Emphasizing the arrow (or in fact the most narrow error) is definitively a good idea in that case. Counting the number of arguments is in general a bit problema

Emphasizing the arrow (or in fact the most narrow error) is definitively a good idea in that case. Counting the number of arguments is in general a bit problematic: it is possible to end with an accidental functional value due to an extra argument send to a sufficiently polymorphic function for instance.