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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