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/*!
@file
Forward declares `boost::hana::fold_right`.
@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_RIGHT_HPP
#define BOOST_HANA_FWD_FOLD_RIGHT_HPP
#include <boost/hana/config.hpp>
#include <boost/hana/core/when.hpp>
BOOST_HANA_NAMESPACE_BEGIN
//! Right-fold of a structure using a binary operation and an optional
//! initial reduction state.
//! @ingroup group-Foldable
//!
//! `fold_right` is a right-associative fold using a binary operation.
//! Given a structure containing `x1, ..., xn`, a function `f` and
//! an optional initial state, `fold_right` applies `f` as follows
//! @code
//! f(x1, f(x2, f(x3, f(x4, ... f(xn-1, xn) ... )))) // without state
//! f(x1, f(x2, f(x3, f(x4, ... f(xn, state) ... )))) // with state
//! @endcode
//!
//! @note
//! It is worth noting that the order in which the binary function should
//! expect its arguments is reversed from `fold_left`.
//!
//! 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_right` are
//! \f[
//! \mathtt{fold\_right} : F(T) \times S \times (T \times S \to S) \to S
//! \f]
//!
//! for the variant with an initial state, and
//! \f[
//! \mathtt{fold\_right} : 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(x, state)`, where `state` is the
//! result accumulated so far and `x` is an element in the structure.
//! For right 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_right.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
constexpr auto fold_right = [](auto&& xs[, auto&& state], auto&& f) -> decltype(auto) {
return tag-dispatched;
};
#else
template <typename T, typename = void>
struct fold_right_impl : fold_right_impl<T, when<true>> { };
struct fold_right_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_right_t fold_right{};
#endif
BOOST_HANA_NAMESPACE_END
#endif // !BOOST_HANA_FWD_FOLD_RIGHT_HPP
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