Core: document and don't require core.hpp for instance and defaults

example/core/instance.cpp

@@ -0,0 +1,59 @@
+/*
+@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 <boost/hana/detail/static_assert.hpp>
+
+#include <type_traits>
+namespace hana = boost::hana;
+
+
+template <typename T, typename Enable = void>
+struct Incrementable;
+
+BOOST_HANA_CONSTEXPR_LAMBDA auto next = [](auto x) {
+    return Incrementable<hana::datatype_t<decltype(x)>>::next_impl(x);
+};
+
+namespace boost { namespace hana {
+    template <>
+    struct instance<Incrementable> {
+        template <typename T, typename Enable = void>
+        struct with { };
+    };
+
+    template <>
+    struct defaults<Incrementable> {
+        template <typename T, typename Enable = void>
+        struct with { };
+    };
+}}
+
+template <typename T, typename Enable>
+struct Incrementable
+    : hana::instance<Incrementable>::template with<T>
+{ };
+
+namespace boost { namespace hana {
+    template <typename T>
+    struct instance<Incrementable>::with<T,
+        std::enable_if_t<std::is_integral<T>{} || std::is_floating_point<T>{}>
+    >
+        : defaults<Incrementable>::template with<T>
+    {
+        template <typename X>
+        static constexpr auto next_impl(X x)
+        { return x + 1; }
+    };
+}}
+
+
+int main() {
+    BOOST_HANA_STATIC_ASSERT(next(1) == 2);
+    BOOST_HANA_STATIC_ASSERT(next(3.3) == 4.3);
+    BOOST_HANA_STATIC_ASSERT(next('a') == 'b');
+}