Core: use BOOST_HANA_TYPECLASS macro to create type classes.

example/core/default_instance.cpp

@@ -0,0 +1,45 @@
+/*
+@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/core.hpp>
+#include <boost/hana/detail/constexpr.hpp>
+
+#include <cassert>
+#include <sstream>
+#include <string>
+namespace hana = boost::hana;
+
+
+struct Showable {
+    BOOST_HANA_TYPECLASS(Showable);
+    struct mcd { };
+};
+
+BOOST_HANA_CONSTEXPR_LAMBDA auto show = [](auto x) {
+    return Showable::instance<hana::datatype_t<decltype(x)>>::show_impl(x);
+};
+
+namespace boost { namespace hana {
+    template <>
+    struct default_instance<Showable> : Showable::mcd {
+        template <typename X>
+        static std::string show_impl(X)
+        { return "<?>"; }
+    };
+}}
+
+template <>
+struct Showable::instance<int> : Showable::mcd {
+    static auto show_impl(int i)
+    { return (std::ostringstream{} << i).str(); }
+};
+
+struct unshowable { };
+
+int main() {
+    assert(show(1) == "1");
+    assert(show(unshowable{}) == "<?>");
+}