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Jparsec is now ported to Ruby, with a way lighter syntax than Java.
Calculator demo
The following is the ruby version of calculator:
require 'rparsec'
class Calculator
include Parsers
include Functors
def parser
ops = OperatorTable.new.
infixl(char(?+) >> Plus, 20).
infixl(char(?-) >> Minus, 20).
infixl(char(?*) >> Mul, 40).
infixl(char(?/) >> Div, 40).
prefix(char(?-) >> Neg, 60)
expr = nil
term = integer.map(&To_i) | char('(') >> lazy{expr} << char (')')
delim = whitespace.many_
expr = delim >> Expressions.build(term, ops, delim)
end
end
Calculator.new.parser.parse '1+2*(3-1)' # => 5
As simple as that!
S-expression demo
The above example utilizes the pre-built Expressions class to help building expression parser. Another example is for a simpler s-expression parser (in lisp syntax, "- (+ 1 (* 2 2)) (1)" sort of thing). As s-expression is way simpler to parse expression with infix operators, we will build the parser without using Expressions class:
class SExpressionTestCase < RUNIT::TestCase
include Parsers
include Functors
def delim
whitespace.many_
end
def ignore parser
parser << delim
end
def lparen
ignore(char('('))
end
def rparen
ignore(char(')'))
end
def parser
expr = nil
lazy_expr = lazy{expr}
term = number.map(&To_f) | lparen >> lazy_expr << rparen
binop = char('+') >> Plus | char('-') >> Minus | char('*') >> Mul | char('/') >> Div
binop = ignore binop
term = ignore term
binary = sequence(binop, lazy_expr, lazy_expr) do |op, e1, e2|
op.call(e1, e2)
end
expr = delim >> (term | binary)
end
def test1
assert_equal(4, parser.parse('- + 1 (* 2 2.0) (1)'))
end
end
The code is pretty self-explanatory. The only thing to note is that we had to manually skip whitespaces using the "ignore" function. (Whitespace skipping can be more elegant if we do a 2-phase parsing, where lexer takes care of them before grammar rule kicks in. Such use is demonstrated in detail in the test code.)
More examples
Take a look at the SQL Parser for a more sophisticated parser, where more traditional lex/parse approach is used.
Installing rparsec
gem install rparsec
rparsec can be downloaded at http://rubyforge.org/frs/?group_id=2326
Rdoc is available online.
Created by Ben Yu
On Sun Oct 08 14:53:56 CDT 2006
Using TimTam