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1 | /*! | |
2 | @file | |
3 | Defines `boost::hana::while_`. | |
4 | ||
5 | @copyright Louis Dionne 2013-2016 | |
6 | Distributed 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_WHILE_HPP | |
11 | #define BOOST_HANA_WHILE_HPP | |
12 | ||
13 | #include <boost/hana/fwd/while.hpp> | |
14 | ||
15 | #include <boost/hana/bool.hpp> | |
16 | #include <boost/hana/concept/constant.hpp> | |
17 | #include <boost/hana/concept/constant.hpp> | |
18 | #include <boost/hana/concept/logical.hpp> | |
19 | #include <boost/hana/config.hpp> | |
20 | #include <boost/hana/core/to.hpp> | |
21 | #include <boost/hana/core/dispatch.hpp> | |
22 | #include <boost/hana/detail/canonical_constant.hpp> | |
23 | ||
24 | #include <type_traits> | |
25 | ||
26 | ||
27 | BOOST_HANA_NAMESPACE_BEGIN | |
28 | //! @cond | |
29 | template <typename Pred, typename State, typename F> | |
30 | constexpr decltype(auto) while_t::operator()(Pred&& pred, State&& state, F&& f) const { | |
31 | using Cond = decltype(pred(state)); | |
32 | using Bool = typename hana::tag_of<Cond>::type; | |
33 | using While = BOOST_HANA_DISPATCH_IF(while_impl<Bool>, | |
34 | hana::Logical<Bool>::value | |
35 | ); | |
36 | ||
37 | #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS | |
38 | static_assert(hana::Logical<Bool>::value, | |
39 | "hana::while_(pred, state, f) requires 'pred(state)' to be a Logical"); | |
40 | #endif | |
41 | ||
42 | return While::apply(static_cast<Pred&&>(pred), | |
43 | static_cast<State&&>(state), | |
44 | static_cast<F&&>(f)); | |
45 | } | |
46 | //! @endcond | |
47 | ||
48 | template <typename L, bool condition> | |
49 | struct while_impl<L, hana::when<condition>> : hana::default_ { | |
50 | template <typename ...Args> | |
51 | static constexpr auto apply(Args&& ...) = delete; | |
52 | }; | |
53 | ||
54 | template <typename L> | |
55 | struct while_impl<L, hana::when<std::is_arithmetic<L>::value>> { | |
56 | template <typename Pred, typename State, typename F> | |
57 | static auto apply(Pred&& pred, State&& state, F&& f) | |
58 | -> decltype( | |
59 | true ? f(static_cast<State&&>(state)) | |
60 | : static_cast<State&&>(state) | |
61 | ) | |
62 | { | |
63 | if (pred(state)) { | |
64 | decltype(auto) r = f(static_cast<State&&>(state)); | |
65 | return hana::while_(static_cast<Pred&&>(pred), | |
66 | static_cast<decltype(r)&&>(r), | |
67 | static_cast<F&&>(f)); | |
68 | } | |
69 | else { | |
70 | return static_cast<State&&>(state); | |
71 | } | |
72 | } | |
73 | }; | |
74 | ||
75 | template <typename C> | |
76 | struct while_impl<C, hana::when< | |
77 | hana::Constant<C>::value && | |
78 | hana::Logical<typename C::value_type>::value | |
79 | >> { | |
80 | template <typename Pred, typename State, typename F> | |
81 | static constexpr State | |
82 | while_helper(hana::false_, Pred&&, State&& state, F&&) { | |
83 | return static_cast<State&&>(state); | |
84 | } | |
85 | ||
86 | template <typename Pred, typename State, typename F> | |
87 | static constexpr decltype(auto) | |
88 | while_helper(hana::true_, Pred&& pred, State&& state, F&& f) { | |
89 | decltype(auto) r = f(static_cast<State&&>(state)); | |
90 | return hana::while_(static_cast<Pred&&>(pred), | |
91 | static_cast<decltype(r)&&>(r), | |
92 | static_cast<F&&>(f)); | |
93 | } | |
94 | ||
95 | template <typename Pred, typename State, typename F> | |
96 | static constexpr decltype(auto) | |
97 | apply(Pred&& pred, State&& state, F&& f) { | |
98 | // Since `pred(state)` returns a `Constant`, we do not actually | |
99 | // need to call it; we only need its decltype. However, we still | |
100 | // call it to run potential side effects. I'm not sure whether | |
101 | // that is desirable, since we pretty much take for granted that | |
102 | // functions are pure, but we'll do it like this for now. Also, I | |
103 | // think there is something rather deep hidden behind this, and | |
104 | // understanding what must be done here should give us a better | |
105 | // understanding of something non-trivial. | |
106 | auto cond_ = pred(state); | |
107 | constexpr auto cond = hana::value(cond_); | |
108 | constexpr bool truth_value = hana::if_(cond, true, false); | |
109 | return while_helper(hana::bool_c<truth_value>, | |
110 | static_cast<Pred&&>(pred), | |
111 | static_cast<State&&>(state), | |
112 | static_cast<F&&>(f)); | |
113 | } | |
114 | }; | |
115 | BOOST_HANA_NAMESPACE_END | |
116 | ||
117 | #endif // !BOOST_HANA_WHILE_HPP |