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+/*!
+@file
+Defines `boost::hana::empty`.
+
+@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_EMPTY_HPP
+#define BOOST_HANA_EMPTY_HPP
+
+#include <boost/hana/fwd/empty.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>
+
+
+BOOST_HANA_NAMESPACE_BEGIN
+ template <typename M>
+ struct empty_t {
+ #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
+ static_assert(hana::MonadPlus<M>::value,
+ "hana::empty<M>() requires 'M' to be a MonadPlus");
+ #endif
+
+ constexpr auto operator()() const {
+ using Empty = BOOST_HANA_DISPATCH_IF(empty_impl<M>,
+ hana::MonadPlus<M>::value
+ );
+
+ return Empty::apply();
+ }
+ };
+
+ template <typename M, bool condition>
+ struct empty_impl<M, when<condition>> : default_ {
+ template <typename ...Args>
+ static constexpr auto apply(Args&& ...) = delete;
+ };
+
+ template <typename S>
+ struct empty_impl<S, when<Sequence<S>::value>> {
+ static constexpr auto apply() {
+ return hana::make<S>();
+ }
+ };
+BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_EMPTY_HPP