54 lines
1.4 KiB
C++
54 lines
1.4 KiB
C++
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/*!
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@file
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Defines `boost::hana::empty`.
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@copyright Louis Dionne 2013-2017
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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_EMPTY_HPP
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#define BOOST_HANA_EMPTY_HPP
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#include <boost/hana/fwd/empty.hpp>
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#include <boost/hana/concept/monad_plus.hpp>
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#include <boost/hana/concept/sequence.hpp>
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#include <boost/hana/config.hpp>
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#include <boost/hana/core/dispatch.hpp>
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#include <boost/hana/core/make.hpp>
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namespace boost { namespace hana {
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//! @cond
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template <typename M>
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constexpr auto empty_t<M>::operator()() const {
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#ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
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static_assert(hana::MonadPlus<M>::value,
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"hana::empty<M>() requires 'M' to be a MonadPlus");
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#endif
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using Empty = BOOST_HANA_DISPATCH_IF(empty_impl<M>,
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hana::MonadPlus<M>::value
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);
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return Empty::apply();
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}
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//! @endcond
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template <typename M, bool condition>
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struct empty_impl<M, when<condition>> : default_ {
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template <typename ...Args>
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static constexpr auto apply(Args&& ...) = delete;
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};
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template <typename S>
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struct empty_impl<S, when<Sequence<S>::value>> {
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static constexpr auto apply() {
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return hana::make<S>();
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}
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};
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}} // end namespace boost::hana
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#endif // !BOOST_HANA_EMPTY_HPP
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