1/*!
2@file
3Defines `boost::hana::monadic_fold_left`.
4
5Copyright Louis Dionne 2013-2022
6Distributed under the Boost Software License, Version 1.0.
7(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
8 */
9
10#ifndef BOOST_HANA_MONADIC_FOLD_LEFT_HPP
11#define BOOST_HANA_MONADIC_FOLD_LEFT_HPP
12
13#include <boost/hana/fwd/monadic_fold_left.hpp>
14
15#include <boost/hana/chain.hpp>
16#include <boost/hana/concept/foldable.hpp>
17#include <boost/hana/concept/monad.hpp>
18#include <boost/hana/config.hpp>
19#include <boost/hana/core/dispatch.hpp>
20#include <boost/hana/detail/decay.hpp>
21#include <boost/hana/fold_right.hpp>
22#include <boost/hana/functional/curry.hpp>
23#include <boost/hana/functional/partial.hpp>
24#include <boost/hana/lift.hpp>
25
26#include <type_traits>
27
28
29namespace boost { namespace hana {
30 //! @cond
31 template <typename M>
32 template <typename Xs, typename State, typename F>
33 constexpr decltype(auto) monadic_fold_left_t<M>::operator()(Xs&& xs, State&& state, F&& f) const {
34 using S = typename hana::tag_of<Xs>::type;
35 using MonadicFoldLeft = BOOST_HANA_DISPATCH_IF(monadic_fold_left_impl<S>,
36 hana::Foldable<S>::value
37 );
38
39 #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
40 static_assert(hana::Monad<M>::value,
41 "hana::monadic_fold_left<M> requires 'M' to be a Monad");
42
43 static_assert(hana::Foldable<S>::value,
44 "hana::monadic_fold_left<M>(xs, state, f) requires 'xs' to be Foldable");
45 #endif
46
47 return MonadicFoldLeft::template apply<M>(static_cast<Xs&&>(xs),
48 static_cast<State&&>(state),
49 static_cast<F&&>(f));
50 }
51 //! @endcond
52
53 //! @cond
54 template <typename M>
55 template <typename Xs, typename F>
56 constexpr decltype(auto) monadic_fold_left_t<M>::operator()(Xs&& xs, F&& f) const {
57 using S = typename hana::tag_of<Xs>::type;
58 using MonadicFoldLeft = BOOST_HANA_DISPATCH_IF(monadic_fold_left_impl<S>,
59 hana::Foldable<S>::value
60 );
61
62 #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
63 static_assert(hana::Monad<M>::value,
64 "hana::monadic_fold_left<M> requires 'M' to be a Monad");
65
66 static_assert(hana::Foldable<S>::value,
67 "hana::monadic_fold_left<M>(xs, f) requires 'xs' to be Foldable");
68 #endif
69
70 return MonadicFoldLeft::template apply<M>(static_cast<Xs&&>(xs),
71 static_cast<F&&>(f));
72 }
73 //! @endcond
74
75 namespace detail {
76 struct foldlM_helper {
77 template <typename F, typename X, typename K, typename Z>
78 constexpr decltype(auto) operator()(F&& f, X&& x, K&& k, Z&& z) const {
79 return hana::chain(
80 static_cast<F&&>(f)(
81 static_cast<Z&&>(z),
82 static_cast<X&&>(x)
83 ),
84 static_cast<K&&>(k)
85 );
86 }
87 };
88
89 template <typename End, typename M, typename F>
90 struct monadic_foldl1_helper {
91 F f;
92 template <typename X, typename Y>
93 constexpr decltype(auto) operator()(X&& x, Y&& y) const
94 { return f(static_cast<X&&>(x), static_cast<Y&&>(y)); }
95 template <typename Y>
96 constexpr decltype(auto) operator()(End, Y&& y) const
97 { return hana::lift<M>(static_cast<Y&&>(y)); }
98 };
99 }
100
101 template <typename T, bool condition>
102 struct monadic_fold_left_impl<T, when<condition>> : default_ {
103 // with state
104 template <typename M, typename Xs, typename S, typename F>
105 static constexpr decltype(auto) apply(Xs&& xs, S&& s, F&& f) {
106 return hana::fold_right(
107 static_cast<Xs&&>(xs),
108 hana::lift<M>,
109 hana::curry<3>(hana::partial(
110 detail::foldlM_helper{}, static_cast<F&&>(f)
111 ))
112 )(static_cast<S&&>(s));
113 }
114
115 // without state
116 template <typename M, typename Xs, typename F>
117 static constexpr decltype(auto) apply(Xs&& xs, F&& f) {
118 struct end { };
119 using G = detail::monadic_foldl1_helper<end, M, typename detail::decay<F>::type>;
120 decltype(auto) result = hana::monadic_fold_left<M>(
121 static_cast<Xs&&>(xs),
122 end{},
123 G{static_cast<F&&>(f)}
124 );
125
126 static_assert(!std::is_same<
127 std::remove_reference_t<decltype(result)>,
128 decltype(hana::lift<M>(end{}))
129 >{},
130 "hana::monadic_fold_left<M>(xs, f) requires 'xs' to be non-empty");
131 return result;
132 }
133 };
134}} // end namespace boost::hana
135
136#endif // !BOOST_HANA_MONADIC_FOLD_LEFT_HPP
137

source code of boost/libs/hana/include/boost/hana/monadic_fold_left.hpp