1 files changed, 57 insertions, 0 deletions
diff --git a/boost/hana/length.hpp b/boost/hana/length.hpp
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+++ b/boost/hana/length.hpp
@@ -0,0 +1,57 @@
+/*!
+@file
+Defines `boost::hana::length`.
+
+@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_LENGTH_HPP
+#define BOOST_HANA_LENGTH_HPP
+
+#include <boost/hana/fwd/length.hpp>
+
+#include <boost/hana/concept/foldable.hpp>
+#include <boost/hana/config.hpp>
+#include <boost/hana/core/dispatch.hpp>
+#include <boost/hana/integral_constant.hpp>
+#include <boost/hana/unpack.hpp>
+
+
+BOOST_HANA_NAMESPACE_BEGIN
+    //! @cond
+    template <typename Xs>
+    constexpr auto length_t::operator()(Xs const& xs) const {
+        using S = typename hana::tag_of<Xs>::type;
+        using Length = BOOST_HANA_DISPATCH_IF(length_impl<S>,
+            hana::Foldable<S>::value
+        );
+
+    #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
+        static_assert(hana::Foldable<S>::value,
+        "hana::length(xs) requires 'xs' to be Foldable");
+    #endif
+
+        return Length::apply(xs);
+    }
+    //! @endcond
+
+    namespace detail {
+        struct argn {
+            template <typename ...Xs>
+            constexpr auto operator()(Xs const& ...) const
+            { return hana::size_c<sizeof...(Xs)>; }
+        };
+    }
+
+    template <typename T, bool condition>
+    struct length_impl<T, when<condition>> : default_ {
+        template <typename Xs>
+        static constexpr auto apply(Xs const& xs) {
+            return hana::unpack(xs, detail::argn{});
+        }
+    };
+BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_LENGTH_HPP