[plt-scheme] Reference to an identifier before its definition

As I try to start the following program, I receive the message

  reference to an identifier before its definition: handle-ok

I see that handle-ok is defined after its use, and if I move the definition before its use,
(e.g at the top of the program), it works.

However, in the definition of handle-ok, I CAN refer to msg, even if it is defined after its use.

So the question is :

Are forward references to definitions allowed ?

Regards
Maurizio


---- This doesn't works --------------

#lang scheme/gui

(define frame (new frame% [label "Example"]))

(define msg (new message% [parent frame]
                 [label "No events so far..."]))

(new button% [parent frame]
     [label "Click Me"]
     (callback handle-ok))    ;  <--- ERROR

(define (handle-ok button event)
                 (send msg set-label "Button click"))

(send frame show #t)

---- This works --------------

#lang scheme/gui

(define (handle-ok button event)
                 (send msg set-label "Button click"))  ; <----  REFERENCE TO MSG IS OK !!!!!!

(define frame (new frame% [label "Example"]))

(define msg (new message% [parent frame]
                 [label "No events so far..."]))

(new button% [parent frame]
     [label "Click Me"]
     (callback handle-ok))


(send frame show #t)
-------------- next part --------------


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