ceph/src/boost/libs/hana/include/boost/hana/then.hpp

   1 /*!
   2 @file
   3 Defines `boost::hana::then`.
   4
   5 @copyright Louis Dionne 2013-2016
   6 Distributed under the Boost Software License, Version 1.0.
   7 (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
   8  */
   9
  10 #ifndef BOOST_HANA_THEN_HPP
  11 #define BOOST_HANA_THEN_HPP
  12
  13 #include <boost/hana/fwd/then.hpp>
  14
  15 #include <boost/hana/chain.hpp>
  16 #include <boost/hana/concept/monad.hpp>
  17 #include <boost/hana/config.hpp>
  18 #include <boost/hana/core/dispatch.hpp>
  19 #include <boost/hana/functional/always.hpp>
  20
  21
  22 BOOST_HANA_NAMESPACE_BEGIN
  23     //! @cond
  24     template <typename Before, typename Xs>
  25     constexpr decltype(auto) then_t::operator()(Before&& before, Xs&& xs) const {
  26         using M = typename hana::tag_of<Before>::type;
  27         using Then = BOOST_HANA_DISPATCH_IF(then_impl<M>,
  28             hana::Monad<M>::value &&
  29             hana::Monad<Xs>::value
  30         );
  31
  32     #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
  33         static_assert(hana::Monad<M>::value,
  34         "hana::then(before, xs) requires 'before' to be a Monad");
  35
  36         static_assert(hana::Monad<Xs>::value,
  37         "hana::then(before, xs) requires 'xs' to be a Monad");
  38     #endif
  39
  40         return Then::apply(static_cast<Before&&>(before),
  41                            static_cast<Xs&&>(xs));
  42     }
  43     //! @endcond
  44
  45     template <typename M, bool condition>
  46     struct then_impl<M, when<condition>> : default_ {
  47         template <typename Xs, typename Ys>
  48         static constexpr decltype(auto) apply(Xs&& xs, Ys&& ys) {
  49             return hana::chain(static_cast<Xs&&>(xs),
  50                                hana::always(static_cast<Ys&&>(ys)));
  51         }
  52     };
  53 BOOST_HANA_NAMESPACE_END
  54
  55 #endif // !BOOST_HANA_THEN_HPP