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Diffstat (limited to 'boost/hana/prepend.hpp')
-rw-r--r-- | boost/hana/prepend.hpp | 74 |
1 files changed, 74 insertions, 0 deletions
diff --git a/boost/hana/prepend.hpp b/boost/hana/prepend.hpp new file mode 100644 index 0000000000..c4f5f0e0e3 --- /dev/null +++ b/boost/hana/prepend.hpp @@ -0,0 +1,74 @@ +/*! +@file +Defines `boost::hana::prepend`. + +@copyright Louis Dionne 2013-2016 +Distributed under the Boost Software License, Version 1.0. +(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt) + */ + +#ifndef BOOST_HANA_PREPEND_HPP +#define BOOST_HANA_PREPEND_HPP + +#include <boost/hana/fwd/prepend.hpp> + +#include <boost/hana/at.hpp> +#include <boost/hana/concat.hpp> +#include <boost/hana/concept/monad_plus.hpp> +#include <boost/hana/concept/sequence.hpp> +#include <boost/hana/config.hpp> +#include <boost/hana/core/dispatch.hpp> +#include <boost/hana/core/make.hpp> +#include <boost/hana/length.hpp> +#include <boost/hana/lift.hpp> + +#include <cstddef> +#include <utility> + + +BOOST_HANA_NAMESPACE_BEGIN + //! @cond + template <typename Xs, typename X> + constexpr auto prepend_t::operator()(Xs&& xs, X&& x) const { + using M = typename hana::tag_of<Xs>::type; + using Prepend = BOOST_HANA_DISPATCH_IF(prepend_impl<M>, + hana::MonadPlus<M>::value + ); + + #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS + static_assert(hana::MonadPlus<M>::value, + "hana::prepend(xs, x) requires 'xs' to be a MonadPlus"); + #endif + + return Prepend::apply(static_cast<Xs&&>(xs), static_cast<X&&>(x)); + } + //! @endcond + + template <typename M, bool condition> + struct prepend_impl<M, when<condition>> : default_ { + template <typename Xs, typename X> + static constexpr auto apply(Xs&& xs, X&& x) { + return hana::concat(hana::lift<M>(static_cast<X&&>(x)), + static_cast<Xs&&>(xs)); + } + }; + + template <typename S> + struct prepend_impl<S, when<Sequence<S>::value>> { + template <typename Xs, typename X, std::size_t ...i> + static constexpr auto prepend_helper(Xs&& xs, X&& x, std::index_sequence<i...>) { + return hana::make<S>( + static_cast<X&&>(x), hana::at_c<i>(static_cast<Xs&&>(xs))... + ); + } + + template <typename Xs, typename X> + static constexpr auto apply(Xs&& xs, X&& x) { + constexpr std::size_t N = decltype(hana::length(xs))::value; + return prepend_helper(static_cast<Xs&&>(xs), static_cast<X&&>(x), + std::make_index_sequence<N>{}); + } + }; +BOOST_HANA_NAMESPACE_END + +#endif // !BOOST_HANA_PREPEND_HPP |