1 /*! 2 @file 3 Forward declares `boost::hana::then`. 4 5 @copyright Louis Dionne 2013-2017 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 53