1/*!
2@file
3Defines `boost::hana::then`.
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_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
22namespace boost { namespace hana {
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}} // end namespace boost::hana
54
55#endif // !BOOST_HANA_THEN_HPP
56

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