boost/hana/append.hpp

   1 /*!
   2 @file
   3 Defines `boost::hana::append`.
   4
   5 @copyright Louis Dionne 2013-2017
   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_APPEND_HPP
  11 #define BOOST_HANA_APPEND_HPP
  12
  13 #include <boost/hana/fwd/append.hpp>
  14
  15 #include <boost/hana/at.hpp>
  16 #include <boost/hana/concat.hpp>
  17 #include <boost/hana/concept/monad_plus.hpp>
  18 #include <boost/hana/concept/sequence.hpp>
  19 #include <boost/hana/config.hpp>
  20 #include <boost/hana/core/dispatch.hpp>
  21 #include <boost/hana/core/make.hpp>
  22 #include <boost/hana/length.hpp>
  23 #include <boost/hana/lift.hpp>
  24
  25 #include <cstddef>
  26 #include <utility>
  27
  28
  29 BOOST_HANA_NAMESPACE_BEGIN
  30     //! @cond
  31     template <typename Xs, typename X>
  32     constexpr auto append_t::operator()(Xs&& xs, X&& x) const {
  33         using M = typename hana::tag_of<Xs>::type;
  34         using Append = BOOST_HANA_DISPATCH_IF(append_impl<M>,
  35             hana::MonadPlus<M>::value
  36         );
  37
  38     #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
  39         static_assert(hana::MonadPlus<M>::value,
  40         "hana::append(xs, x) requires 'xs' to be a MonadPlus");
  41     #endif
  42
  43         return Append::apply(static_cast<Xs&&>(xs), static_cast<X&&>(x));
  44     }
  45     //! @endcond
  46
  47     template <typename M, bool condition>
  48     struct append_impl<M, when<condition>> : default_ {
  49         template <typename Xs, typename X>
  50         static constexpr auto apply(Xs&& xs, X&& x) {
  51             return hana::concat(static_cast<Xs&&>(xs),
  52                                 hana::lift<M>(static_cast<X&&>(x)));
  53         }
  54     };
  55
  56     template <typename S>
  57     struct append_impl<S, when<Sequence<S>::value>> {
  58         template <typename Xs, typename X, std::size_t ...i>
  59         static constexpr auto append_helper(Xs&& xs, X&& x, std::index_sequence<i...>) {
  60             return hana::make<S>(
  61                 hana::at_c<i>(static_cast<Xs&&>(xs))..., static_cast<X&&>(x)
  62             );
  63         }
  64
  65         template <typename Xs, typename X>
  66         static constexpr auto apply(Xs&& xs, X&& x) {
  67             constexpr std::size_t N = decltype(hana::length(xs))::value;
  68             return append_helper(static_cast<Xs&&>(xs), static_cast<X&&>(x),
  69                                  std::make_index_sequence<N>{});
  70         }
  71     };
  72 BOOST_HANA_NAMESPACE_END
  73
  74 #endif // !BOOST_HANA_APPEND_HPP