Comparable: add the comparing helper function

example/comparable/comparing.cpp

@@ -0,0 +1,41 @@
+/*
+@copyright Louis Dionne 2014
+Distributed under the Boost Software License, Version 1.0.
+(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
+ */
+
+#include <boost/hana/detail/assert.hpp>
+#include <boost/hana/detail/constexpr.hpp>
+#include <boost/hana/list/instance.hpp>
+#include <boost/hana/range.hpp>
+#include <boost/hana/type_list.hpp>
+using namespace boost::hana;
+
+
+int main() {
+    //! [main]
+    BOOST_HANA_CONSTEXPR_LAMBDA auto grouped = group_by(comparing(length), list(
+        list(1, 2, 3),
+        list('x', 'y', 'z'),
+        range_c<long, 0, 1>,
+        type_list<char, int>,
+        range_c<int, 0, 2>,
+        list(123.4, nullptr)
+    ));
+
+    BOOST_HANA_CONSTEXPR_ASSERT(grouped == list(
+        list(
+            list(1, 2, 3),
+            list('x', 'y', 'z')
+        ),
+        list(
+            range_c<long, 0, 1>
+        ),
+        list(
+            type_list<char, int>,
+            range_c<int, 0, 2>,
+            list(123.4, nullptr)
+        )
+    ));
+    //! [main]
+}