75 lines
2.4 KiB
C++
75 lines
2.4 KiB
C++
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/*!
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@file
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Defines `boost::hana::prepend`.
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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_PREPEND_HPP
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#define BOOST_HANA_PREPEND_HPP
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#include <boost/hana/fwd/prepend.hpp>
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#include <boost/hana/at.hpp>
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#include <boost/hana/concat.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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#include <boost/hana/length.hpp>
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#include <boost/hana/lift.hpp>
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#include <cstddef>
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#include <utility>
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BOOST_HANA_NAMESPACE_BEGIN
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//! @cond
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template <typename Xs, typename X>
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constexpr auto prepend_t::operator()(Xs&& xs, X&& x) const {
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using M = typename hana::tag_of<Xs>::type;
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using Prepend = BOOST_HANA_DISPATCH_IF(prepend_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::prepend(xs, x) requires 'xs' to be a MonadPlus");
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#endif
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return Prepend::apply(static_cast<Xs&&>(xs), static_cast<X&&>(x));
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}
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//! @endcond
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template <typename M, bool condition>
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struct prepend_impl<M, when<condition>> : default_ {
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template <typename Xs, typename X>
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static constexpr auto apply(Xs&& xs, X&& x) {
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return hana::concat(hana::lift<M>(static_cast<X&&>(x)),
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static_cast<Xs&&>(xs));
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}
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};
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template <typename S>
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struct prepend_impl<S, when<Sequence<S>::value>> {
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template <typename Xs, typename X, std::size_t ...i>
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static constexpr auto prepend_helper(Xs&& xs, X&& x, std::index_sequence<i...>) {
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return hana::make<S>(
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static_cast<X&&>(x), hana::at_c<i>(static_cast<Xs&&>(xs))...
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);
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}
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template <typename Xs, typename X>
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static constexpr auto apply(Xs&& xs, X&& x) {
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constexpr std::size_t N = decltype(hana::length(xs))::value;
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return prepend_helper(static_cast<Xs&&>(xs), static_cast<X&&>(x),
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std::make_index_sequence<N>{});
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}
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};
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_PREPEND_HPP
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