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+/*!
+@file
+Forward declares `boost::hana::ap`.
+
+@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_AP_HPP
+#define BOOST_HANA_FWD_AP_HPP
+
+#include <boost/hana/config.hpp>
+#include <boost/hana/core/when.hpp>
+
+
+BOOST_HANA_NAMESPACE_BEGIN
+    //! Lifted application.
+    //! @ingroup group-Applicative
+    //!
+    //! Specifically, `ap` applies a structure containing functions to a
+    //! structure containing values, and returns a new structure containing
+    //! values. The exact way in which the functions are applied to the values
+    //! depends on the `Applicative`.
+    //!
+    //! `ap` can be called with two arguments or more; the functions in the `f`
+    //! structure are curried and then applied to the values in each `x...`
+    //! structure using the binary form of `ap`. Note that this requires the
+    //! number of `x...` must match the arity of the functions in the `f`
+    //! structure. In other words, `ap(f, x1, ..., xN)` is equivalent to
+    //! @code
+    //!     ((f' <ap> x1) <ap> x2) ... <ap> xN
+    //! @endcode
+    //! where `f'` is `f` but containing curried functions instead and
+    //! `x <ap> y` is just `ap(x, y)` written in infix notation to emphasize
+    //! the left associativity.
+    //!
+    //!
+    //! Signature
+    //! ---------
+    //! Given an Applicative `A`, the signature is
+    //! @f$ \mathtt{ap} : A(T_1 \times \cdots \times T_n \to U)
+    //!                   \times A(T_1) \times \cdots \times A(T_n)
+    //!                   \to A(U) @f$.
+    //!
+    //! @param f
+    //! A structure containing function(s).
+    //!
+    //! @param x...
+    //! Structure(s) containing value(s) and on which `f` is applied. The
+    //! number of structures must match the arity of the functions in the
+    //! `f` structure.
+    //!
+    //!
+    //! Example
+    //! -------
+    //! @include example/ap.cpp
+    //!
+    //! @todo
+    //! Consider giving access to all the arguments to the tag-dispatched
+    //! implementation for performance purposes.
+#ifdef BOOST_HANA_DOXYGEN_INVOKED
+    constexpr auto ap = [](auto&& f, auto&& ...x) -> decltype(auto) {
+        return tag-dispatched;
+    };
+#else
+    template <typename A, typename = void>
+    struct ap_impl : ap_impl<A, when<true>> { };
+
+    struct ap_t {
+        template <typename F, typename X>
+        constexpr decltype(auto) operator()(F&& f, X&& x) const;
+
+        template <typename F, typename ...Xs>
+        constexpr decltype(auto) operator()(F&& f, Xs&& ...xs) const;
+    };
+
+    constexpr ap_t ap{};
+#endif
+BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_FWD_AP_HPP