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Diffstat (limited to 'boost/hana/fwd/empty.hpp')
| -rw-r--r-- | boost/hana/fwd/empty.hpp | 51 |
1 files changed, 51 insertions, 0 deletions
diff --git a/boost/hana/fwd/empty.hpp b/boost/hana/fwd/empty.hpp new file mode 100644 index 0000000000..90a3696357 --- /dev/null +++ b/boost/hana/fwd/empty.hpp @@ -0,0 +1,51 @@ +/*! +@file +Forward declares `boost::hana::empty`. + +@copyright Louis Dionne 2013-2016 +Distributed under the Boost Software License, Version 1.0. +(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt) + */ + +#ifndef BOOST_HANA_FWD_EMPTY_HPP +#define BOOST_HANA_FWD_EMPTY_HPP + +#include <boost/hana/config.hpp> +#include <boost/hana/core/when.hpp> + + +BOOST_HANA_NAMESPACE_BEGIN + //! Identity of the monadic combination `concat`. + //! @ingroup group-MonadPlus + //! + //! Signature + //! --------- + //! Given a MonadPlus `M`, the signature is + //! @f$ \mathtt{empty}_M : \emptyset \to M(T) @f$. + //! + //! @tparam M + //! The tag of the monadic structure to return. This must be + //! a model of the MonadPlus concept. + //! + //! + //! Example + //! ------- + //! @include example/empty.cpp +#ifdef BOOST_HANA_DOXYGEN_INVOKED + template <typename M> + constexpr auto empty = []() { + return tag-dispatched; + }; +#else + template <typename M, typename = void> + struct empty_impl : empty_impl<M, when<true>> { }; + + template <typename M> + struct empty_t; + + template <typename M> + constexpr empty_t<M> empty{}; +#endif +BOOST_HANA_NAMESPACE_END + +#endif // !BOOST_HANA_FWD_EMPTY_HPP |