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1 | /*! |
2 | @file | |
3 | Defines `boost::hana::then`. | |
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_THEN_HPP | |
11 | #define BOOST_HANA_THEN_HPP | |
12 | ||
13 | #include <boost/hana/fwd/then.hpp> | |
14 | ||
15 | #include <boost/hana/chain.hpp> | |
16 | #include <boost/hana/concept/monad.hpp> | |
17 | #include <boost/hana/config.hpp> | |
18 | #include <boost/hana/core/dispatch.hpp> | |
19 | #include <boost/hana/functional/always.hpp> | |
20 | ||
21 | ||
22 | BOOST_HANA_NAMESPACE_BEGIN | |
23 | //! @cond | |
24 | template <typename Before, typename Xs> | |
25 | constexpr decltype(auto) then_t::operator()(Before&& before, Xs&& xs) const { | |
26 | using M = typename hana::tag_of<Before>::type; | |
27 | using Then = BOOST_HANA_DISPATCH_IF(then_impl<M>, | |
28 | hana::Monad<M>::value && | |
29 | hana::Monad<Xs>::value | |
30 | ); | |
31 | ||
32 | #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS | |
33 | static_assert(hana::Monad<M>::value, | |
34 | "hana::then(before, xs) requires 'before' to be a Monad"); | |
35 | ||
36 | static_assert(hana::Monad<Xs>::value, | |
37 | "hana::then(before, xs) requires 'xs' to be a Monad"); | |
38 | #endif | |
39 | ||
40 | return Then::apply(static_cast<Before&&>(before), | |
41 | static_cast<Xs&&>(xs)); | |
42 | } | |
43 | //! @endcond | |
44 | ||
45 | template <typename M, bool condition> | |
46 | struct then_impl<M, when<condition>> : default_ { | |
47 | template <typename Xs, typename Ys> | |
48 | static constexpr decltype(auto) apply(Xs&& xs, Ys&& ys) { | |
49 | return hana::chain(static_cast<Xs&&>(xs), | |
50 | hana::always(static_cast<Ys&&>(ys))); | |
51 | } | |
52 | }; | |
53 | BOOST_HANA_NAMESPACE_END | |
54 | ||
55 | #endif // !BOOST_HANA_THEN_HPP |