85 lines
2.9 KiB
C++
85 lines
2.9 KiB
C++
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/*!
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@file
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Forward declares `boost::hana::reverse_fold`.
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@copyright Louis Dionne 2013-2016
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Distributed under the Boost Software License, Version 1.0.
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(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
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*/
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#ifndef BOOST_HANA_FWD_REVERSE_FOLD_HPP
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#define BOOST_HANA_FWD_REVERSE_FOLD_HPP
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#include <boost/hana/config.hpp>
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BOOST_HANA_NAMESPACE_BEGIN
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//! Equivalent to `reverse_fold` in Boost.Fusion and Boost.MPL.
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//! @ingroup group-Foldable
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//!
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//! This method has the same semantics as `reverse_fold` in Boost.Fusion
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//! and Boost.MPL, with the extension that an initial state is not
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//! required. This method is equivalent to `fold_right`, except that
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//! the accumulating function must take its arguments in reverse order,
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//! to match the order used in Fusion. In other words,
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//! @code
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//! reverse_fold(sequence, state, f) == fold_right(sequence, state, flip(f))
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//! reverse_fold(sequence, f) == fold_right(sequence, flip(f))
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//! @endcode
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//!
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//! @note
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//! This method is a convenience alias to `fold_right`. As an alias,
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//! `reverse_fold` is not tag-dispatched on its own and `fold_right`
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//! should be customized instead.
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//!
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//!
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//! Signature
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//! ---------
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//! Given a `Foldable` `F` and an optional initial state of tag `S`,
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//! the signatures for `reverse_fold` are
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//! \f[
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//! \mathtt{reverse\_fold} : F(T) \times S \times (S \times T \to S) \to S
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//! \f]
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//!
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//! for the variant with an initial state, and
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//! \f[
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//! \mathtt{reverse\_fold} : F(T) \times (T \times T \to T) \to T
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//! \f]
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//!
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//! for the variant without an initial state.
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//!
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//! @param xs
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//! The structure to fold.
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//!
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//! @param state
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//! The initial value used for folding.
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//!
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//! @param f
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//! A binary function called as `f(state, x)`, where `state` is the
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//! result accumulated so far and `x` is an element in the structure.
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//! For reverse folds without an initial state, the function is called as
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//! `f(x1, x2)`, where `x1` and `x2` are elements of the structure.
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//!
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//!
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//! Example
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//! -------
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//! @include example/reverse_fold.cpp
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#ifdef BOOST_HANA_DOXYGEN_INVOKED
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constexpr auto reverse_fold = [](auto&& xs[, auto&& state], auto&& f) -> decltype(auto) {
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return fold_right(forwarded(xs), forwarded(state), flip(forwarded(f)));
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};
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#else
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struct reverse_fold_t {
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template <typename Xs, typename S, typename F>
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constexpr decltype(auto) operator()(Xs&& xs, S&& s, F&& f) const;
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template <typename Xs, typename F>
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constexpr decltype(auto) operator()(Xs&& xs, F&& f) const;
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};
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constexpr reverse_fold_t reverse_fold{};
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#endif
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_FWD_REVERSE_FOLD_HPP
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