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+/*!
+@file
+Forward declares `boost::hana::equal`.
+
+@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_EQUAL_HPP
+#define BOOST_HANA_FWD_EQUAL_HPP
+
+#include <boost/hana/config.hpp>
+#include <boost/hana/core/when.hpp>
+#include <boost/hana/detail/nested_to_fwd.hpp>
+
+
+BOOST_HANA_NAMESPACE_BEGIN
+    //! Returns a `Logical` representing whether `x` is equal to `y`.
+    //! @ingroup group-Comparable
+    //!
+    //! The `equal` function can be called in two different ways. First, it
+    //! can be called like a normal function:
+    //! @code
+    //!     equal(x, y)
+    //! @endcode
+    //!
+    //! However, it may also be partially applied to an argument by using
+    //! `equal.to`:
+    //! @code
+    //!     equal.to(x)(y) == equal(x, y)
+    //! @endcode
+    //!
+    //! In other words, `equal.to(x)` is a function object that is equivalent
+    //! to `partial(equal, x)`. This is provided to enhance the readability of
+    //! some constructs, especially when using higher order algorithms.
+    //!
+    //!
+    //! Signature
+    //! ---------
+    //! Given a Logical `Bool` and two Comparables `A` and `B` that
+    //! share a common embedding, the signature is
+    //! @f$ \mathtt{equal} : A \times B \to Bool @f$.
+    //!
+    //! @param x, y
+    //! Two objects to compare for equality.
+    //!
+    //!
+    //! Example
+    //! -------
+    //! @include example/equal.cpp
+    //!
+    //!
+    //! > #### Rationale for the arity of `equal`
+    //! > It is a valid question whether `equal` should accept more than 2
+    //! > arguments and have semantics matching those of Python's `==`. This
+    //! > is not supported right now for the following reasons:
+    //! > - It was implemented in the MPL11, but it was not shown to be useful
+    //! >   so far.
+    //! > - It does not make sense for `not_equal` to have an arity of more
+    //! >   than 2, only `equal` could maybe have those semantics, which would
+    //! >   break symmetry.
+#ifdef BOOST_HANA_DOXYGEN_INVOKED
+    constexpr auto equal = [](auto&& x, auto&& y) {
+        return tag-dispatched;
+    };
+#else
+    template <typename T, typename U, typename = void>
+    struct equal_impl : equal_impl<T, U, when<true>> { };
+
+    struct equal_t : detail::nested_to<equal_t> {
+        template <typename X, typename Y>
+        constexpr auto operator()(X&& x, Y&& y) const;
+    };
+
+    constexpr equal_t equal{};
+#endif
+BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_FWD_EQUAL_HPP