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+/*!
+@file
+Defines `boost::hana::take_front` and `boost::hana::take_front_c`.
+
+@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_TAKE_FRONT_HPP
+#define BOOST_HANA_TAKE_FRONT_HPP
+
+#include <boost/hana/fwd/take_front.hpp>
+
+#include <boost/hana/at.hpp>
+#include <boost/hana/concept/integral_constant.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/integral_constant.hpp>
+#include <boost/hana/length.hpp>
+
+#include <cstddef>
+#include <utility>
+
+
+BOOST_HANA_NAMESPACE_BEGIN
+ //! @cond
+ template <typename Xs, typename N>
+ constexpr auto take_front_t::operator()(Xs&& xs, N const& n) const {
+ using S = typename hana::tag_of<Xs>::type;
+ using TakeFront = BOOST_HANA_DISPATCH_IF(take_front_impl<S>,
+ hana::Sequence<S>::value &&
+ hana::IntegralConstant<N>::value
+ );
+
+#ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
+ static_assert(hana::Sequence<S>::value,
+ "hana::take_front(xs, n) requires 'xs' to be a Sequence");
+
+ static_assert(hana::IntegralConstant<N>::value,
+ "hana::take_front(xs, n) requires 'n' to be an IntegralConstant");
+#endif
+
+ return TakeFront::apply(static_cast<Xs&&>(xs), n);
+ }
+ //! @endcond
+
+ template <typename S, bool condition>
+ struct take_front_impl<S, when<condition>> : default_ {
+ template <typename Xs, std::size_t ...n>
+ static constexpr auto take_front_helper(Xs&& xs, std::index_sequence<n...>) {
+ return hana::make<S>(hana::at_c<n>(static_cast<Xs&&>(xs))...);
+ }
+
+ template <typename Xs, typename N>
+ static constexpr auto apply(Xs&& xs, N const&) {
+ constexpr std::size_t n = N::value;
+ constexpr std::size_t size = decltype(hana::length(xs))::value;
+ return take_front_helper(static_cast<Xs&&>(xs),
+ std::make_index_sequence<(n < size ? n : size)>{});
+ }
+ };
+
+ template <std::size_t n>
+ struct take_front_c_t {
+ template <typename Xs>
+ constexpr auto operator()(Xs&& xs) const {
+ return hana::take_front(static_cast<Xs&&>(xs), hana::size_c<n>);
+ }
+ };
+BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_TAKE_FRONT_HPP