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- /*!
- @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
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