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+/*!
+@file
+Defines `boost::hana::front`.
+
+@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_FRONT_HPP
+#define BOOST_HANA_FRONT_HPP
+
+#include <boost/hana/fwd/front.hpp>
+
+#include <boost/hana/at.hpp>
+#include <boost/hana/concept/iterable.hpp>
+#include <boost/hana/config.hpp>
+#include <boost/hana/core/dispatch.hpp>
+
+
+BOOST_HANA_NAMESPACE_BEGIN
+ //! @cond
+ template <typename Xs>
+ constexpr decltype(auto) front_t::operator()(Xs&& xs) const {
+ using It = typename hana::tag_of<Xs>::type;
+ using Front = BOOST_HANA_DISPATCH_IF(front_impl<It>,
+ hana::Iterable<It>::value
+ );
+
+ #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
+ static_assert(hana::Iterable<It>::value,
+ "hana::front(xs) requires 'xs' to be an Iterable");
+ #endif
+
+ return Front::apply(static_cast<Xs&&>(xs));
+ }
+ //! @endcond
+
+ template <typename It, bool condition>
+ struct front_impl<It, when<condition>> : default_ {
+ template <typename Xs>
+ static constexpr decltype(auto) apply(Xs&& xs)
+ { return hana::at_c<0>(static_cast<Xs&&>(xs)); }
+ };
+BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_FRONT_HPP