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- /*!
- @file
- Defines `boost::hana::then`.
- @copyright Louis Dionne 2013-2017
- 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_THEN_HPP
- #define BOOST_HANA_THEN_HPP
- #include <boost/hana/fwd/then.hpp>
- #include <boost/hana/chain.hpp>
- #include <boost/hana/concept/monad.hpp>
- #include <boost/hana/config.hpp>
- #include <boost/hana/core/dispatch.hpp>
- #include <boost/hana/functional/always.hpp>
- BOOST_HANA_NAMESPACE_BEGIN
- //! @cond
- template <typename Before, typename Xs>
- constexpr decltype(auto) then_t::operator()(Before&& before, Xs&& xs) const {
- using M = typename hana::tag_of<Before>::type;
- using Then = BOOST_HANA_DISPATCH_IF(then_impl<M>,
- hana::Monad<M>::value &&
- hana::Monad<Xs>::value
- );
- #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
- static_assert(hana::Monad<M>::value,
- "hana::then(before, xs) requires 'before' to be a Monad");
- static_assert(hana::Monad<Xs>::value,
- "hana::then(before, xs) requires 'xs' to be a Monad");
- #endif
- return Then::apply(static_cast<Before&&>(before),
- static_cast<Xs&&>(xs));
- }
- //! @endcond
- template <typename M, bool condition>
- struct then_impl<M, when<condition>> : default_ {
- template <typename Xs, typename Ys>
- static constexpr decltype(auto) apply(Xs&& xs, Ys&& ys) {
- return hana::chain(static_cast<Xs&&>(xs),
- hana::always(static_cast<Ys&&>(ys)));
- }
- };
- BOOST_HANA_NAMESPACE_END
- #endif // !BOOST_HANA_THEN_HPP
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