boost/hana/prefix.hpp

0001 /*!
0002 @file
0003 Defines `boost::hana::prefix`.
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_PREFIX_HPP
0011 #define BOOST_HANA_PREFIX_HPP
0012
0013 #include <boost/hana/fwd/prefix.hpp>
0014
0015 #include <boost/hana/append.hpp>
0016 #include <boost/hana/chain.hpp>
0017 #include <boost/hana/concept/monad_plus.hpp>
0018 #include <boost/hana/config.hpp>
0019 #include <boost/hana/core/dispatch.hpp>
0020 #include <boost/hana/functional/partial.hpp>
0021 #include <boost/hana/lift.hpp>
0022
0023
0024 namespace boost { namespace hana {
0025     //! @cond
0026     template <typename Xs, typename Pref>
0027     constexpr auto prefix_t::operator()(Xs&& xs, Pref&& pref) const {
0028         using M = typename hana::tag_of<Xs>::type;
0029         using Prefix = BOOST_HANA_DISPATCH_IF(prefix_impl<M>,
0030             hana::MonadPlus<M>::value
0031         );
0032
0033     #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
0034         static_assert(hana::MonadPlus<M>::value,
0035         "hana::prefix(xs, pref) requires 'xs' to be a MonadPlus");
0036     #endif
0037
0038         return Prefix::apply(static_cast<Xs&&>(xs), static_cast<Pref&&>(pref));
0039     }
0040     //! @endcond
0041
0042     template <typename M, bool condition>
0043     struct prefix_impl<M, when<condition>> : default_ {
0044         template <typename Xs, typename Z>
0045         static constexpr decltype(auto) apply(Xs&& xs, Z&& z) {
0046             return hana::chain(static_cast<Xs&&>(xs),
0047                 hana::partial(hana::append, hana::lift<M>(static_cast<Z&&>(z))));
0048         }
0049     };
0050 }} // end namespace boost::hana
0051
0052 #endif // !BOOST_HANA_PREFIX_HPP