I added is_functional.

Meta/is_functional.h

@@ -0,0 +1,37 @@
+static_assert( __cplusplus >  201700, "C++17 Required" );
+
+#pragma once
+
+#include <Alepha/Alepha.h>
+
+#include <functional>
+
+namespace Alepha::Hydrogen::Meta
+{
+	inline namespace exports { inline namespace type_traits {} }
+
+	namespace detail::type_traits::is_functional
+	{
+		inline namespace exports {}
+
+		template< typename T, typename= void >
+		struct is_functional : std::false_type {};
+
+		template< typename T >
+		struct is_functional< T, std::void_t< decltype( std::function< std::declval< T >() > ) > > : std::true_type {};
+
+		namespace exports
+		{
+			template< typename T >
+			constexpr bool is_functional_v= is_functional< T >::value;
+
+			template< typename T >
+			struct is_functional : std::bool_constant< bool, is_functional_v< T > > {};
+		}
+	}
+
+	namespace exports::type_traits
+	{
+		using namespace detail::type_traits::is_functional::exports;
+	}
+}