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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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