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Diffstat (limited to 'boost/hana/monadic_fold_right.hpp')
-rw-r--r-- | boost/hana/monadic_fold_right.hpp | 130 |
1 files changed, 130 insertions, 0 deletions
diff --git a/boost/hana/monadic_fold_right.hpp b/boost/hana/monadic_fold_right.hpp new file mode 100644 index 0000000000..814343a336 --- /dev/null +++ b/boost/hana/monadic_fold_right.hpp @@ -0,0 +1,130 @@ +/*! +@file +Defines `boost::hana::monadic_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_MONADIC_FOLD_RIGHT_HPP +#define BOOST_HANA_MONADIC_FOLD_RIGHT_HPP + +#include <boost/hana/fwd/monadic_fold_right.hpp> + +#include <boost/hana/chain.hpp> +#include <boost/hana/concept/foldable.hpp> +#include <boost/hana/concept/monad.hpp> +#include <boost/hana/config.hpp> +#include <boost/hana/core/dispatch.hpp> +#include <boost/hana/fold_left.hpp> +#include <boost/hana/functional/curry.hpp> +#include <boost/hana/functional/partial.hpp> +#include <boost/hana/lift.hpp> + +#include <type_traits> + + +BOOST_HANA_NAMESPACE_BEGIN + template <typename M> + struct monadic_fold_right_t { + #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS + static_assert(hana::Monad<M>::value, + "hana::monadic_fold_right<M> requires 'M' to be a Monad"); + #endif + + template <typename Xs, typename State, typename F> + constexpr decltype(auto) operator()(Xs&& xs, State&& state, F&& f) const { + using S = typename hana::tag_of<Xs>::type; + using MonadicFoldRight = BOOST_HANA_DISPATCH_IF(monadic_fold_right_impl<S>, + hana::Foldable<S>::value + ); + + #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS + static_assert(hana::Foldable<S>::value, + "hana::monadic_fold_right<M>(xs, state, f) requires 'xs' to be Foldable"); + #endif + + return MonadicFoldRight::template apply<M>(static_cast<Xs&&>(xs), + static_cast<State&&>(state), + static_cast<F&&>(f)); + } + + template <typename Xs, typename F> + constexpr decltype(auto) operator()(Xs&& xs, F&& f) const { + using S = typename hana::tag_of<Xs>::type; + using MonadicFoldRight = BOOST_HANA_DISPATCH_IF(monadic_fold_right_impl<S>, + hana::Foldable<S>::value + ); + + #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS + static_assert(hana::Foldable<S>::value, + "hana::monadic_fold_right<M>(xs, f) requires 'xs' to be Foldable"); + #endif + return MonadicFoldRight::template apply<M>(static_cast<Xs&&>(xs), + static_cast<F&&>(f)); + } + }; + + namespace detail { + struct foldrM_helper { + template <typename F, typename K, typename X, typename Z> + constexpr decltype(auto) operator()(F&& f, K&& k, X&& x, Z&& z) const { + return hana::chain( + static_cast<F&&>(f)( + static_cast<X&&>(x), + static_cast<Z&&>(z) + ), + static_cast<K&&>(k) + ); + } + }; + + template <typename End, typename M, typename F> + struct monadic_foldr1_helper { + F f; + template <typename X, typename Y> + constexpr decltype(auto) operator()(X&& x, Y&& y) const + { return f(static_cast<X&&>(x), static_cast<Y&&>(y)); } + template <typename X> + constexpr decltype(auto) operator()(X&& x, End) const + { return hana::lift<M>(static_cast<X&&>(x)); } + }; + } + + template <typename T, bool condition> + struct monadic_fold_right_impl<T, when<condition>> : default_ { + // with state + template <typename M, typename Xs, typename S, typename F> + static constexpr decltype(auto) apply(Xs&& xs, S&& s, F&& f) { + return hana::fold_left( + static_cast<Xs&&>(xs), + hana::lift<M>, + hana::curry<3>(hana::partial( + detail::foldrM_helper{}, static_cast<F&&>(f) + )) + )(static_cast<S&&>(s)); + } + + // without state + template <typename M, typename Xs, typename F> + static constexpr decltype(auto) apply(Xs&& xs, F&& f) { + struct end { }; + using G = detail::monadic_foldr1_helper<end, M, typename std::decay<F>::type>; + decltype(auto) result = hana::monadic_fold_right<M>( + static_cast<Xs&&>(xs), + end{}, + G{static_cast<F&&>(f)} + ); + + static_assert(!std::is_same< + std::remove_reference_t<decltype(result)>, + decltype(hana::lift<M>(end{})) + >{}, + "hana::monadic_fold_right<M>(xs, f) requires 'xs' to be non-empty"); + return result; + } + }; +BOOST_HANA_NAMESPACE_END + +#endif // !BOOST_HANA_MONADIC_FOLD_RIGHT_HPP |