1 /*! 2 @file 3 Defines `boost::hana::prepend`. 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_PREPEND_HPP 11 #define BOOST_HANA_PREPEND_HPP 12 13 #include <boost/hana/fwd/prepend.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> operator ()(Xs && xs,X && x) const32 constexpr auto prepend_t::operator()(Xs&& xs, X&& x) const { 33 using M = typename hana::tag_of<Xs>::type; 34 using Prepend = BOOST_HANA_DISPATCH_IF(prepend_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::prepend(xs, x) requires 'xs' to be a MonadPlus"); 41 #endif 42 43 return Prepend::apply(static_cast<Xs&&>(xs), static_cast<X&&>(x)); 44 } 45 //! @endcond 46 47 template <typename M, bool condition> 48 struct prepend_impl<M, when<condition>> : default_ { 49 template <typename Xs, typename X> applyprepend_impl50 static constexpr auto apply(Xs&& xs, X&& x) { 51 return hana::concat(hana::lift<M>(static_cast<X&&>(x)), 52 static_cast<Xs&&>(xs)); 53 } 54 }; 55 56 template <typename S> 57 struct prepend_impl<S, when<Sequence<S>::value>> { 58 template <typename Xs, typename X, std::size_t ...i> prepend_helperprepend_impl59 static constexpr auto prepend_helper(Xs&& xs, X&& x, std::index_sequence<i...>) { 60 return hana::make<S>( 61 static_cast<X&&>(x), hana::at_c<i>(static_cast<Xs&&>(xs))... 62 ); 63 } 64 65 template <typename Xs, typename X> applyprepend_impl66 static constexpr auto apply(Xs&& xs, X&& x) { 67 constexpr std::size_t N = decltype(hana::length(xs))::value; 68 return prepend_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_PREPEND_HPP 75