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+/*!
+@file
+Forward declares `boost::hana::chain`.
+
+@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_CHAIN_HPP
+#define BOOST_HANA_FWD_CHAIN_HPP
+
+#include <boost/hana/config.hpp>
+#include <boost/hana/core/when.hpp>
+
+
+BOOST_HANA_NAMESPACE_BEGIN
+    //! Feed a monadic value into a monadic computation.
+    //! @ingroup group-Monad
+    //!
+    //! Given a monadic value and a monadic function, `chain` feeds the
+    //! monadic value into the function, thus performing some Monad-specific
+    //! effects, and returns the result. An implementation of `chain` must
+    //! satisfy
+    //! @code
+    //!     chain(xs, f) == flatten(transform(xs, f))
+    //! @endcode
+    //!
+    //!
+    //! Signature
+    //! ---------
+    //! For a monad `M`, given a monadic value of type `M(A)` and a monadic
+    //! function @f$ f : A \to M(B) @f$, `chain` has the signature
+    //! @f$
+    //!     \mathtt{chain} : M(A) \times (A \to M(B)) \to M(B)
+    //! @f$.
+    //!
+    //! @param xs
+    //! A monadic value to be fed to the function `f`.
+    //!
+    //! @param f
+    //! A function taking a normal value in the `xs` structure, and returning
+    //! a monadic value. This function is called as  `f(x)`, where `x` is an
+    //! element of the structure `xs`.
+    //!
+    //!
+    //! Example
+    //! -------
+    //! @include example/chain.cpp
+#ifdef BOOST_HANA_DOXYGEN_INVOKED
+    constexpr auto chain = [](auto&& xs, auto&& f) -> decltype(auto) {
+        return tag-dispatched;
+    };
+#else
+    template <typename M, typename = void>
+    struct chain_impl : chain_impl<M, when<true>> { };
+
+    struct chain_t {
+        template <typename Xs, typename F>
+        constexpr decltype(auto) operator()(Xs&& xs, F&& f) const;
+    };
+
+    constexpr chain_t chain{};
+#endif
+BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_FWD_CHAIN_HPP