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56 lines
1.7 KiB
C++
56 lines
1.7 KiB
C++
/*!
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@file
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Defines `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_THEN_HPP
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#define BOOST_HANA_THEN_HPP
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#include <boost/hana/fwd/then.hpp>
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#include <boost/hana/chain.hpp>
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#include <boost/hana/concept/monad.hpp>
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#include <boost/hana/config.hpp>
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#include <boost/hana/core/dispatch.hpp>
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#include <boost/hana/functional/always.hpp>
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BOOST_HANA_NAMESPACE_BEGIN
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//! @cond
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template <typename Before, typename Xs>
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constexpr decltype(auto) then_t::operator()(Before&& before, Xs&& xs) const {
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using M = typename hana::tag_of<Before>::type;
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using Then = BOOST_HANA_DISPATCH_IF(then_impl<M>,
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hana::Monad<M>::value &&
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hana::Monad<Xs>::value
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);
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#ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
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static_assert(hana::Monad<M>::value,
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"hana::then(before, xs) requires 'before' to be a Monad");
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static_assert(hana::Monad<Xs>::value,
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"hana::then(before, xs) requires 'xs' to be a Monad");
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#endif
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return Then::apply(static_cast<Before&&>(before),
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static_cast<Xs&&>(xs));
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}
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//! @endcond
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template <typename M, bool condition>
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struct then_impl<M, when<condition>> : default_ {
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template <typename Xs, typename Ys>
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static constexpr decltype(auto) apply(Xs&& xs, Ys&& ys) {
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return hana::chain(static_cast<Xs&&>(xs),
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hana::always(static_cast<Ys&&>(ys)));
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}
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};
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_THEN_HPP
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