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+/*!
+@file
+Forward declares `boost::hana::extend`.
+
+@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_EXTEND_HPP
+#define BOOST_HANA_FWD_EXTEND_HPP
+
+#include <boost/hana/config.hpp>
+#include <boost/hana/core/when.hpp>
+
+
+BOOST_HANA_NAMESPACE_BEGIN
+    //! Comonadic application of a function to a comonadic value.
+    //! @ingroup group-Comonad
+    //!
+    //! Given a comonadic value and a function accepting a comonadic input,
+    //! `extend` returns the result of applying the function to that input
+    //! inside the comonadic context.
+    //!
+    //!
+    //! Signature
+    //! ---------
+    //! Given a Comonad `W` and a function of type \f$ W(T) \to U \f$, the
+    //! signature is
+    //! \f$
+    //!     \mathtt{extend} : W(T) \times (W(T) \to U) \to W(U)
+    //! \f$
+    //!
+    //! @param w
+    //! A comonadic value to call the function with.
+    //!
+    //! @param f
+    //! A function of signature \f$ W(T) \to U \f$ to be applied to its
+    //! comonadic argument inside the comonadic context.
+    //!
+    //!
+    //! Example
+    //! -------
+    //! @include example/extend.cpp
+#ifdef BOOST_HANA_DOXYGEN_INVOKED
+    constexpr auto extend = [](auto&& w, auto&& f) -> decltype(auto) {
+        return tag-dispatched;
+    };
+#else
+    template <typename W, typename = void>
+    struct extend_impl : extend_impl<W, when<true>> { };
+
+    struct extend_t {
+        template <typename W_, typename F>
+        constexpr decltype(auto) operator()(W_&& w, F&& f) const;
+    };
+
+    constexpr extend_t extend{};
+#endif
+BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_FWD_EXTEND_HPP