boost/hana/empty.hpp

0001 /*!
0002 @file
0003 Defines `boost::hana::empty`.
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_EMPTY_HPP
0011 #define BOOST_HANA_EMPTY_HPP
0012
0013 #include <boost/hana/fwd/empty.hpp>
0014
0015 #include <boost/hana/concept/monad_plus.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
0021
0022 namespace boost { namespace hana {
0023     //! @cond
0024     template <typename M>
0025     constexpr auto empty_t<M>::operator()() const {
0026     #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
0027         static_assert(hana::MonadPlus<M>::value,
0028         "hana::empty<M>() requires 'M' to be a MonadPlus");
0029     #endif
0030
0031         using Empty = BOOST_HANA_DISPATCH_IF(empty_impl<M>,
0032             hana::MonadPlus<M>::value
0033         );
0034
0035         return Empty::apply();
0036     }
0037     //! @endcond
0038
0039     template <typename M, bool condition>
0040     struct empty_impl<M, when<condition>> : default_ {
0041         template <typename ...Args>
0042         static constexpr auto apply(Args&& ...) = delete;
0043     };
0044
0045     template <typename S>
0046     struct empty_impl<S, when<Sequence<S>::value>> {
0047         static constexpr auto apply() {
0048             return hana::make<S>();
0049         }
0050     };
0051 }} // end namespace boost::hana
0052
0053 #endif // !BOOST_HANA_EMPTY_HPP