[plt-scheme] Using constructors

Hi all,

How can I make a constructor for a given class?
I mean, I need to make a method/function that will show up a GUI 
(depends on object to create), receives input from user and returns a 
new object.

I could use a normal function but then I would not be able to use 
polymorphism.

Any ideas?

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!