expected: add trivial then method.

expected.hpp

@@ -9,7 +9,7 @@
 #pragma once
 
 #include <exception>
-
+#include <functional>
 
 ///////////////////////////////////////////////////////////////////////////////
 namespace cruft {
@@ -83,7 +83,16 @@ namespace cruft {
         ValueT* operator-> (void)& { return &value (); }
         ValueT& operator*  (void)& { return  value (); }
 
-        operator bool() const noexcept;
+        template <typename FunctionT, typename ...ArgsT>
+        expected then (FunctionT &&func, ArgsT &&...args)
+        {
+            if (!m_valid)
+                return *this;
+            else
+                return expected (std::invoke (func, args..., value ()));
+        }
+
+        explicit operator bool() const noexcept { return m_valid; }
 
     private:
         union storage_t {