boost/hana/concat.hpp

0001 /*!
0002 @file
0003 Defines `boost::hana::concat`.
0004
0005 Copyright Louis Dionne 2013-2022
0006 Distributed under the Boost Software License, Version 1.0.
0007 (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
0008  */
0009
0010 #ifndef BOOST_HANA_CONCAT_HPP
0011 #define BOOST_HANA_CONCAT_HPP
0012
0013 #include <boost/hana/fwd/concat.hpp>
0014
0015 #include <boost/hana/at.hpp>
0016 #include <boost/hana/concept/monad_plus.hpp>
0017 #include <boost/hana/concept/sequence.hpp>
0018 #include <boost/hana/config.hpp>
0019 #include <boost/hana/core/dispatch.hpp>
0020 #include <boost/hana/core/make.hpp>
0021 #include <boost/hana/length.hpp>
0022
0023 #include <cstddef>
0024 #include <type_traits>
0025 #include <utility>
0026
0027
0028 namespace boost { namespace hana {
0029     //! @cond
0030     template <typename Xs, typename Ys>
0031     constexpr auto concat_t::operator()(Xs&& xs, Ys&& ys) const {
0032         using M = typename hana::tag_of<Xs>::type;
0033         using Concat = BOOST_HANA_DISPATCH_IF(concat_impl<M>,
0034             hana::MonadPlus<M>::value &&
0035             std::is_same<typename hana::tag_of<Ys>::type, M>::value
0036         );
0037
0038     #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
0039         static_assert(std::is_same<typename hana::tag_of<Ys>::type, M>::value,
0040         "hana::concat(xs, ys) requires 'xs' and 'ys' to have the same tag");
0041
0042         static_assert(hana::MonadPlus<M>::value,
0043         "hana::concat(xs, ys) requires 'xs' and 'ys' to be MonadPlus");
0044     #endif
0045
0046         return Concat::apply(static_cast<Xs&&>(xs), static_cast<Ys&&>(ys));
0047     }
0048     //! @endcond
0049
0050     template <typename M, bool condition>
0051     struct concat_impl<M, when<condition>> : default_ {
0052         template <typename ...Args>
0053         static constexpr auto apply(Args&& ...) = delete;
0054     };
0055
0056     template <typename S>
0057     struct concat_impl<S, when<Sequence<S>::value>> {
0058         template <typename Xs, typename Ys, std::size_t ...xi, std::size_t ...yi>
0059         static constexpr auto
0060         concat_helper(Xs&& xs, Ys&& ys, std::index_sequence<xi...>,
0061                                         std::index_sequence<yi...>)
0062         {
0063             return hana::make<S>(
0064                 hana::at_c<xi>(static_cast<Xs&&>(xs))...,
0065                 hana::at_c<yi>(static_cast<Ys&&>(ys))...
0066             );
0067         }
0068
0069         template <typename Xs, typename Ys>
0070         static constexpr auto apply(Xs&& xs, Ys&& ys) {
0071             constexpr std::size_t xi = decltype(hana::length(xs))::value;
0072             constexpr std::size_t yi = decltype(hana::length(ys))::value;
0073             return concat_helper(static_cast<Xs&&>(xs), static_cast<Ys&&>(ys),
0074                                  std::make_index_sequence<xi>{},
0075                                  std::make_index_sequence<yi>{});
0076         }
0077     };
0078 }} // end namespace boost::hana
0079
0080 #endif // !BOOST_HANA_CONCAT_HPP