trees
	rb_visit/2	7.10 Red-Black Trees
	reconsult/1	4.1 Program loading and updating
	recorda/3	6.8 Internal Data Base
	recorda_at/3	6.8 Internal Data Base
	recordaifnot/3	6.8 Internal Data Base
	recordz_at/3	6.8 Internal Data Base
	recordzifnot/3	6.8 Internal Data Base
	regexp/3	7.11 Regular Expressions
	regexp/4	7.11 Regular Expressions
	remove_from_path/1	4.2 Changing the Compiler's Behavior
	rename/2	6.12 Access to Operating System Functionality
	reset_op_counters/0	18. Profiling the Abstract Machine
	resize_static_array/3	6.16 Arrays
S
	save/1	4.3 Saving and Loading Prolog States
	save/2	4.3 Saving and Loading Prolog States
	selectlist/3	7.1 Apply Macros
	set_prolog_flag/2	6.18 Miscellaneous
	set_value/2	6.8 Internal Data Base
	setarg/3n	6.13 Term Modification
	setenv/2	8. SWI-Prolog Emulation
	sh/0	6.12 Access to Operating System Functionality
	show_op_counters/1	18. Profiling the Abstract Machine
	show_ops_by_group/1	18. Profiling the Abstract Machine
	show_table/1	16. Tabling
	source/0	4.2 Changing the Compiler's Behavior
	source_mode/2	4.2 Changing the Compiler's Behavior
	splay_access/5	7.12 Splay Trees
	splay_delete/4	7.12 Splay Trees
	splay_init/3	7.12 Splay Trees
	splay_insert/4	7.12 Splay Trees
	splay_join/3	7.12 Splay Trees
	splay_split/5	7.12 Splay Trees
	spy_write/2	19.1 Debugging Predicates
	srandom/1	6.5 Arithmetic
	start_low_level_trace/0	17. Tracing at Low Level
	start_low_level_trace/0	17. Tracing at Low Level
	static_array/3	6.16 Arrays
	static_array/3	6.16 Arrays
	static_array_location/4	6.16 Arrays
	static_array_properties/3	6.16 Arrays
	static_array_to_term/3	6.16 Arrays
	stream_property/2	6.6.1 Handling Streams and Files
	style_check/1	4.2 Changing the Compiler's Behavior
	style_check/1	4.2 Changing the Compiler's Behavior
	sub_atom/5	6.3 Predicates on terms
	sumargs/4	7.1 Apply Macros
	sumlist/4	7.1 Apply Macros
	sumnodes/4	7.1 Apply Macros
	system/1	6.12 Access to Operating System Functionality
T
	table_statistics/1	16. Tabling
	term_to_atom/2	8. SWI-Prolog Emulation
	thread_at_exit/1	14.1 Creating and Destroying Prolog Threads
	thread_create/3	14.1 Creating and Destroying Prolog Threads
	thread_detach/1	14.1 Creating and Destroying Prolog Threads
	thread_exit/1	14.1 Creating and Destroying Prolog Threads
	thread_get_message/1	14.3.1 Message Queues
	thread_get_message/2	14.3.1 Message Queues
	thread_join/2	14.1 Creating and Destroying Prolog Threads
	thread_local/1 (directive)	14.3.3 Threads and Dynamic Predicates
	thread_message_queue_create/1	14.3.1 Message Queues
	thread_message_queue_destroy/1	14.3.1 Message Queues
	thread_peek_message/1	14.3.1 Message Queues
	thread_self/1	14.1 Creating and Destroying Prolog Threads
	thread_send_message/2	14.3.1 Message Queues
	thread_setconcurrency/2	14.1 Creating and Destroying Prolog Threads
	thread_signal/2	14.3.2 Signalling Threads
	thread_statistics/3	14.2 Monitoring Threads
	throw/1	6.1 Control Predicates
U
	undgraph_add_edges/3	7.21 Undirected Graphs
	undgraph_add_vertices/3	7.21 Undirected Graphs
	undgraph_complement/2	7.21 Undirected Graphs
	undgraph_del_edges/3	7.21 Undirected Graphs
	undgraph_edges/2	7.21 Undirected Graphs
	undgraph_neighbors/3	7.21 Undirected Graphs
	undgraph_neighbours/3	7.21 Undirected Graphs
	undgraph_new/1	7.21 Undirected Graphs
	undgraph_vertices/2	7.21 Undirected Graphs
	unhide/1	4.2 Changing the Compiler's Behavior
	unix/1	6.12 Access to Operating System Functionality
	update_array/3	6.16 Arrays
V
	variable_in_term/2	7.15 Utilities On Terms
W
	with_mutex/2	14.4 Thread Synchronisation
	working_directory/2	8. SWI-Prolog Emulation
	write_depth/2	6.6.7 Controlling Input/Output
	write_depth/3	6.6.7 Controlling Input/Output
Y
	yap_flag/2	6.18 Miscellaneous

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23.3 Compatibility with the ISO Prolog standard

The Prolog standard was developed by ISO/IEC JTC1/SC22/WG17, the international standardization working group for the programming language Prolog. The book "Prolog: The Standard" by Deransart, Ed-Dbali and Cervoni gives a complete description of this standard. Development in YAP from YAP4.1.6 onwards have striven at making YAP compatible with ISO Prolog. As such:

• YAP now supports all of the built-ins required by the ISO-standard, and,
• Error-handling is as required by the standard.

YAP by default is not fully ISO standard compliant. You can set the language flag to iso to obtain very good compatibility. Setting this flag changes the following:

• By default, YAP uses "immediate update semantics" for its database, and not "logical update semantics", as per the standard, (see section Compatibility with the Quintus and SICStus Prolog systems). This affects assert/1, retract/1, and friends.

  Calling set_prolog_flag(update_semantics,logical) will switch YAP to use logical update semantics.

• By default, YAP implements the atom_chars/2 (see section Predicates on terms), and number_chars/2, (see section Predicates on terms), built-ins as per the original Quintus Prolog definition, and not as per the ISO definition.

  Calling set_prolog_flag(to_chars_mode,iso) will switch YAP to use the ISO definition for atom_chars/2 and number_chars/2.

• By default, YAP fails on undefined predicates. To follow the ISO Prolog standard use:

  :- set_prolog_flag(unknown,error).

• By default, YAP allows executable goals in directives. In ISO mode most directives can only be called from top level (the exceptions are set_prolog_flag/2 and op/3).
• Error checking for meta-calls under ISO Prolog mode is stricter than by default.
• The strict_iso flag automatically enables the ISO Prolog standard. This feature should disable all features not present in the standard.

The following incompatibilities between YAP and the ISO standard are known to still exist:

• Currently, YAP does not handle overflow errors in integer operations, and handles floating-point errors only in some architectures. Otherwise, YAP follows IEEE arithmetic.

Please inform the authors on other incompatibilities that may still exist.

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A. Summary of Yap Predefined Operators

The Prolog syntax caters for operators of three main kinds:

• prefix;
• infix;
• postfix.

Each operator has precedence in the range 1 to 1200, and this precedence is used to disambiguate expressions where the structure of the term denoted is not made explicit using brackets. The operator of higher precedence is the main functor.

If there are two operators with the highest precedence, the ambiguity is solved analyzing the types of the operators. The possible infix types are: xfx, xfy, yfx.

With an operator of type xfx both sub-expressions must have lower precedence than the operator itself, unless they are bracketed (which assigns to them zero precedence). With an operator type xfy only the left-hand sub-expression must have lower precedence. The opposite happens for yfx type.

A prefix operator can be of type fx or fy, and a postfix operator, xf, yf. The meaning of the notation is analogous to the above.

a + b * c

means

a + (b * c)

as + and * have the following types and precedences:

:-op(500,yfx,'+'). :-op(400,yfx,'*').

Now defining

:-op(700,xfy,'++'). :-op(700,xfx,'=:='). a ++ b =:= c

means

a ++ (b =:= c)

The following is the list of the declarations of the predefined operators:

:-op(1200,fx,['?-', ':-']). :-op(1200,xfx,[':-','-->']). :-op(1150,fx,[block,dynamic,mode,public,multifile,meta_predicate, sequential,table,initialization]). :-op(1100,xfy,[';','|']). :-op(1050,xfy,->). :-op(1000,xfy,','). :-op(999,xfy,'.'). :-op(900,fy,['\+', not]). :-op(900,fx,[nospy, spy]). :-op(700,xfx,[@>=,@=<,@<,@>,<,=,>,=:=,=\=,\==,>=,=<,==,\=,=..,is]). :-op(500,yfx,['\/','/\','+','-']). :-op(500,fx,['+','-']). :-op(400,yfx,['<<','>>','//','*','/']). :-op(300,xfx,mod). :-op(200,xfy,['^','**']). :-op(50,xfx,same).

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Predicate Index

Jump to:	!   ,   -   ;   <   =   >   @   [   \   A   B   C   D   E   F   G   H   I   J   K   L   M   N   O   P   Q   R   S   T   U   V   W   Y

	Index Entry	Section
!
	!/0	6.1 Control Predicates
,
	,/2	6.1 Control Predicates
-
	->/2	6.1 Control Predicates
;
	;/2	6.1 Control Predicates
<
	</2	6.5 Arithmetic
=
	=../2	6.3 Predicates on terms
	=/2	6.3 Predicates on terms
	=:=/2	6.5 Arithmetic
	=</2	6.5 Arithmetic
	==/2	6.4 Comparing Terms
	=\=/2	6.5 Arithmetic
>
	>/2	6.5 Arithmetic
	>=/2	6.5 Arithmetic
@
	@</2	6.4 Comparing Terms
	@=</2	6.4 Comparing Terms
	@>/2	6.4 Comparing Terms
	@>=/2	6.4 Comparing Terms
[
	[-]/1	4.1 Program loading and updating
	[]/1	4.1 Program loading and updating
\
	\+/1	6.1 Control Predicates
	\=/2	6.3 Predicates on terms
	\==/2	6.4 Comparing Terms
A
	abolish/1	6.7.1 Modification of the Data Base
	abolish/2	6.7.1 Modification of the Data Base
	abolish_table/1	16. Tabling
	abort/0	6.1 Control Predicates
	absolute_file_name/2	6.6.1 Handling Streams and Files
	add_edges/3	7.19 Unweighted Graphs
	add_to_array_element/4	6.16 Arrays
	add_to_heap/4	7.4 Heaps
	add_to_path/1	4.2 Changing the Compiler's Behavior
	add_to_path/2	4.2 Changing the Compiler's Behavior
	add_vertices/3	7.19 Unweighted Graphs
	alarm/3	6.12 Access to Operating System Functionality
	all/3	6.10 Collecting Solutions to a Goal
	always_prompt_user/0	6.6.7 Controlling Input/Output
	append/3	7.5 List Manipulation
	append/3	8. SWI-Prolog Emulation
	arg/3	6.3 Predicates on terms
	argv (yap_flag/2 option)	6.18 Miscellaneous
	array/2	6.16 Arrays
	array_element/3	6.16 Arrays
	assert/1	6.7.1 Modification of the Data Base
	assert/2	6.7.3 Using Data Base References
	assert_static/1	6.7.1 Modification of the Data Base
	asserta/1	6.7.1 Modification of the Data Base
	asserta/2	6.7.3 Using Data Base References
	asserta_static/1	6.7.1 Modification of the Data Base
	assertz/1	6.7.1 Modification of the Data Base
	assertz/2	6.7.3 Using Data Base References
	assertz_static/1	6.7.1 Modification of the Data Base
	assoc_to_list/2	7.2 Association Lists
	at_end_of_stream/0	6.6.1 Handling Streams and Files
	at_end_of_stream/1	6.6.1 Handling Streams and Files
	atom/1	6.3 Predicates on terms
	atom_chars/2	6.3 Predicates on terms
	atom_codes/2	6.3 Predicates on terms
	atom_concat/2	6.3 Predicates on terms
	atom_concat/3	6.3 Predicates on terms
	atom_concat/3	6.3 Predicates on terms
	atom_length/2	6.3 Predicates on terms
	atom_to_chars/2	7.13 Reading From and Writing To Strings
	atom_to_chars/3	7.13 Reading From and Writing To Strings
	atomic/1	6.3 Predicates on terms
	atomic_concat/2	6.3 Predicates on terms
	attr_unify_hook/2	8.3 hProlog and SWI-Prolog Attributed Variables
	attribute/1 (declaration)	10.1 Attribute Declarations
	attribute_goal/2	10.4 Displaying Attributes
	attvar/1	10.3 Attributed Unification
	avl_insert/4	7.3 AVL Trees
	avl_lookup/3	7.3 AVL Trees
B
	b_getval/2	8.4 SWI Global variables
	b_setval/2	8.4 SWI Global variables
	bagof/3	6.10 Collecting Solutions to a Goal
	bb_delete/2	6.9 The Blackboard
	bb_get/2	6.9 The Blackboard
	bb_put/2	6.9 The Blackboard
	bb_update/3	6.9 The Blackboard
	between/3	8. SWI-Prolog Emulation
	bounded (yap_flag/2 option)	6.18 Miscellaneous
	break/0	6.1 Control Predicates
C
	C/3	6.11 Grammar Rules
	call/1	6.1 Control Predicates
	call_cleanup/1	7.16 Call Cleanup
	call_cleanup/2	7.16 Call Cleanup
	call_count_data/0	6.15 Counting Calls
	call_count_data/3	6.15 Counting Calls
	call_count_data/3	6.15 Counting Calls
	call_counting (yap_flag/2 option)	6.18 Miscellaneous
	call_residue/2	9.2 Coroutining
	call_with_args/n	6.1 Control Predicates
	callable/1	6.3 Predicates on terms
	catch/3	6.1 Control Predicates
	cd/1	6.12 Access to Operating System Functionality
	char_code/2	6.3 Predicates on terms
	char_conversion (yap_flag/2 option)	6.18 Miscellaneous
	char_conversion/2	6.6.3 Handling Input/Output of Terms
	character_escapes (yap_flag/2 option)	6.18 Miscellaneous
	chdir/1	8. SWI-Prolog Emulation
	checklist/2	7.1 Apply Macros
	checknodes/3	7.1 Apply Macros
	clause/2	6.7.1 Modification of the Data Base
	clause/3	6.7.1 Modification of the Data Base
	cleanup_all/0	7.16 Call Cleanup
	close/1	6.6.1 Handling Streams and Files
	close/2	6.6.1 Handling Streams and Files
	close_static_array/1	6.16 Arrays
	compare/3	6.4 Comparing Terms
	compile/1	4.1 Program loading and updating
	compile_expressions/0	4.2 Changing the Compiler's Behavior
	complement/2	7.19 Unweighted Graphs
	compose/3	7.19 Unweighted Graphs
	compound/1	6.3 Predicates on terms
	concat_atom/2	8. SWI-Prolog Emulation
	concat_atom/3	8. SWI-Prolog Emulation
	consult/1	4.1 Program loading and updating
	convlist/3	7.1 Apply Macros
	copy_term/2	6.3 Predicates on terms
	copy_term_nat/2	8.3.1 Special Purpose SWI Predicates for Attributes
	cputime (statistics/2 option)	6.18 Miscellaneous
	create_mutable/2	6.13 Term Modification
	current_atom/1	6.7.2 Looking at the Data Base
	current_char_conversion/2	6.6.3 Handling Input/Output of Terms
	current_input/1	6.6.1 Handling Streams and Files
	current_key/2	6.8 Internal Data Base
	current_module/1	6.17 Predicate Information
	current_module/2	6.17 Predicate Information
	current_mutex/3	14.4 Thread Synchronisation
	current_op/3	6.18 Miscellaneous
	current_output/1	6.6.1 Handling Streams and Files
	current_predicate/1	6.7.2 Looking at the Data Base
	current_predicate/2	6.7.2 Looking at the Data Base
	current_prolog_flag/2	6.18 Miscellaneous
	current_stream/3	6.6.1 Handling Streams and Files
	current_thread/2	14.2 Monitoring Threads
	cyclic_term/1	7.15 Utilities On Terms
	cyclic_term/1	7.15 Utilities On Terms
D
	datime/1	7.14 Calling The Operating System from YAP
	db_reference/1C	6.3 Predicates on terms
	debug (yap_flag/2 option)	6.18 Miscellaneous
	debug/0	19.1 Debugging Predicates
	debugging/0	19.1 Debugging Predicates
	del_assoc/4	7.2 Association Lists
	del_attr/2	8.3 hProlog and SWI-Prolog Attributed Variables
	del_edges/3	7.19 Unweighted Graphs
	del_max_assoc/4	7.2 Association Lists
	del_min_assoc/4	7.2 Association Lists
	del_vertices/3	7.19 Unweighted Graphs
	delete/3	7.5 List Manipulation
	delete_file/1	7.14 Calling The Operating System from YAP
	delete_file/2	7.14 Calling The Operating System from YAP
	dgraph_add_edges/3	7.20 Directed Graphs
	dgraph_add_vertices/3	7.20 Directed Graphs
	dgraph_complement/2	7.20 Directed Graphs
	dgraph_compose/3	7.20 Directed Graphs
	dgraph_del_edges/3	7.20 Directed Graphs
	dgraph_del_vertices/3	7.20 Directed Graphs
	dgraph_edges/2	7.20 Directed Graphs
	dgraph_neighbors/3	7.20 Directed Graphs
	dgraph_neighbours/3	7.20 Directed Graphs
	dgraph_new/1	7.20 Directed Graphs
	dgraph_symmetric_closure/2	7.20 Directed Graphs
	dgraph_top_sort/2	7.20 Directed Graphs
	dgraph_transitive_closure/2	7.20 Directed Graphs
	dgraph_transpose/2	7.20 Directed Graphs
	dgraph_vertices/2	7.20 Directed Graphs
	dif/2	9.2 Coroutining
	directory (prolog_load_context/2 option)	6.18 Miscellaneous
	directory_files/2	7.14 Calling The Operating System from YAP
	discontiguous/1 (directive)	4.2 Changing the Compiler's Behavior
	discontiguous_warnings (yap_flag/2 option)	6.18 Miscellaneous
	discontiguous_warnings (yap_flag/2 option)	6.18 Miscellaneous
	display/1	6.6.3 Handling Input/Output of Terms
	display/1	6.6.3 Handling Input/Output of Terms
	display/1	6.6.5 Input/Output Predicates applied to Streams
	display/2	6.6.5 Input/Output Predicates applied to Streams
	do_not_compile_expressions/0	4.2 Changing the Compiler's Behavior
	dollar_as_lower_case (yap_flag/2 option)	6.18 Miscellaneous
	double_quotes (yap_flag/2 option)	6.18 Miscellaneous
	dynamic/1	6.7 Using the Clausal Data Base
	dynamic_predicate/2	6.7 Using the Clausal Data Base
E
	edges/2	7.19 Unweighted Graphs
	empty_assoc/1	7.2 Association Lists
	empty_heap/1	7.4 Heaps
	empty_queue/1	7.8 Queues
	ensure_loaded/1	4.1 Program loading and updating
	environ/2	6.12 Access to Operating System Functionality
	environ/2	7.14 Calling The Operating System from YAP
	erase/1	6.8 Internal Data Base
	eraseall/1	6.8 Internal Data Base
	erased/1	6.8 Internal Data Base
	exec/3	7.14 Calling The Operating System from YAP
	exists/1	6.6.7 Controlling Input/Output
	expand_exprs/2	4.2 Changing the Compiler's Behavior
	expand_term/2	6.11 Grammar Rules
F
	fail/0	6.1 Control Predicates
	false/0	6.1 Control Predicates
	fast (yap_flag/2 option)	6.18 Miscellaneous
	file (prolog_load_context/2 option)	6.18 Miscellaneous
	file (prolog_load_context/2 option)	6.18 Miscellaneous
	file (prolog_load_context/2 option)	6.18 Miscellaneous
	file (prolog_load_context/2 option)	6.18 Miscellaneous
	file_exists/1	7.14 Calling The Operating System from YAP
	file_exists/2	7.14 Calling The Operating System from YAP
	file_property/2	7.14 Calling The Operating System from YAP
	file_search_path/2	4.2 Changing the Compiler's Behavior
	fileerrors (yap_flag/2 option)	6.18 Miscellaneous
	fileerrors/0	6.6.7 Controlling Input/Output
	findall/3	6.10 Collecting Solutions to a Goal
	findall/4	6.10 Collecting Solutions to a Goal
	flatten/2	7.5 List Manipulation
	float/1	6.3 Predicates on terms
	float_format (yap_flag/2 option)	6.18 Miscellaneous
	flush_output/0	6.6.1 Handling Streams and Files
	flush_output/1	6.6.1 Handling Streams and Files
	forall/2	8.2 Forall
	format/2	6.6.3 Handling Input/Output of Terms
	format/3	6.6.3 Handling Input/Output of Terms
	format_to_chars/3	7.13 Reading From and Writing To Strings
	format_to_chars/4	7.13 Reading From and Writing To Strings
	fragile	7.16 Call Cleanup
	freeze/2	9.2 Coroutining
	frozen/2	9.2 Coroutining
	functor/3	6.3 Predicates on terms
G
	garbage_collect/0	6.1 Control Predicates
	garbage_collect_atoms/0	6.1 Control Predicates
	garbage_collection (statistics/2 option)	6.18 Miscellaneous
	gc (yap_flag/2 option)	6.18 Miscellaneous
	gc/0	6.1 Control Predicates
	gc_margin (yap_flag/2 option)	6.18 Miscellaneous
	gc_trace (yap_flag/2 option)	6.18 Miscellaneous
	gen_assoc/3	7.2 Association Lists
	get/1	6.6.4 Handling Input/Output of Characters
	get/2	6.6.5 Input/Output Predicates applied to Streams
	get0/1	6.6.4 Handling Input/Output of Characters
	get0/2	6.6.5 Input/Output Predicates applied to Streams
	get_assoc/3	7.2 Association Lists
	get_assoc/5	7.2 Association Lists
	get_attr/3	8.3 hProlog and SWI-Prolog Attributed Variables
	get_attrs/2	8.3.1 Special Purpose SWI Predicates for Attributes
	get_atts/2	10.2 Attribute Manipulation
	get_byte/1	6.6.4 Handling Input/Output of Characters
	get_byte/2	6.6.5 Input/Output Predicates applied to Streams
	get_char/1	6.6.4 Handling Input/Output of Characters
	get_char/2	6.6.5 Input/Output Predicates applied to Streams
	get_code/1	6.6.4 Handling Input/Output of Characters
	get_code/2	6.6.5 Input/Output Predicates applied to Streams
	get_from_heap/4	7.4 Heaps
	get_label/3	7.18 Updatable Binary Trees
	get_mutable/2	6.13 Term Modification
	get_mutable/2	6.13 Term Modification
	get_next_assoc/4	7.2 Association Lists
	get_prev_assoc/4	7.2 Association Lists
	get_value/2	6.8 Internal Data Base
	getcwd/1	6.12 Access to Operating System Functionality
	getrand/1	7.9 Random Number Generator
	global_stack (statistics/2 option)	6.18 Miscellaneous
	goal_expansion/3	6.11 Grammar Rules
	ground/1	6.3 Predicates on terms
	grow_stack/1	6.1 Control Predicates
H
	halt/0	6.1 Control Predicates
	halt/1	6.1 Control Predicates
	head_queue/2	7.8 Queues
	heap (statistics/2 option)	6.18 Miscellaneous
	heap_size/2	7.4 Heaps
	heap_to_list/2	7.4 Heaps
	hide/1	4.2 Changing the Compiler's Behavior
	hide_predicate/1	4.2 Changing the Compiler's Behavior
	host_id/1	7.14 Calling The Operating System from YAP
	host_name/1	7.14 Calling The Operating System from YAP
	host_type (yap_flag/2 option)	6.18 Miscellaneous
I
	if/3	6.1 Control Predicates
	include/1 (directive)	4.1 Program loading and updating
	incore/1	6.1 Control Predicates
	index (yap_flag/2 option)	6.18 Miscellaneous
	informational_messages (yap_flag/2 option)	6.18 Miscellaneous
	initialization/0	6.18 Miscellaneous
	initialization/1 (directive)	4.2 Changing the Compiler's Behavior
	instance/2	6.8 Internal Data Base
	integer/1	6.3 Predicates on terms
	integer_rounding_function (yap_flag/2 option)	6.18 Miscellaneous
	is/2	6.5 Arithmetic
	is_assoc/1	7.2 Association Lists
	is_list/1	7.5 List Manipulation
	is_mutable/1	6.13 Term Modification
	is_rbtree/1	7.10 Red-Black Trees
	is_tabled/1	16. Tabling
	is_tabled/1	16. Tabling
J
	join_queue/3	7.8 Queues
	jump_queue/3	7.8 Queues
K
	key_statistics/3	6.8 Internal Data Base
	key_statistics/4	6.8 Internal Data Base
	keysort/2	6.4 Comparing Terms
	kill/2	7.14 Calling The Operating System from YAP
L
	language (yap_flag/2 option)	6.18 Miscellaneous
	last/2	7.5 List Manipulation
	leash/1	19.1 Debugging Predicates
	length/2	6.4 Comparing Terms
	length_queue/2	7.8 Queues
	library_directory/1	4.2 Changing the Compiler's Behavior
	library_directory/1	4.2 Changing the Compiler's Behavior
	list_concat/2	7.5 List Manipulation
	list_join_queue/3	7.8 Queues
	list_jump_queue/3	7.8 Queues
	list_to_assoc/2	7.2 Association Lists
	list_to_heap/2	7.4 Heaps
	list_to_ord_set/2	7.6 Ordered Sets
	list_to_queue/2	7.8 Queues
	list_to_rbtree/2	7.10 Red-Black Trees
	list_to_rbtree/2	7.10 Red-Black Trees
	list_to_tree/2	7.18 Updatable Binary Trees
	listing/0	6.7.2 Looking at the Data Base
	listing/1	6.7.2 Looking at the Data Base
	local_stack (statistics/2 option)	6.18 Miscellaneous
M
	make_directory/2	7.14 Calling The Operating System from YAP
	make_queue/1	7.8 Queues
	map_assoc/2	7.2 Association Lists
	map_assoc/3	7.2 Association Lists
	map_tree/3	7.18 Updatable Binary Trees
	mapargs/3	7.1 Apply Macros
	maplist/2	8.1 Invoking Predicates on all Members of a List
	maplist/3	7.1 Apply Macros
	maplist/3	8.1 Invoking Predicates on all Members of a List
	maplist/4	8.1 Invoking Predicates on all Members of a List
	mapnodes/3	7.1 Apply Macros
	max_arity (yap_flag/2 option)	6.18 Miscellaneous
	max_assoc/3	7.2 Association Lists
	max_integer (yap_flag/2 option)	6.18 Miscellaneous
	max_list/2	7.5 List Manipulation
	max_tagged_integer (yap_flag/2 option)	6.18 Miscellaneous
	max_tagged_integer (yap_flag/2 option)	6.18 Miscellaneous
	member/2	7.5 List Manipulation
	memberchk/2	7.5 List Manipulation
	merge/3	7.6 Ordered Sets
	meta_predicate/1 (directive)	5.4 Meta-Predicates in Modules
	min_assoc/3	7.2 Association Lists
	min_integer (yap_flag/2 option)	6.18 Miscellaneous
	min_list/2	7.5 List Manipulation
	min_of_heap/3	7.4 Heaps
	min_of_heap/5	7.4 Heaps
	mktemp/2	7.14 Calling The Operating System from YAP
	mktime/2	7.14 Calling The Operating System from YAP
	module (prolog_load_context/2 option)	6.18 Miscellaneous
	module/1	5.2 Defining a New Module
	module/2 (directive)	5.2 Defining a New Module
	module/3 (directive)	5.2 Defining a New Module
	multifile/1 (directive)	4.2 Changing the Compiler's Behavior
	mutex_create/1	14.4 Thread Synchronisation
	mutex_destroy/1	14.4 Thread Synchronisation
	mutex_lock/1	14.4 Thread Synchronisation
	mutex_statistics/0	14.2 Monitoring Threads
	mutex_trylock/1	14.4 Thread Synchronisation
	mutex_unlock/1	14.4 Thread Synchronisation
	mutex_unlock_all/0	14.4 Thread Synchronisation
N
	n_of_integer_keys_in_bb (yap_flag/2 option)	6.18 Miscellaneous
	n_of_integer_keys_in_db (yap_flag/2 option)	6.18 Miscellaneous
	name/2	6.3 Predicates on terms
	nb_current/2	8.4 SWI Global variables
	nb_delete/1	8.4 SWI Global variables
	nb_getval/2	8.4 SWI Global variables
	nb_setval/2	8.4 SWI Global variables
	neighbors/3	7.19 Unweighted Graphs
	neighbours/3	7.19 Unweighted Graphs
	nl/0	6.6.4 Handling Input/Output of Characters
	nl/1	6.6.5 Input/Output Predicates applied to Streams
	no_source/0	4.2 Changing the Compiler's Behavior
	no_style_check/1	4.2 Changing the Compiler's Behavior
	nodebug/0	19.1 Debugging Predicates
	nofileerrors/0	6.6.7 Controlling Input/Output
	nogc/0	6.1 Control Predicates
	nonvar/1	6.3 Predicates on terms
	nospy/1	19.1 Debugging Predicates
	nospyall/0	19.1 Debugging Predicates
	not/1	6.1 Control Predicates
	nth/2	7.5 List Manipulation
	nth/4	7.5 List Manipulation
	nth0/2	7.5 List Manipulation
	nth0/4	7.5 List Manipulation
	nth1/3	8. SWI-Prolog Emulation
	nth_clause/3	6.7.1 Modification of the Data Base
	nth_recorded/3	6.8 Internal Data Base
	number/1	6.3 Predicates on terms
	number_atom/2	6.3 Predicates on terms
	number_chars/2	6.3 Predicates on terms
	number_codes/2	6.3 Predicates on terms
	number_to_chars/2	7.13 Reading From and Writing To Strings
	number_to_chars/3	7.13 Reading From and Writing To Strings
	numbervars/3	6.3 Predicates on terms
O
	on_cleanup/1	7.16 Call Cleanup
	on_signal/3	6.12 Access to Operating System Functionality
	once/1	6.1 Control Predicates
	op/3	6.18 Miscellaneous
	open/3	6.6.1 Handling Streams and Files
	open/4	6.6.1 Handling Streams and Files
	open_chars_stream/2	7.13 Reading From and Writing To Strings
	ord_add_element/3	7.6 Ordered Sets
	ord_del_element/3	7.6 Ordered Sets
	ord_disjoint/2	7.6 Ordered Sets
	ord_insert/3	7.6 Ordered Sets
	ord_intersect/2	7.6 Ordered Sets
	ord_intersect/3	7.6 Ordered Sets
	ord_intersect/4	7.6 Ordered Sets
	ord_list_to_assoc/2	7.2 Association Lists
	ord_member/2	7.6 Ordered Sets
	ord_seteq/2	7.6 Ordered Sets
	ord_setproduct/3	7.6 Ordered Sets
	ord_subtract/3	7.6 Ordered Sets
	ord_symdiff/3	7.6 Ordered Sets
	ord_union/2	7.6 Ordered Sets
	ord_union/3	7.6 Ordered Sets
	ord_union/4	7.6 Ordered Sets
	ordsubset/2	7.6 Ordered Sets
P
	path/1	4.2 Changing the Compiler's Behavior
	peek_byte/1	6.6.4 Handling Input/Output of Characters
	peek_byte/2	6.6.5 Input/Output Predicates applied to Streams
	peek_char/1	6.6.4 Handling Input/Output of Characters
	peek_char/2	6.6.5 Input/Output Predicates applied to Streams
	peek_code/1	6.6.4 Handling Input/Output of Characters
	peek_code/2	6.6.5 Input/Output Predicates applied to Streams
	permutation/2	7.5 List Manipulation
	phrase/2	6.11 Grammar Rules
	phrase/3	6.11 Grammar Rules
	pid/1	7.14 Calling The Operating System from YAP
	popen/3	7.14 Calling The Operating System from YAP
	portray_clause/1	6.7.2 Looking at the Data Base
	portray_clause/2	6.7.2 Looking at the Data Base
	predicate_property/2	6.7.2 Looking at the Data Base
	primitive/1	6.3 Predicates on terms
	print/1	6.6.3 Handling Input/Output of Terms
	print/2	6.6.5 Input/Output Predicates applied to Streams
	profile_data/3	6.14 Profiling Prolog Programs
	profiled_reset/0	6.14 Profiling Prolog Programs
	profiling (yap_flag/2 option)	6.18 Miscellaneous
	program (statistics/2 option)	6.18 Miscellaneous
	project_attributes/2	10.4 Displaying Attributes
	project_attributes/2	10.5 Projecting Attributes
	prolog_file_name/2	4.2 Changing the Compiler's Behavior
	prolog_flag/3	6.18 Miscellaneous
	prolog_initialization/1	6.18 Miscellaneous
	prolog_load_context/2	6.18 Miscellaneous
	prompt/2	6.18 Miscellaneous
	public/1 (directive)	4.2 Changing the Compiler's Behavior
	put/1	6.6.4 Handling Input/Output of Characters
	put/2	6.6.5 Input/Output Predicates applied to Streams
	put_assoc/4	7.2 Association Lists
	put_attr/3	8.3 hProlog and SWI-Prolog Attributed Variables
	put_attrs/2	8.3.1 Special Purpose SWI Predicates for Attributes
	put_atts/2	10.2 Attribute Manipulation
	put_byte/1	6.6.4 Handling Input/Output of Characters
	put_byte/2	6.6.5 Input/Output Predicates applied to Streams
	put_char/1	6.6.4 Handling Input/Output of Characters
	put_char/2	6.6.5 Input/Output Predicates applied to Streams
	put_code/1	6.6.4 Handling Input/Output of Characters
	put_code/2	6.6.5 Input/Output Predicates applied to Streams
	put_label/4	7.18 Updatable Binary Trees
	putenv/2	6.12 Access to Operating System Functionality
Q
	queue_to_list/2	7.8 Queues
R
	random/1	7.9 Random Number Generator
	random/3	7.9 Random Number Generator
	randseq/3	7.9 Random Number Generator
	randset/3	7.9 Random Number Generator
	rannum/1	7.7 Pseudo Random Number Integer Generator
	ranstart/0	7.7 Pseudo Random Number Integer Generator
	ranstart/1	7.7 Pseudo Random Number Integer Generator
	ranunif/2	7.7 Pseudo Random Number Integer Generator
	rb_apply/4	7.10 Red-Black Trees
	rb_clone/3	7.10 Red-Black Trees
	rb_del_max/4	7.10 Red-Black Trees
	rb_del_min/4	7.10 Red-Black Trees
	rb_delete/3	7.10 Red-Black Trees
	rb_delete/4	7.10 Red-Black Trees
	rb_empty/1	7.10 Red-Black Trees
	rb_insert/4	7.10 Red-Black Trees
	rb_keys/2	7.10 Red-Black Trees
	rb_lookup/3	7.10 Red-Black Trees
	rb_lookupall/3	7.10 Red-Black Trees
	rb_map/3	7.10 Red-Black Trees
	rb_max/3	7.10 Red-Black Trees
	rb_min/3	7.10 Red-Black Trees
	rb_new/1	7.10 Red-Black Trees
	rb_next/4	7.10 Red-Black Trees
	rb_partial_map/4	7.10 Red-Black Trees
	rb_previous/4	7.10 Red-Black Trees
	rb_size/2	7.10 Red-Black Trees
	rb_update/4	7.10 Red-Black Trees
	rb_visit/2	7.10 Red-Black Trees
	reachable/3	7.19 Unweighted Graphs
	read/1	6.6.3 Handling Input/Output of Terms
	read/2	6.6.5 Input/Output Predicates applied to Streams
	read_from_chars/2	7.13 Reading From and Writing To Strings
	read_term/2	6.6.3 Handling Input/Output of Terms
	read_term/3	6.6.5 Input/Output Predicates applied to Streams
	reconsult/1	4.1 Program loading and updating
	recorda/3	6.8 Internal Data Base
	recorda/3	6.8 Internal Data Base
	recorda_at/3	6.8 Internal Data Base
	recordaifnot/3	6.8 Internal Data Base
	recordaifnot/3	6.8 Internal Data Base
	recorded/3	6.8 Internal Data Base
	recordz/3	6.8 Internal Data Base
	recordz_at/3	6.8 Internal Data Base
	recordzifnot/3	6.8 Internal Data Base
	regexp/3	7.11 Regular Expressions
	regexp/4	7.11 Regular Expressions
	remove_duplicates/2	7.5 List Manipulation
	remove_from_path/1	4.2 Changing the Compiler's Behavior
	rename/2	6.12 Access to Operating System Functionality
	rename_file/2	7.14 Calling The Operating System from YAP
	repeat/0	6.1 Control Predicates
	reset_op_counters/0	18. Profiling the Abstract Machine
	resize_static_array/3	6.16 Arrays
	restore/1	4.3 Saving and Loading Prolog States
	retract/1	6.7.1 Modification of the Data Base
	retract/2	6.7.3 Using Data Base References
	retractall/1	6.7.1 Modification of the Data Base
	reverse/2	7.5 List Manipulation
	runtime (statistics/2 option)	6.18 Miscellaneous
S
	same_length/2	7.5 List Manipulation
	save/1	4.3 Saving and Loading Prolog States
	save/2	4.3 Saving and Loading Prolog States
	save_program/1	4.3 Saving and Loading Prolog States
	save_program/2	4.3 Saving and Loading Prolog States
	see/1	6.6.2 Handling Streams and Files
	seeing/1	6.6.2 Handling Streams and Files
	seen/0	6.6.2 Handling Streams and Files
	select/3	7.5 List Manipulation
	selectlist/3	7.1 Apply Macros
	serve_queue/3	7.8 Queues
	set_input/1	6.6.1 Handling Streams and Files
	set_output/1	6.6.1 Handling Streams and Files
	set_prolog_flag/2	6.18 Miscellaneous
	set_stream_position/2	6.6.1 Handling Streams and Files
	set_value/2	6.8 Internal Data Base
	setarg/3n	6.13 Term Modification
	setenv/2	8. SWI-Prolog Emulation
	setof/3	6.10 Collecting Solutions to a Goal
	setrand/1	7.9 Random Number Generator
	sh/0	6.12 Access to Operating System Functionality
	shell/0	7.14 Calling The Operating System from YAP
	shell/1	7.14 Calling The Operating System from YAP
	shell/1	7.14 Calling The Operating System from YAP
	show_op_counters/1	18. Profiling the Abstract Machine
	show_ops_by_group/1	18. Profiling the Abstract Machine
	show_table/1	16. Tabling
	simple/1	6.3 Predicates on terms
	single_var_warnings (yap_flag/2 option)	6.18 Miscellaneous
	singletons/1 (read_term/2 option)	6.6.3 Handling Input/Output of Terms
	skip/1	6.6.4 Handling Input/Output of Characters
	skip/2	6.6.5 Input/Output Predicates applied to Streams
	sleep/1	7.14 Calling The Operating System from YAP
	socket/2	6.6.8 Using Sockets From Yap
	socket/4	6.6.8 Using Sockets From Yap
	socket_accept/2	6.6.8 Using Sockets From Yap
	socket_accept/2	6.6.8 Using Sockets From Yap
	socket_accept/3	6.6.8 Using Sockets From Yap
	socket_bind/2	6.6.8 Using Sockets From Yap
	socket_buffering/4	6.6.8 Using Sockets From Yap
	socket_close/1	6.6.8 Using Sockets From Yap
	socket_connect/3	6.6.8 Using Sockets From Yap
	socket_listen/2	6.6.8 Using Sockets From Yap
	socket_select/5	6.6.8 Using Sockets From Yap
	sort/2	6.4 Comparing Terms
	source/0	4.2 Changing the Compiler's Behavior
	source_mode/2	4.2 Changing the Compiler's Behavior
	splay_access/5	7.12 Splay Trees
	splay_delete/4	7.12 Splay Trees
	splay_init/3	7.12 Splay Trees
	splay_insert/4	7.12 Splay Trees
	splay_join/3	7.12 Splay Trees
	splay_split/5	7.12 Splay Trees
	spy/1	19.1 Debugging Predicates
	spy_write/2	19.1 Debugging Predicates
	srandom/1	6.5 Arithmetic
	stack_dump_on_error (yap_flag/2 option)	6.18 Miscellaneous
	stack_shifts (stack_shifts/3 option)	6.18 Miscellaneous
	start_low_level_trace/0	17. Tracing at Low Level
	start_low_level_trace/0	17. Tracing at Low Level
	static_array/3	6.16 Arrays
	static_array/3	6.16 Arrays
	static_array_location/4	6.16 Arrays
	static_array_properties/3	6.16 Arrays
	static_array_to_term/3	6.16 Arrays
	statistics/0	6.18 Miscellaneous
	statistics/2	6.18 Miscellaneous
	stream_property/2	6.6.1 Handling Streams and Files
	stream_select/3	6.6.1 Handling Streams and Files
	strict_iso (prolog_flag/2 option)	6.18 Miscellaneous
	style_check/1	4.2 Changing the Compiler's Behavior
	sub_atom/5	6.3 Predicates on terms
	sublist/2	7.5 List Manipulation
	subsumes/2	7.15 Utilities On Terms
	subsumes_chk/2	7.15 Utilities On Terms
	suffix/2	7.5 List Manipulation
	sum_list/2	7.5 List Manipulation
	sumargs/4	7.1 Apply Macros
	sumlist/2	7.5 List Manipulation
	sumlist/4	7.1 Apply Macros
	sumnodes/4	7.1 Apply Macros
	syntax_errors (yap_flag/2 option)	6.18 Miscellaneous
	syntax_errors/1 (read_term/2 option)	6.6.3 Handling Input/Output of Terms
	system/0	7.14 Calling The Operating System from YAP
	system/1	6.12 Access to Operating System Functionality
	system/2	7.14 Calling The Operating System from YAP
	system_options (yap_flag/2 option)	6.18 Miscellaneous
	system_predicate/2	6.7.2 Looking at the Data Base
T
	tab/1	6.6.4 Handling Input/Output of Characters
	tab/2	6.6.5 Input/Output Predicates applied to Streams
	table_statistics/1	16. Tabling
	tell/1	6.6.2 Handling Streams and Files
	telling/1	6.6.2 Handling Streams and Files
	term_expansion/2	6.11 Grammar Rules
	term_hash/2	7.15 Utilities On Terms
	term_hash/4	7.15 Utilities On Terms
	term_to_atom/2	8. SWI-Prolog Emulation
	term_variables/2	7.15 Utilities On Terms
	thread_at_exit/1	14.1 Creating and Destroying Prolog Threads
	thread_create/3	14.1 Creating and Destroying Prolog Threads
	thread_detach/1	14.1 Creating and Destroying Prolog Threads
	thread_exit/1	14.1 Creating and Destroying Prolog Threads
	thread_get_message/1	14.3.1 Message Queues
	thread_get_message/2	14.3.1 Message Queues
	thread_join/2	14.1 Creating and Destroying Prolog Threads
	thread_local/1 (directive)	14.3.3 Threads and Dynamic Predicates
	thread_message_queue_create/1	14.3.1 Message Queues
	thread_message_queue_destroy/1	14.3.1 Message Queues
	thread_peek_message/1	14.3.1 Message Queues
	thread_self/1	14.1 Creating and Destroying Prolog Threads
	thread_send_message/2	14.3.1 Message Queues
	thread_setconcurrency/2	14.1 Creating and Destroying Prolog Threads
	thread_signal/2	14.3.2 Signalling Threads
	thread_statistics/3	14.2 Monitoring Threads
	throw/1	6.1 Control Predicates
	time_out/3	7.17 Calls With Timeout
	tmpnam/1	7.14 Calling The Operating System from YAP
	to_chars_modes (yap_flag/2 option)	6.18 Miscellaneous
	told/0	6.6.2 Handling Streams and Files
	top_sort/2	7.19 Unweighted Graphs
	top_sort/3	7.19 Unweighted Graphs
	toplevel_hook (yap_flag/2 option)	6.18 Miscellaneous
	trail (statistics/2 option)	6.18 Miscellaneous
	transitive_closure/2	7.19 Unweighted Graphs
	transpose/3	7.19 Unweighted Graphs
	tree_size/2	7.18 Updatable Binary Trees
	tree_to_list/2	7.18 Updatable Binary Trees
	true/0	6.1 Control Predicates
	ttyget/1	6.6.6 Compatible C-Prolog predicates for Terminal I/O
	ttyget0/1	6.6.6 Compatible C-Prolog predicates for Terminal I/O
	ttynl/0	6.6.6 Compatible C-Prolog predicates for Terminal I/O
	ttyput/1	6.6.6 Compatible C-Prolog predicates for Terminal I/O
	ttyskip/1	6.6.6 Compatible C-Prolog predicates for Terminal I/O
	ttytab/1	6.6.6 Compatible C-Prolog predicates for Terminal I/O
	typein_module (yap_flag/2 option)	6.18 Miscellaneous
U
	undgraph_add_edges/3	7.21 Undirected Graphs
	undgraph_add_vertices/3	7.21 Undirected Graphs
	undgraph_complement/2	7.21 Undirected Graphs
	undgraph_del_edges/3	7.21 Undirected Graphs
	undgraph_del_vertices/3	7.21 Undirected Graphs
	undgraph_edges/2	7.21 Undirected Graphs
	undgraph_neighbors/3	7.21 Undirected Graphs
	undgraph_neighbours/3	7.21 Undirected Graphs
	undgraph_new/1	7.21 Undirected Graphs
	undgraph_vertices/2	7.21 Undirected Graphs
	unhide/1	4.2 Changing the Compiler's Behavior
	unify_with_occurs_check/2	6.3 Predicates on terms
	unix/1	6.12 Access to Operating System Functionality
	unknown (yap_flag/2 option)	6.18 Miscellaneous
	unknown/2	6.2 Handling Undefined Procedures
	unknown_predicate_handler/3	6.2 Handling Undefined Procedures
	update_array/3	6.16 Arrays
	update_mutable/2	6.13 Term Modification
	update_semantics (yap_flag/2 option)	6.18 Miscellaneous
	use_module/1	5.3 Using Modules
	use_module/2	5.3 Using Modules
	use_module/3	5.3 Using Modules
	user_error (yap_flag/2 option)	6.18 Miscellaneous
	user_input (yap_flag/2 option)	6.18 Miscellaneous
	user_output (yap_flag/2 option)	6.18 Miscellaneous
V
	var/1	6.3 Predicates on terms
	variable_in_term/2	7.15 Utilities On Terms
	variable_names/1 (read_term/2 option)	6.6.3 Handling Input/Output of Terms
	variables/1 (read_term/2 option)	6.6.3 Handling Input/Output of Terms
	variant/2	7.15 Utilities On Terms
	verify_attributes/3	10.3 Attributed Unification
	version (yap_flag/2 option)	6.18 Miscellaneous
	version/0	6.18 Miscellaneous
	version/1	6.18 Miscellaneous
	vertices/2	7.19 Unweighted Graphs
	vertices_edges_to_ugraph/3	7.19 Unweighted Graphs
W
	wait/2	7.14 Calling The Operating System from YAP
	walltime (statistics/2 option)	6.18 Miscellaneous
	when/2	9.2 Coroutining
	with_mutex/2	14.4 Thread Synchronisation
	with_output_to_chars/2	7.13 Reading From and Writing To Strings
	with_output_to_chars/3	7.13 Reading From and Writing To Strings
	with_output_to_chars/4	7.13 Reading From and Writing To Strings
	working_directory/2	7.14 Calling The Operating System from YAP
	working_directory/2	8. SWI-Prolog Emulation
	write/1	6.6.3 Handling Input/Output of Terms
	write/2	6.6.5 Input/Output Predicates applied to Streams
	write_depth/2	6.6.7 Controlling Input/Output
	write_depth/3	6.6.7 Controlling Input/Output
	write_strings (yap_flag/2 option)	6.18 Miscellaneous
	write_term/2	6.6.3 Handling Input/Output of Terms
	write_term/3	6.6.5 Input/Output Predicates applied to Streams
	write_to_chars/2	7.13 Reading From and Writing To Strings
	write_to_chars/3	7.13 Reading From and Writing To Strings
	writeq/1	6.6.3 Handling Input/Output of Terms
	writeq/2	6.6.5 Input/Output Predicates applied to Streams
Y
	YAP_AllocSpaceFromYap (C-Interface function)	21.4 Memory Allocation
	YAP_ArgOfTerm (C-Interface function)	21.1 Terms
	YAP_ArityOfFunctor (C-Interface function)	21.1 Terms
	YAP_AtomName (C-Interface function)	21.1 Terms
	YAP_AtomOfTerm (C-Interface function)	21.1 Terms
	YAP_BigNumOfTerm (C-Interface function)	21.1 Terms
	YAP_BufferToAtomList (C-Interface function)	21.3 Strings
	YAP_BufferToString (C-Interface function)	21.3 Strings
	YAP_CallProlog (C-Interface function)	21.6 From C back to Prolog
	YAP_ClearExceptions/0	22. Using YAP as a Library
	YAP_CloseAllOpenStreams (C-Interface function)	21.5 Controlling Yap Streams from C
	YAP_CompileClause/1	22. Using YAP as a Library
	YAP_ContinueGoal/0	22. Using YAP as a Library
	YAP_cutfail (C-Interface function)	21.7 Writing predicates in C
	YAP_cutsucceed (C-Interface function)	21.7 Writing predicates in C
	YAP_EndConsult/0	22. Using YAP as a Library
	YAP_Error/1	22. Using YAP as a Library
	YAP_Exit/1	22. Using YAP as a Library
	YAP_FastInit/1	22. Using YAP as a Library
	yap_flag/2	6.18 Miscellaneous
	YAP_FloatOfTerm (C-Interface function)	21.1 Terms
	YAP_FreeSpaceFromYap (C-Interface function)	21.4 Memory Allocation
	YAP_FullLookupAtom (C-Interface function)	21.1 Terms
	YAP_FunctorOfTerm (C-Interface function)	21.1 Terms
	YAP_GetValue/1	22. Using YAP as a Library
	YAP_HeadOfTerm (C-Interface function)	21.1 Terms
	YAP_Init/1	22. Using YAP as a Library
	YAP_InitConsult/2	22. Using YAP as a Library
	YAP_IntOfTerm (C-Interface function)	21.1 Terms
	YAP_IsApplTerm (C-Interface function)	21.1 Terms
	YAP_IsAtomTerm (C-Interface function)	21.1 Terms
	YAP_IsBigNumTerm (C-Interface function)	21.1 Terms
	YAP_IsDBRefTerm (C-Interface function)	21.1 Terms
	YAP_IsFloatTerm (C-Interface function)	21.1 Terms
	YAP_IsIntTerm (C-Interface function)	21.1 Terms
	YAP_IsNonVarTerm (C-Interface function)	21.1 Terms
	YAP_IsPairTerm (C-Interface function)	21.1 Terms
	YAP_IsVarTerm (C-Interface function)	21.1 Terms
	YAP_LookupAtom (C-Interface function)	21.1 Terms
	YAP_MkApplTerm (C-Interface function)	21.1 Terms
	YAP_MkAtomTerm (C-Interface function)	21.1 Terms
	YAP_MkBigNumTerm (C-Interface function)	21.1 Terms
	YAP_MkFloatTerm (C-Interface function)	21.1 Terms
	YAP_MkFunctor (C-Interface function)	21.1 Terms
	YAP_MkIntTerm (C-Interface function)	21.1 Terms
	YAP_MkNewApplTerm (C-Interface function)	21.1 Terms
	YAP_MkNewPairTerm (C-Interface function)	21.1 Terms
	YAP_MkPairTerm (C-Interface function)	21.1 Terms
	YAP_NameOfFunctor (C-Interface function)	21.1 Terms
	YAP_OpenStream (C-Interface function)	21.5 Controlling Yap Streams from C
	YAP_PRESERVE_DATA (C-Interface function)	21.7 Writing predicates in C
	YAP_PRESERVED_DATA (C-Interface function)	21.7 Writing predicates in C
	YAP_PutValue/2	22. Using YAP as a Library
	YAP_Read/1	22. Using YAP as a Library
	YAP_ReadBuffer (C-Interface function)	21.3 Strings
	YAP_Reset/0	22. Using YAP as a Library
	YAP_RestartGoal/0	22. Using YAP as a Library
	YAP_RestartGoal/1	22. Using YAP as a Library
	YAP_RunGoal/1	22. Using YAP as a Library
	YAP_StreamToFileNo (C-Interface function)	21.5 Controlling Yap Streams from C
	YAP_StringToBuffer (C-Interface function)	21.3 Strings
	YAP_TailOfTerm (C-Interface function)	21.1 Terms
	YAP_Unify (C-Interface function)	21.2 Unification
	YAP_UserBackCPredicate (C-Interface function)	21.7 Writing predicates in C
	YAP_UserCPredicate (C-Interface function)	21.7 Writing predicates in C
	YAP_Write/3	22. Using YAP as a Library
	YAP_WriteBuffer/4	22. Using YAP as a Library
	YAPBINDIR	2.1 Running Yap Interactively
	YAPLIBDIR	2.1 Running Yap Interactively
	YAPSHAREDIR	2.1 Running Yap Interactively

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Concept Index

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	Index Entry	Section
A
	anonymous variable	3.2.4 Variables
	association list	7.2 Association Lists
	atom	3.2.3 Atoms
	attribute declaration	10.1 Attribute Declarations
	attributed variables	10. Attributed Variables
	AVL trees	7.3 AVL Trees
B
	booting	2. Running YAP
C
	cleanup	7.16 Call Cleanup
	CLPQ	11. Constraint Logic Programming over Reals
	CLPR	11. Constraint Logic Programming over Reals
	comment	3.2.6 Layout
	Counting Calls	6.15 Counting Calls
D
	declaration, attribute	10.1 Attribute Declarations
E
	Efficient Directed Graphs	7.20 Directed Graphs
	end of term	3.1 Syntax of Terms
	environment variables	2.1 Running Yap Interactively
F
	floating-point number	3.2.1.2 Floating-point Numbers
H
	heap	7.4 Heaps
	hProlog Attributed Variables	8.3 hProlog and SWI-Prolog Attributed Variables
I
	installation	1. Installing YAP
	integer	3.2.1.1 Integers
L
	list manipulation	7.5 List Manipulation
	Logtalk	13. Logtalk
M
	machine optimizations	1.2 Tuning YAP for a Particular Machine and Compiler
	macros	7.1 Apply Macros
	mutable variables	6.13 Term Modification
N
	number	3.2.1 Numbers
O
	Operating System Utilities	7.14 Calling The Operating System from YAP
	or-parallelism	15. Parallelism
	ordered set	7.6 Ordered Sets
P
	parallelism	15. Parallelism
	profiling	6.14 Profiling Prolog Programs
	pseudo random	7.7 Pseudo Random Number Integer Generator
	punctuation token	3.2.5 Punctuation Tokens
Q
	queue	7.8 Queues
	queue	7.9 Random Number Generator
R
	Red-Black Trees	7.10 Red-Black Trees
	regular expressions	7.11 Regular Expressions
S
	splay trees	7.12 Splay Trees
	string	3.2.2 Character Strings
	string I/O	7.13 Reading From and Writing To Strings
	SWI-Prolog	8. SWI-Prolog Emulation
	syntax	1.1 Tuning the Functionality of YAP
	syntax	3.1 Syntax of Terms
T
	tabling	16. Tabling
	timeout	7.17 Calls With Timeout
	token	3.2 Prolog Tokens
U
	undrected graphs	7.21 Undirected Graphs
	unweighted graphs	7.19 Unweighted Graphs
	updatable tree	7.18 Updatable Binary Trees
	update semantics	23.2.1 Major Differences between YAP and SICStus Prolog.
	updating terms	6.13 Term Modification
	utilities on terms	7.15 Utilities On Terms
V
	variable	3.2.4 Variables

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Table of Contents

• Introduction
• 1. Installing YAP
  • 1.1 Tuning the Functionality of YAP
  • 1.2 Tuning YAP for a Particular Machine and Compiler
  • 1.3 Tuning YAP for GCC.
    • 1.3.1 Compiling Under Visual C++
    • 1.3.2 Compiling Under SGI's cc
• 2. Running YAP
  • 2.1 Running Yap Interactively
  • 2.2 Running Prolog Files
• 3. Syntax
  • 3.1 Syntax of Terms
  • 3.2 Prolog Tokens
    • 3.2.1 Numbers
      • 3.2.1.1 Integers
      • 3.2.1.2 Floating-point Numbers
    • 3.2.2 Character Strings
    • 3.2.3 Atoms
    • 3.2.4 Variables
    • 3.2.5 Punctuation Tokens
    • 3.2.6 Layout
• 4. Loading Programs
  • 4.1 Program loading and updating
  • 4.2 Changing the Compiler's Behavior
  • 4.3 Saving and Loading Prolog States
• 5. The Module System
  • 5.1 Module Concepts
  • 5.2 Defining a New Module
  • 5.3 Using Modules
  • 5.4 Meta-Predicates in Modules
• 6. Built-In Predicates
  • 6.1 Control Predicates
  • 6.2 Handling Undefined Procedures
  • 6.3 Predicates on terms
  • 6.4 Comparing Terms
  • 6.5 Arithmetic
  • 6.6 I/O Predicates
    • 6.6.1 Handling Streams and Files
    • 6.6.2 Handling Streams and Files
    • 6.6.3 Handling Input/Output of Terms
    • 6.6.4 Handling Input/Output of Characters
    • 6.6.5 Input/Output Predicates applied to Streams
    • 6.6.6 Compatible C-Prolog predicates for Terminal I/O
    • 6.6.7 Controlling Input/Output
    • 6.6.8 Using Sockets From Yap
  • 6.7 Using the Clausal Data Base
    • 6.7.1 Modification of the Data Base
    • 6.7.2 Looking at the Data Base
    • 6.7.3 Using Data Base References
  • 6.8 Internal Data Base
  • 6.9 The Blackboard
  • 6.10 Collecting Solutions to a Goal
  • 6.11 Grammar Rules
  • 6.12 Access to Operating System Functionality
  • 6.13 Term Modification
  • 6.14 Profiling Prolog Programs
  • 6.15 Counting Calls
  • 6.16 Arrays
  • 6.17 Predicate Information
  • 6.18 Miscellaneous
• 7. Library Predicates
  • 7.1 Apply Macros
  • 7.2 Association Lists
  • 7.3 AVL Trees
  • 7.4 Heaps
  • 7.5 List Manipulation
  • 7.6 Ordered Sets
  • 7.7 Pseudo Random Number Integer Generator
  • 7.8 Queues
  • 7.9 Random Number Generator
  • 7.10 Red-Black Trees
  • 7.11 Regular Expressions
  • 7.12 Splay Trees
  • 7.13 Reading From and Writing To Strings
  • 7.14 Calling The Operating System from YAP
  • 7.15 Utilities On Terms
  • 7.16 Call Cleanup
  • 7.17 Calls With Timeout
  • 7.18 Updatable Binary Trees
  • 7.19 Unweighted Graphs
  • 7.20 Directed Graphs
  • 7.21 Undirected Graphs
• 8. SWI-Prolog Emulation
  • 8.1 Invoking Predicates on all Members of a List
  • 8.2 Forall
  • 8.3 hProlog and SWI-Prolog Attributed Variables
    • 8.3.1 Special Purpose SWI Predicates for Attributes
  • 8.4 SWI Global variables
    • 8.4.1 Compatibility of SWI-Prolog Global Variables
• 9. Extensions to Prolog
  • 9.1 Rational Trees
  • 9.2 Coroutining
• 10. Attributed Variables
  • 10.1 Attribute Declarations
  • 10.2 Attribute Manipulation
  • 10.3 Attributed Unification
  • 10.4 Displaying Attributes
  • 10.5 Projecting Attributes
  • 10.6 Attribute Examples
• 11. Constraint Logic Programming over Reals
  • 11.1 Solver Predicates
  • 11.2 Syntax of the predicate arguments
  • 11.3 Use of unification
  • 11.4 Non-Linear Constraints
• 12. CHR: Constraint Handling Rules
  • 12.1 Introduction
  • 12.2 Syntax and Semantics
    • 12.2.1 Syntax
    • 12.2.2 Semantics
      • Rule Types
      • Rule Names
      • Pragmas
      • Options
  • 12.3 CHR in YAP Programs
    • 12.3.1 Embedding in Prolog Programs
    • 12.3.2 Constraint declaration
    • 12.3.3 Compilation
  • 12.4 Debugging
    • 12.4.1 Ports
    • 12.4.2 Tracing
    • 12.4.3 CHR Debugging Predicates
  • 12.5 Examples
  • 12.6 Compatibility with SICStus CHR
  • 12.7 Guidelines
• 13. Logtalk
• 14. Threads
  • 14.1 Creating and Destroying Prolog Threads
  • 14.2 Monitoring Threads
  • 14.3 Thread communication
    • 14.3.1 Message Queues
    • 14.3.2 Signalling Threads
    • 14.3.3 Threads and Dynamic Predicates
  • 14.4 Thread Synchronisation
• 15. Parallelism
• 16. Tabling
• 17. Tracing at Low Level
• 18. Profiling the Abstract Machine
• 19. Debugging
  • 19.1 Debugging Predicates
  • 19.2 Interacting with the debugger
• 20. Indexing
• 21. C Language interface to YAP
  • 21.1 Terms
  • 21.2 Unification
  • 21.3 Strings
  • 21.4 Memory Allocation
  • 21.5 Controlling Yap Streams from C
  • 21.6 From C back to Prolog
  • 21.7 Writing predicates in C
  • 21.8 Loading Object Files
  • 21.9 Saving and Restoring
  • 21.10 Changes to the C-Interface in Yap4
• 22. Using YAP as a Library
• 23. Compatibility with Other Prolog systems
  • 23.1 Compatibility with the C-Prolog interpreter
    • 23.1.1 Major Differences between YAP and C-Prolog.
    • 23.1.2 Yap predicates fully compatible with C-Prolog
    • 23.1.3 Yap predicates not strictly compatible with C-Prolog
    • 23.1.4 Yap predicates not available in C-Prolog
    • 23.1.5 Yap predicates not available in C-Prolog
  • 23.2 Compatibility with the Quintus and SICStus Prolog systems
    • 23.2.1 Major Differences between YAP and SICStus Prolog.
    • 23.2.2 Yap predicates fully compatible with SICStus Prolog
    • 23.2.3 Yap predicates not strictly compatible with SICStus Prolog
    • 23.2.4 Yap predicates not available in SICStus Prolog
  • 23.3 Compatibility with the ISO Prolog standard
• A. Summary of Yap Predefined Operators
• Predicate Index
• Concept Index

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