| 1 | /*! |
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| 2 | @file |
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| 3 | Forward declares `boost::hana::empty`. |
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| 4 | |
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| 5 | Copyright Louis Dionne 2013-2022 |
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| 6 | Distributed under the Boost Software License, Version 1.0. |
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| 7 | (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt) |
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| 8 | */ |
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| 9 | |
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| 10 | #ifndef BOOST_HANA_FWD_EMPTY_HPP |
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| 11 | #define BOOST_HANA_FWD_EMPTY_HPP |
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| 12 | |
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| 13 | #include <boost/hana/config.hpp> |
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| 14 | #include <boost/hana/core/when.hpp> |
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| 15 | |
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| 16 | |
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| 17 | namespace boost { namespace hana { |
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| 18 | //! Identity of the monadic combination `concat`. |
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| 19 | //! @ingroup group-MonadPlus |
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| 20 | //! |
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| 21 | //! Signature |
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| 22 | //! --------- |
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| 23 | //! Given a MonadPlus `M`, the signature is |
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| 24 | //! @f$ \mathtt{empty}_M : \emptyset \to M(T) @f$. |
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| 25 | //! |
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| 26 | //! @tparam M |
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| 27 | //! The tag of the monadic structure to return. This must be |
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| 28 | //! a model of the MonadPlus concept. |
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| 29 | //! |
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| 30 | //! |
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| 31 | //! Example |
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| 32 | //! ------- |
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| 33 | //! @include example/empty.cpp |
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| 34 | #ifdef BOOST_HANA_DOXYGEN_INVOKED |
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| 35 | template <typename M> |
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| 36 | constexpr auto empty = []() { |
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| 37 | return tag-dispatched; |
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| 38 | }; |
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| 39 | #else |
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| 40 | template <typename M, typename = void> |
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| 41 | struct empty_impl : empty_impl<M, when<true>> { }; |
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| 42 | |
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| 43 | template <typename M> |
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| 44 | struct empty_t { |
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| 45 | constexpr auto operator()() const; |
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| 46 | }; |
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| 47 | |
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| 48 | template <typename M> |
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| 49 | BOOST_HANA_INLINE_VARIABLE constexpr empty_t<M> empty{}; |
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| 50 | #endif |
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| 51 | }} // end namespace boost::hana |
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| 52 | |
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| 53 | #endif // !BOOST_HANA_FWD_EMPTY_HPP |
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| 54 | |
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