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53 lines
1.6 KiB
C++
53 lines
1.6 KiB
C++
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/*!
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@file
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Defines `boost::hana::prefix`.
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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_PREFIX_HPP
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#define BOOST_HANA_PREFIX_HPP
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#include <boost/hana/fwd/prefix.hpp>
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#include <boost/hana/append.hpp>
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#include <boost/hana/chain.hpp>
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#include <boost/hana/concept/monad_plus.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/functional/partial.hpp>
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#include <boost/hana/lift.hpp>
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BOOST_HANA_NAMESPACE_BEGIN
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//! @cond
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template <typename Xs, typename Pref>
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constexpr auto prefix_t::operator()(Xs&& xs, Pref&& pref) const {
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using M = typename hana::tag_of<Xs>::type;
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using Prefix = BOOST_HANA_DISPATCH_IF(prefix_impl<M>,
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hana::MonadPlus<M>::value
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);
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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::prefix(xs, pref) requires 'xs' to be a MonadPlus");
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#endif
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return Prefix::apply(static_cast<Xs&&>(xs), static_cast<Pref&&>(pref));
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}
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//! @endcond
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template <typename M, bool condition>
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struct prefix_impl<M, when<condition>> : default_ {
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template <typename Xs, typename Z>
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static constexpr decltype(auto) apply(Xs&& xs, Z&& z) {
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return hana::chain(static_cast<Xs&&>(xs),
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hana::partial(hana::append, hana::lift<M>(static_cast<Z&&>(z))));
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}
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};
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_PREFIX_HPP
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