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+/*!
+@file
+Forward declares `boost::hana::extract`.
+
+@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_FWD_EXTRACT_HPP
+#define BOOST_HANA_FWD_EXTRACT_HPP
+
+#include <boost/hana/config.hpp>
+#include <boost/hana/core/when.hpp>
+
+
+BOOST_HANA_NAMESPACE_BEGIN
+ //! Extract a value in a given comonadic context.
+ //! @ingroup group-Comonad
+ //!
+ //! Given a value inside a comonadic context, extract it from that
+ //! context, performing whatever effects are mandated by that context.
+ //! This can be seen as the dual operation to the `lift` method of the
+ //! Applicative concept.
+ //!
+ //!
+ //! Signature
+ //! ---------
+ //! Given a Comonad `W`, the signature is
+ //! \f$
+ //! \mathtt{extract} : W(T) \to T
+ //! \f$
+ //!
+ //! @param w
+ //! The value to be extracted inside a comonadic context.
+ //!
+ //!
+ //! Example
+ //! -------
+ //! @include example/extract.cpp
+#ifdef BOOST_HANA_DOXYGEN_INVOKED
+ constexpr auto extract = [](auto&& w) -> decltype(auto) {
+ return tag-dispatched;
+ };
+#else
+ template <typename W, typename = void>
+ struct extract_impl : extract_impl<W, when<true>> { };
+
+ struct extract_t {
+ template <typename W_>
+ constexpr decltype(auto) operator()(W_&& w) const;
+ };
+
+ constexpr extract_t extract{};
+#endif
+BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_FWD_EXTRACT_HPP