# [plt-scheme] Typed scheme: Cannot apply expression of type Procedure, since it is not a function type

Hi,
continuing on the stuff from my last question (where I was asking for a way to find out a function's return type), I now try representing a function by a structure
(define-struct: Fun ((name : Symbol) (proc : Procedure) (args : (Listof Symbol)) (ret : Symbol)))
But when I try
(define f1 (make-Fun 'plus + '(n n) 'n))
((Fun-proc f1) 1 2)
or
(define f2 (make-Fun 'plus (lambda: ((x : Real) (y : Real)) (+ x y)) '(n n) 'n))
((Fun-proc f2) 1 2)
I get the error "Cannot apply expression of type Procedure, since it is not a function type"
What can I do to really get a function type? If all functions were of the same type, I'd try something like
(define-struct: Fun ((name : Symbol) (proc : (Any Any -> Any)) (args : (Listof Symbol)) (ret : Symbol)))
... but as things are, I have no idea what to do.
Thanks a lot in advance for any help,
Sigrid