I'm playing with encapsulated state in Advanced Student Language, and have noticed that sometimes I see "(void)" explicitly as a result in the interactions pane, and other times I see nothing at all.
Specifically, I was playing with two different ways to implement a posn with its own "swap" and "format" methods. One way, storing the methods as fields of a struct, I get the following behavior: > (define here (build-with-swapper-and-formatter 3 4)) > (define there (build-with-swapper-and-formatter 5 -2)) > (my-posn-x here) 3 > (my-posn-y there) -2 > ((my-posn-formatter here)) "(3, 4)" > ((my-posn-swapper here)) > ((my-posn-formatter here)) "(4, 3)" Note the lack of any visible result after the call to (my-posn-swapper here). The other approach, wrapping everything up in a function that dispatches on its argument, gives me the following behavior: > (define here (make-my-posn 3 4)) > (define there (make-my-posn 5 -2)) > (here 'x) 3 > (there 'y) -2 > (here 'format) "(3, 4)" > (here 'swap) (void) > (here 'format) "(4, 3)" Note that this time the "swap" produced an explicit "(void)". When does "(void)" print out, and when does it not? Why? Stephen Bloch [email protected] _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/users
