1 files changed, 68 insertions, 0 deletions
diff --git a/boost/hana/fwd/hash.hpp b/boost/hana/fwd/hash.hpp
new file mode 100644
index 0000000000..38602db518
--- /dev/null
+++ b/boost/hana/fwd/hash.hpp
@@ -0,0 +1,68 @@
+/*!
+@file
+Forward declares `boost::hana::hash`.
+
+@copyright Louis Dionne 2016
+@copyright Jason Rice 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_HASH_HPP
+#define BOOST_HANA_FWD_HASH_HPP
+
+#include <boost/hana/config.hpp>
+#include <boost/hana/core/when.hpp>
+
+
+BOOST_HANA_NAMESPACE_BEGIN
+    //! Returns a `hana::type` representing the compile-time hash of an object.
+    //! @ingroup group-Hashable
+    //!
+    //! Given an arbitrary object `x`, `hana::hash` returns a `hana::type`
+    //! representing the hash of `x`. In normal programming, hashes are
+    //! usually numerical values that can be used e.g. as indices in an
+    //! array as part of the implementation of a hash table. In the context
+    //! of metaprogramming, we are interested in type-level hashes instead.
+    //! Thus, `hana::hash` must return a `hana::type` object instead of an
+    //! integer. This `hana::type` must somehow summarize the object being
+    //! hashed, but that summary may of course lose some information.
+    //!
+    //! In order for the `hash` function to be defined properly, it must be
+    //! the case that whenever `x` is equal to `y`, then `hash(x)` is equal
+    //! to `hash(y)`. This ensures that `hana::hash` is a function in the
+    //! mathematical sense of the term.
+    //!
+    //!
+    //! Signature
+    //! ---------
+    //! Given a `Hashable` `H`, the signature is
+    //! \f$
+    //!     \mathtt{hash} : H \to \mathtt{type\_tag}
+    //! \f$
+    //!
+    //! @param x
+    //! An object whose hash is to be computed.
+    //!
+    //!
+    //! Example
+    //! -------
+    //! @include example/hash.cpp
+#ifdef BOOST_HANA_DOXYGEN_INVOKED
+    constexpr auto hash = [](auto const& x) {
+        return tag-dispatched;
+    };
+#else
+    template <typename T, typename = void>
+    struct hash_impl : hash_impl<T, when<true>> { };
+
+    struct hash_t {
+        template <typename X>
+        constexpr auto operator()(X const& x) const;
+    };
+
+    constexpr hash_t hash{};
+#endif
+BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_FWD_HASH_HPP