52 lines
1.3 KiB
C++
52 lines
1.3 KiB
C++
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/*!
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@file
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Forward declares `boost::hana::empty`.
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@copyright Louis Dionne 2013-2016
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Distributed under the Boost Software License, Version 1.0.
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(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
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*/
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#ifndef BOOST_HANA_FWD_EMPTY_HPP
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#define BOOST_HANA_FWD_EMPTY_HPP
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#include <boost/hana/config.hpp>
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#include <boost/hana/core/when.hpp>
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BOOST_HANA_NAMESPACE_BEGIN
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//! Identity of the monadic combination `concat`.
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//! @ingroup group-MonadPlus
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//!
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//! Signature
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//! ---------
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//! Given a MonadPlus `M`, the signature is
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//! @f$ \mathtt{empty}_M : \emptyset \to M(T) @f$.
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//!
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//! @tparam M
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//! The tag of the monadic structure to return. This must be
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//! a model of the MonadPlus concept.
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//!
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//!
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//! Example
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//! -------
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//! @include example/empty.cpp
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#ifdef BOOST_HANA_DOXYGEN_INVOKED
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template <typename M>
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constexpr auto empty = []() {
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return tag-dispatched;
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};
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#else
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template <typename M, typename = void>
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struct empty_impl : empty_impl<M, when<true>> { };
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template <typename M>
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struct empty_t;
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template <typename M>
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constexpr empty_t<M> empty{};
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#endif
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_FWD_EMPTY_HPP
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