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//! Function module contains typing and signature for built-in and user defined functions.

use crate::Accumulator;
use crate::ColumnarValue;
use arrow::datatypes::DataType;
use datafusion_common::Result;
use std::sync::Arc;

/// Scalar function
///
/// The Fn param is the wrapped function but be aware that the function will
/// be passed with the slice / vec of columnar values (either scalar or array)
/// with the exception of zero param function, where a singular element vec
/// will be passed. In that case the single element is a null array to indicate
/// the batch's row count (so that the generative zero-argument function can know
/// the result array size).
pub type ScalarFunctionImplementation =
    Arc<dyn Fn(&[ColumnarValue]) -> Result<ColumnarValue> + Send + Sync>;

/// A function's return type
pub type ReturnTypeFunction =
    Arc<dyn Fn(&[DataType]) -> Result<Arc<DataType>> + Send + Sync>;

/// the implementation of an aggregate function
pub type AccumulatorFunctionImplementation =
    Arc<dyn Fn() -> Result<Box<dyn Accumulator>> + Send + Sync>;

/// This signature corresponds to which types an aggregator serializes
/// its state, given its return datatype.
pub type StateTypeFunction =
    Arc<dyn Fn(&DataType) -> Result<Arc<Vec<DataType>>> + Send + Sync>;