53 lines
1.4 KiB
C++
53 lines
1.4 KiB
C++
/*!
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@file
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Forward declares `boost::hana::then`.
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@copyright Louis Dionne 2013-2016
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Distributed under the Boost Software License, Version 1.0.
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(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
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*/
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#ifndef BOOST_HANA_FWD_THEN_HPP
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#define BOOST_HANA_FWD_THEN_HPP
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#include <boost/hana/config.hpp>
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#include <boost/hana/core/when.hpp>
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BOOST_HANA_NAMESPACE_BEGIN
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//! Sequentially compose two monadic actions, discarding any value
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//! produced by the first but not its effects.
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//! @ingroup group-Monad
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//!
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//!
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//! @param before
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//! The first `Monad` in the monadic composition chain. The result of
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//! this monad is ignored, but its effects are combined with that of the
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//! second monad.
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//!
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//! @param xs
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//! The second `Monad` in the monadic composition chain.
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//!
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//!
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//! Example
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//! -------
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//! @include example/then.cpp
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#ifdef BOOST_HANA_DOXYGEN_INVOKED
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constexpr auto then = [](auto&& before, auto&& xs) -> decltype(auto) {
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return tag-dispatched;
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};
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#else
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template <typename M, typename = void>
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struct then_impl : then_impl<M, when<true>> { };
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struct then_t {
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template <typename Before, typename Xs>
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constexpr decltype(auto) operator()(Before&& before, Xs&& xs) const;
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};
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constexpr then_t then{};
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#endif
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_FWD_THEN_HPP
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