From fb4e70d1467845610c3b9f6ab9cc51b9eedab4bb Mon Sep 17 00:00:00 2001
From: Danny Robson <danny@nerdcruft.net>
Date: Tue, 25 Feb 2020 11:16:16 +1100
Subject: [PATCH] concepts: add initial invocable concept

---
 concepts.hpp | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/concepts.hpp b/concepts.hpp
index f1380e1c..03baca26 100644
--- a/concepts.hpp
+++ b/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...>>;
 }