1/*!
2@file
3Defines `boost::hana::lazy`.
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_LAZY_HPP
11#define BOOST_HANA_LAZY_HPP
12
13#include <boost/hana/fwd/lazy.hpp>
14
15#include <boost/hana/basic_tuple.hpp>
16#include <boost/hana/config.hpp>
17#include <boost/hana/core/make.hpp>
18#include <boost/hana/detail/decay.hpp>
19#include <boost/hana/detail/operators/adl.hpp>
20#include <boost/hana/detail/operators/monad.hpp>
21#include <boost/hana/functional/apply.hpp>
22#include <boost/hana/functional/compose.hpp>
23#include <boost/hana/functional/on.hpp>
24#include <boost/hana/fwd/ap.hpp>
25#include <boost/hana/fwd/duplicate.hpp>
26#include <boost/hana/fwd/eval.hpp>
27#include <boost/hana/fwd/extend.hpp>
28#include <boost/hana/fwd/extract.hpp>
29#include <boost/hana/fwd/flatten.hpp>
30#include <boost/hana/fwd/lift.hpp>
31#include <boost/hana/fwd/transform.hpp>
32
33#include <cstddef>
34#include <type_traits>
35#include <utility>
36
37
38namespace boost { namespace hana {
39 //////////////////////////////////////////////////////////////////////////
40 // lazy
41 //////////////////////////////////////////////////////////////////////////
42 template <typename Indices, typename F, typename ...Args>
43 struct lazy_apply_t;
44
45 namespace detail { struct lazy_secret { }; }
46
47 template <std::size_t ...n, typename F, typename ...Args>
48 struct lazy_apply_t<std::index_sequence<n...>, F, Args...>
49 : detail::operators::adl<>
50 {
51 template <typename ...T>
52 constexpr lazy_apply_t(detail::lazy_secret, T&& ...t)
53 : storage_{static_cast<T&&>(t)...}
54 { }
55
56 basic_tuple<F, Args...> storage_;
57 using hana_tag = lazy_tag;
58 };
59
60 template <typename X>
61 struct lazy_value_t : detail::operators::adl<> {
62 template <typename Y>
63 constexpr lazy_value_t(detail::lazy_secret, Y&& y)
64 : storage_{static_cast<Y&&>(y)}
65 { }
66
67 basic_tuple<X> storage_;
68 using hana_tag = lazy_tag;
69
70 // If this is called, we assume that `X` is in fact a function.
71 template <typename ...Args>
72 constexpr lazy_apply_t<
73 std::make_index_sequence<sizeof...(Args)>,
74 X, typename detail::decay<Args>::type...
75 > operator()(Args&& ...args) const& {
76 return {detail::lazy_secret{},
77 hana::at_c<0>(storage_), static_cast<Args&&>(args)...};
78 }
79
80 template <typename ...Args>
81 constexpr lazy_apply_t<
82 std::make_index_sequence<sizeof...(Args)>,
83 X, typename detail::decay<Args>::type...
84 > operator()(Args&& ...args) && {
85 return {detail::lazy_secret{},
86 static_cast<X&&>(hana::at_c<0>(storage_)),
87 static_cast<Args&&>(args)...
88 };
89 }
90 };
91
92 //////////////////////////////////////////////////////////////////////////
93 // make<lazy_tag>
94 //////////////////////////////////////////////////////////////////////////
95 template <>
96 struct make_impl<lazy_tag> {
97 template <typename X>
98 static constexpr lazy_value_t<typename detail::decay<X>::type> apply(X&& x) {
99 return {detail::lazy_secret{}, static_cast<X&&>(x)};
100 }
101 };
102
103 //////////////////////////////////////////////////////////////////////////
104 // Operators
105 //////////////////////////////////////////////////////////////////////////
106 namespace detail {
107 template <>
108 struct monad_operators<lazy_tag> { static constexpr bool value = true; };
109 }
110
111 //////////////////////////////////////////////////////////////////////////
112 // eval for lazy_tag
113 //////////////////////////////////////////////////////////////////////////
114 template <>
115 struct eval_impl<lazy_tag> {
116 // lazy_apply_t
117 template <std::size_t ...n, typename F, typename ...Args>
118 static constexpr decltype(auto)
119 apply(lazy_apply_t<std::index_sequence<n...>, F, Args...> const& expr) {
120 return hana::at_c<0>(expr.storage_)(
121 hana::at_c<n+1>(expr.storage_)...
122 );
123 }
124
125 template <std::size_t ...n, typename F, typename ...Args>
126 static constexpr decltype(auto)
127 apply(lazy_apply_t<std::index_sequence<n...>, F, Args...>& expr) {
128 return hana::at_c<0>(expr.storage_)(
129 hana::at_c<n+1>(expr.storage_)...
130 );
131 }
132
133 template <std::size_t ...n, typename F, typename ...Args>
134 static constexpr decltype(auto)
135 apply(lazy_apply_t<std::index_sequence<n...>, F, Args...>&& expr) {
136 return static_cast<F&&>(hana::at_c<0>(expr.storage_))(
137 static_cast<Args&&>(hana::at_c<n+1>(expr.storage_))...
138 );
139 }
140
141 // lazy_value_t
142 template <typename X>
143 static constexpr X const& apply(lazy_value_t<X> const& expr)
144 { return hana::at_c<0>(expr.storage_); }
145
146 template <typename X>
147 static constexpr X& apply(lazy_value_t<X>& expr)
148 { return hana::at_c<0>(expr.storage_); }
149
150 template <typename X>
151 static constexpr X apply(lazy_value_t<X>&& expr)
152 { return static_cast<X&&>(hana::at_c<0>(expr.storage_)); }
153 };
154
155 //////////////////////////////////////////////////////////////////////////
156 // Functor
157 //////////////////////////////////////////////////////////////////////////
158 template <>
159 struct transform_impl<lazy_tag> {
160 template <typename Expr, typename F>
161 static constexpr auto apply(Expr&& expr, F&& f) {
162 return hana::make_lazy(hana::compose(static_cast<F&&>(f), hana::eval))(
163 static_cast<Expr&&>(expr)
164 );
165 }
166 };
167
168 //////////////////////////////////////////////////////////////////////////
169 // Applicative
170 //////////////////////////////////////////////////////////////////////////
171 template <>
172 struct lift_impl<lazy_tag> {
173 template <typename X>
174 static constexpr lazy_value_t<typename detail::decay<X>::type>
175 apply(X&& x) {
176 return {detail::lazy_secret{}, static_cast<X&&>(x)};
177 }
178 };
179
180 template <>
181 struct ap_impl<lazy_tag> {
182 template <typename F, typename X>
183 static constexpr decltype(auto) apply(F&& f, X&& x) {
184 return hana::make_lazy(hana::on(hana::apply, hana::eval))(
185 static_cast<F&&>(f), static_cast<X&&>(x)
186 );
187 }
188 };
189
190 //////////////////////////////////////////////////////////////////////////
191 // Monad
192 //////////////////////////////////////////////////////////////////////////
193 template <>
194 struct flatten_impl<lazy_tag> {
195 template <typename Expr>
196 static constexpr decltype(auto) apply(Expr&& expr) {
197 return hana::make_lazy(hana::compose(hana::eval, hana::eval))(
198 static_cast<Expr&&>(expr)
199 );
200 }
201 };
202
203 //////////////////////////////////////////////////////////////////////////
204 // Comonad
205 //////////////////////////////////////////////////////////////////////////
206 template <>
207 struct extract_impl<lazy_tag> {
208 template <typename Expr>
209 static constexpr decltype(auto) apply(Expr&& expr)
210 { return hana::eval(static_cast<Expr&&>(expr)); }
211 };
212
213 template <>
214 struct duplicate_impl<lazy_tag> {
215 template <typename Expr>
216 static constexpr decltype(auto) apply(Expr&& expr)
217 { return hana::make_lazy(static_cast<Expr&&>(expr)); }
218 };
219
220 template <>
221 struct extend_impl<lazy_tag> {
222 template <typename Expr, typename F>
223 static constexpr decltype(auto) apply(Expr&& expr, F&& f) {
224 return hana::make_lazy(static_cast<F&&>(f))(static_cast<Expr&&>(expr));
225 }
226 };
227}} // end namespace boost::hana
228
229#endif // !BOOST_HANA_LAZY_HPP
230

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