[plt-scheme] Return Value

> There is no good way to return a value from a callback using a typical
> oo/callback based GUI system such as mred's. The control context of the
> callback and the control context of the main program are totally
> disconnected. You have to use mutable state to communicate the value
> and possibly use a semaphore if the code needs to wake up the main
> thread when the button is clicked.

This is really a bad problem.
Sometimes people need to re-check their decisions so I was
creating a predicate that would show a yes/no radio-box and a
button. If when the button is clicked, the radio-box is on Yes we
return #t otherwise #f. 

Since we cannot return values from callbacks I imagine that it
would be a lot of work to implement something like this... right?

Any ideas?

Best regards,

Paulo

> 
> The Haskellites at Yale have noticed this problem, too. They use arrows
> (a functional programming technique for mimicing imperative features)
> to glue together either streams of events or continually varying
> signals.  If you're interested, check out their paper.
> 
>   Antony Courtney and Conal Elliott. Genuinely functional user
>   interfaces. In Proceedings of the 2001 Haskell Workshop, September
>   2001.
> 
> Paul
> 
> P.S.  You can use (instantiate button% () (callback void) ...) if you don't
>       want the callback to do anything.
> 
> At 09 Sep 2002 02:15:32 +0100, Paulo J. Matos wrote:
> >   For list-related administrative tasks:
> >     http://list.cs.brown.edu/mailman/listinfo/plt-scheme
> > 
> > Hi all,
> > 
> > I'm doing a graphical interface that was suggested in my last
> > mail to the list.
> > Somehow I got myself into a mess.
> > 
> > I have a function like the following that should return a struct
> > of type tran:
> > (define read-tran-gui
> >  (lambda ()
> >     (define rtframe
> >       (instantiate frame% ()
> >         (label "New Transaction")))
> >     ...
> > 
> > (instantiate button% ()
> >       (label "Insert Transaction")
> >       (parent rtframe)
> >       (callback (lambda (button event) 
> >                   (make-new-tran 
> >                    (string->number (send tbamount get-value))
> >                    (if (eq? (send rad-type get-selection)
> >                             0)
> >                        *CRED*
> >                        *DEB*)
> >                    (send tbcomment get-value))
> >                   (send rtframe show #f))))
> >     
> >     (send rtframe show #t)))
> > 
> > The function needs to return the value returned by make-new-tran
> > called in the callback function of the button. How can I do that?
> > 
> > Best regards,
> > 
> > -- 
> > Paulo J. Matos : pocm(_at_)mega.ist.utl.pt
> > Instituto Superior Tecnico - Lisbon    
> > Software & Computer Engineering - A.I.
> >  - > http://mega.ist.utl.pt/~pocm 
> >  ---	
> > 	Yes, God had a deadline...
> > 		So, He wrote it all in Lisp!
> > 
> 
> 

-- 
Paulo J. Matos : pocm(_at_)mega.ist.utl.pt
Instituto Superior Tecnico - Lisbon    
Software & Computer Engineering - A.I.
 - > http://mega.ist.utl.pt/~pocm 
 ---	
	Yes, God had a deadline...
		So, He wrote it all in Lisp!