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+/*!
+@file
+Defines `boost::hana::monadic_fold_left`.
+
+@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_LEFT_HPP
+#define BOOST_HANA_MONADIC_FOLD_LEFT_HPP
+
+#include <boost/hana/fwd/monadic_fold_left.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_right.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_left_t {
+ #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
+ static_assert(hana::Monad<M>::value,
+ "hana::monadic_fold_left<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 MonadicFoldLeft = BOOST_HANA_DISPATCH_IF(monadic_fold_left_impl<S>,
+ hana::Foldable<S>::value
+ );
+
+ #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
+ static_assert(hana::Foldable<S>::value,
+ "hana::monadic_fold_left<M>(xs, state, f) requires 'xs' to be Foldable");
+ #endif
+
+ return MonadicFoldLeft::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 MonadicFoldLeft = BOOST_HANA_DISPATCH_IF(monadic_fold_left_impl<S>,
+ hana::Foldable<S>::value
+ );
+
+ #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
+ static_assert(hana::Foldable<S>::value,
+ "hana::monadic_fold_left<M>(xs, f) requires 'xs' to be Foldable");
+ #endif
+
+ return MonadicFoldLeft::template apply<M>(static_cast<Xs&&>(xs),
+ static_cast<F&&>(f));
+ }
+ };
+
+ namespace detail {
+ struct foldlM_helper {
+ template <typename F, typename X, typename K, typename Z>
+ constexpr decltype(auto) operator()(F&& f, X&& x, K&& k, Z&& z) const {
+ return hana::chain(
+ static_cast<F&&>(f)(
+ static_cast<Z&&>(z),
+ static_cast<X&&>(x)
+ ),
+ static_cast<K&&>(k)
+ );
+ }
+ };
+
+ template <typename End, typename M, typename F>
+ struct monadic_foldl1_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 Y>
+ constexpr decltype(auto) operator()(End, Y&& y) const
+ { return hana::lift<M>(static_cast<Y&&>(y)); }
+ };
+ }
+
+ template <typename T, bool condition>
+ struct monadic_fold_left_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_right(
+ static_cast<Xs&&>(xs),
+ hana::lift<M>,
+ hana::curry<3>(hana::partial(
+ detail::foldlM_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_foldl1_helper<end, M, typename std::decay<F>::type>;
+ decltype(auto) result = hana::monadic_fold_left<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_left<M>(xs, f) requires 'xs' to be non-empty");
+ return result;
+ }
+ };
+BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_MONADIC_FOLD_LEFT_HPP