63e88f80
 Hu Chunming
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/*!
@file
Forward declares `boost::hana::then`.
Copyright Louis Dionne 2013-2022
Distributed under the Boost Software License, Version 1.0.
(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
*/
#ifndef BOOST_HANA_FWD_THEN_HPP
#define BOOST_HANA_FWD_THEN_HPP
#include <boost/hana/config.hpp>
#include <boost/hana/core/when.hpp>
namespace boost { namespace hana {
//! Sequentially compose two monadic actions, discarding any value
//! produced by the first but not its effects.
//! @ingroup group-Monad
//!
//!
//! @param before
//! The first `Monad` in the monadic composition chain. The result of
//! this monad is ignored, but its effects are combined with that of the
//! second monad.
//!
//! @param xs
//! The second `Monad` in the monadic composition chain.
//!
//!
//! Example
//! -------
//! @include example/then.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
constexpr auto then = [](auto&& before, auto&& xs) -> decltype(auto) {
return tag-dispatched;
};
#else
template <typename M, typename = void>
struct then_impl : then_impl<M, when<true>> { };
struct then_t {
template <typename Before, typename Xs>
constexpr decltype(auto) operator()(Before&& before, Xs&& xs) const;
};
BOOST_HANA_INLINE_VARIABLE constexpr then_t then{};
#endif
}} // end namespace boost::hana
#endif // !BOOST_HANA_FWD_THEN_HPP
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