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Diffstat (limited to 'boost/hana/fwd/fold_right.hpp')
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diff --git a/boost/hana/fwd/fold_right.hpp b/boost/hana/fwd/fold_right.hpp new file mode 100644 index 0000000000..319bbade1a --- /dev/null +++ b/boost/hana/fwd/fold_right.hpp @@ -0,0 +1,100 @@ +/*! +@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 + //! + //! + //! Benchmarks + //! ---------- + //! <div class="benchmark-chart" + //! style="min-width: 310px; height: 400px; margin: 0 auto" + //! data-dataset="benchmark.fold_right.compile.json"> + //! </div> +#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 |