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1 | /*! |
2 | @file | |
3 | Forward declares `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_FWD_THEN_HPP | |
11 | #define BOOST_HANA_FWD_THEN_HPP | |
12 | ||
13 | #include <boost/hana/config.hpp> | |
14 | #include <boost/hana/core/when.hpp> | |
15 | ||
16 | ||
17 | BOOST_HANA_NAMESPACE_BEGIN | |
18 | //! Sequentially compose two monadic actions, discarding any value | |
19 | //! produced by the first but not its effects. | |
20 | //! @ingroup group-Monad | |
21 | //! | |
22 | //! | |
23 | //! @param before | |
24 | //! The first `Monad` in the monadic composition chain. The result of | |
25 | //! this monad is ignored, but its effects are combined with that of the | |
26 | //! second monad. | |
27 | //! | |
28 | //! @param xs | |
29 | //! The second `Monad` in the monadic composition chain. | |
30 | //! | |
31 | //! | |
32 | //! Example | |
33 | //! ------- | |
34 | //! @include example/then.cpp | |
35 | #ifdef BOOST_HANA_DOXYGEN_INVOKED | |
36 | constexpr auto then = [](auto&& before, auto&& xs) -> decltype(auto) { | |
37 | return tag-dispatched; | |
38 | }; | |
39 | #else | |
40 | template <typename M, typename = void> | |
41 | struct then_impl : then_impl<M, when<true>> { }; | |
42 | ||
43 | struct then_t { | |
44 | template <typename Before, typename Xs> | |
45 | constexpr decltype(auto) operator()(Before&& before, Xs&& xs) const; | |
46 | }; | |
47 | ||
48 | constexpr then_t then{}; | |
49 | #endif | |
50 | BOOST_HANA_NAMESPACE_END | |
51 | ||
52 | #endif // !BOOST_HANA_FWD_THEN_HPP |