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/*!
@file
Forward declares `boost::hana::reverse_fold`.
@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_REVERSE_FOLD_HPP
#define BOOST_HANA_FWD_REVERSE_FOLD_HPP
#include <boost/hana/config.hpp>
BOOST_HANA_NAMESPACE_BEGIN
//! Equivalent to `reverse_fold` in Boost.Fusion and Boost.MPL.
//! @ingroup group-Foldable
//!
//! This method has the same semantics as `reverse_fold` in Boost.Fusion
//! and Boost.MPL, with the extension that an initial state is not
//! required. This method is equivalent to `fold_right`, except that
//! the accumulating function must take its arguments in reverse order,
//! to match the order used in Fusion. In other words,
//! @code
//! reverse_fold(sequence, state, f) == fold_right(sequence, state, flip(f))
//! reverse_fold(sequence, f) == fold_right(sequence, flip(f))
//! @endcode
//!
//! @note
//! This method is a convenience alias to `fold_right`. As an alias,
//! `reverse_fold` is not tag-dispatched on its own and `fold_right`
//! should be customized instead.
//!
//!
//! Signature
//! ---------
//! Given a `Foldable` `F` and an optional initial state of tag `S`,
//! the signatures for `reverse_fold` are
//! \f[
//! \mathtt{reverse\_fold} : F(T) \times S \times (S \times T \to S) \to S
//! \f]
//!
//! for the variant with an initial state, and
//! \f[
//! \mathtt{reverse\_fold} : 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 reverse 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/reverse_fold.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
constexpr auto reverse_fold = [](auto&& xs[, auto&& state], auto&& f) -> decltype(auto) {
return fold_right(forwarded(xs), forwarded(state), flip(forwarded(f)));
};
#else
struct reverse_fold_t {
template <typename Xs, typename S, typename F>
constexpr decltype(auto) operator()(Xs&& xs, S&& s, F&& f) const;
template <typename Xs, typename F>
constexpr decltype(auto) operator()(Xs&& xs, F&& f) const;
};
constexpr reverse_fold_t reverse_fold{};
#endif
BOOST_HANA_NAMESPACE_END
#endif // !BOOST_HANA_FWD_REVERSE_FOLD_HPP
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