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/*!
@file
Forward declares `boost::hana::fold_left`.
@copyright Louis Dionne 2013-2016
Distributed under the Boost Software License, Version 1.0.
(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
*/
#ifndef BOOST_HANA_FWD_FOLD_LEFT_HPP
#define BOOST_HANA_FWD_FOLD_LEFT_HPP
#include <boost/hana/config.hpp>
#include <boost/hana/core/when.hpp>
BOOST_HANA_NAMESPACE_BEGIN
//! Left-fold of a structure using a binary operation and an optional
//! initial reduction state.
//! @ingroup group-Foldable
//!
//! `fold_left` is a left-associative fold using a binary operation.
//! Given a structure containing `x1, ..., xn`, a function `f` and
//! an optional initial state, `fold_left` applies `f` as follows
//! @code
//! f(... f(f(f(x1, x2), x3), x4) ..., xn) // without state
//! f(... f(f(f(f(state, x1), x2), x3), x4) ..., xn) // with state
//! @endcode
//!
//! When the structure is empty, two things may arise. If an initial
//! state was provided, it is returned as-is. Otherwise, if the no-state
//! version of the function was used, an error is triggered. When the
//! stucture contains a single element and the no-state version of the
//! function was used, that single element is returned as is.
//!
//!
//! Signature
//! ---------
//! Given a `Foldable` `F` and an optional initial state of tag `S`,
//! the signatures for `fold_left` are
//! \f[
//! \mathtt{fold\_left} : F(T) \times S \times (S \times T \to S) \to S
//! \f]
//!
//! for the variant with an initial state, and
//! \f[
//! \mathtt{fold\_left} : F(T) \times (T \times T \to T) \to T
//! \f]
//!
//! for the variant without an initial state.
//!
//! @param xs
//! The structure to fold.
//!
//! @param state
//! The initial value used for folding.
//!
//! @param f
//! A binary function called as `f(state, x)`, where `state` is the
//! result accumulated so far and `x` is an element in the structure.
//! For left folds without an initial state, the function is called as
//! `f(x1, x2)`, where `x1` and `x2` are elements of the structure.
//!
//!
//! Example
//! -------
//! @include example/fold_left.cpp
//!
//!
//! Benchmarks
//! ----------
//! <div class="benchmark-chart"
//! style="min-width: 310px; height: 400px; margin: 0 auto"
//! data-dataset="benchmark.fold_left.compile.json">
//! </div>
#ifdef BOOST_HANA_DOXYGEN_INVOKED
constexpr auto fold_left = [](auto&& xs[, auto&& state], auto&& f) -> decltype(auto) {
return tag-dispatched;
};
#else
template <typename T, typename = void>
struct fold_left_impl : fold_left_impl<T, when<true>> { };
struct fold_left_t {
template <typename Xs, typename State, typename F>
constexpr decltype(auto) operator()(Xs&& xs, State&& state, F&& f) const;
template <typename Xs, typename F>
constexpr decltype(auto) operator()(Xs&& xs, F&& f) const;
};
constexpr fold_left_t fold_left{};
#endif
BOOST_HANA_NAMESPACE_END
#endif // !BOOST_HANA_FWD_FOLD_LEFT_HPP
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