text: /*!
  @file
  Defines `boost::hana::concat`.
  
  @copyright Louis Dionne 2013-2016
  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_CONCAT_HPP
  #define BOOST_HANA_CONCAT_HPP
  
  #include <boost/hana/fwd/concat.hpp>
  
  #include <boost/hana/at.hpp>
  #include <boost/hana/concept/monad_plus.hpp>
  #include <boost/hana/concept/sequence.hpp>
  #include <boost/hana/config.hpp>
  #include <boost/hana/core/dispatch.hpp>
  #include <boost/hana/core/make.hpp>
  #include <boost/hana/length.hpp>
  
  #include <cstddef>
  #include <type_traits>
  #include <utility>
  
  
  BOOST_HANA_NAMESPACE_BEGIN
      //! @cond
      template <typename Xs, typename Ys>
      constexpr auto concat_t::operator()(Xs&& xs, Ys&& ys) const {
          using M = typename hana::tag_of<Xs>::type;
          using Concat = BOOST_HANA_DISPATCH_IF(concat_impl<M>,
              hana::MonadPlus<M>::value &&
              std::is_same<typename hana::tag_of<Ys>::type, M>::value
          );
  
      #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
          static_assert(std::is_same<typename hana::tag_of<Ys>::type, M>::value,
          "hana::concat(xs, ys) requires 'xs' and 'ys' to have the same tag");
  
          static_assert(hana::MonadPlus<M>::value,
          "hana::concat(xs, ys) requires 'xs' and 'ys' to be MonadPlus");
      #endif
  
          return Concat::apply(static_cast<Xs&&>(xs), static_cast<Ys&&>(ys));
      }
      //! @endcond
  
      template <typename M, bool condition>
      struct concat_impl<M, when<condition>> : default_ {
          template <typename ...Args>
          static constexpr auto apply(Args&& ...) = delete;
      };
  
      template <typename S>
      struct concat_impl<S, when<Sequence<S>::value>> {
          template <typename Xs, typename Ys, std::size_t ...xi, std::size_t ...yi>
          static constexpr auto
          concat_helper(Xs&& xs, Ys&& ys, std::index_sequence<xi...>,
                                          std::index_sequence<yi...>)
          {
              return hana::make<S>(
                  hana::at_c<xi>(static_cast<Xs&&>(xs))...,
                  hana::at_c<yi>(static_cast<Ys&&>(ys))...
              );
          }
  
          template <typename Xs, typename Ys>
          static constexpr auto apply(Xs&& xs, Ys&& ys) {
              constexpr std::size_t xi = decltype(hana::length(xs))::value;
              constexpr std::size_t yi = decltype(hana::length(ys))::value;
              return concat_helper(static_cast<Xs&&>(xs), static_cast<Ys&&>(ys),
                                   std::make_index_sequence<xi>{},
                                   std::make_index_sequence<yi>{});
          }
      };
  BOOST_HANA_NAMESPACE_END
  
  #endif // !BOOST_HANA_CONCAT_HPP
  