boost/hana/flatten.hpp

0001 /*!
0002 @file
0003 Defines `boost::hana::flatten`.
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_FLATTEN_HPP
0011 #define BOOST_HANA_FLATTEN_HPP
0012
0013 #include <boost/hana/fwd/flatten.hpp>
0014
0015 #include <boost/hana/concept/monad.hpp>
0016 #include <boost/hana/concept/sequence.hpp>
0017 #include <boost/hana/config.hpp>
0018 #include <boost/hana/core/dispatch.hpp>
0019 #include <boost/hana/core/make.hpp>
0020 #include <boost/hana/detail/unpack_flatten.hpp>
0021 #include <boost/hana/functional/id.hpp>
0022 #include <boost/hana/fwd/chain.hpp>
0023
0024 #include <cstddef>
0025 #include <utility>
0026
0027
0028 namespace boost { namespace hana {
0029     //! @cond
0030     template <typename Xs>
0031     constexpr auto flatten_t::operator()(Xs&& xs) const {
0032         using M = typename hana::tag_of<Xs>::type;
0033         using Flatten = BOOST_HANA_DISPATCH_IF(flatten_impl<M>,
0034             hana::Monad<M>::value
0035         );
0036
0037 #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
0038         static_assert(hana::Monad<M>::value,
0039         "hana::flatten(xs) requires 'xs' to be a Monad");
0040 #endif
0041
0042         return Flatten::apply(static_cast<Xs&&>(xs));
0043     }
0044     //! @endcond
0045
0046     template <typename M, bool condition>
0047     struct flatten_impl<M, when<condition>> : default_ {
0048         template <typename Xs>
0049         static constexpr auto apply(Xs&& xs)
0050         { return hana::chain(static_cast<Xs&&>(xs), hana::id); }
0051     };
0052
0053     template <typename S>
0054     struct flatten_impl<S, when<Sequence<S>::value>> {
0055         template <typename Xs>
0056         static constexpr auto apply(Xs&& xs) {
0057             return detail::unpack_flatten(static_cast<Xs&&>(xs), hana::make<S>);
0058         }
0059     };
0060 }} // end namespace boost::hana
0061
0062 #endif // !BOOST_HANA_FLATTEN_HPP