concepts: add initial invocable concept

concepts.hpp

@@ -11,6 +11,7 @@
 #include <type_traits>
 #include <iterator>
 #include <utility>
+#include <functional>
 
 
 ///////////////////////////////////////////////////////////////////////////////
@@ -134,6 +135,25 @@ namespace cruft::concepts {
         { b == a } -> boolean;
         { b != a } -> boolean;
     };
+
+
+    template <typename FunctionT, typename ...ArgsT>
+    concept invocable =
+        requires (FunctionT &&function, ArgsT &&...args)
+    {
+        std::invoke (
+            std::forward<FunctionT> (function),
+            std::forward<ArgsT> (args)...
+        );
+    };
+
+    template <typename FunctionT, typename ...ArgsT>
+    concept regular_invocable = invocable<FunctionT, ArgsT...>;
+
+    template <typename FunctionT, typename ...ArgsT>
+    concept predicate =
+        regular_invocable<FunctionT, ArgsT...> &&
+        boolean<std::invoke_result_t<FunctionT, ArgsT...>>;
 }