[racket] On (void) and interactions

Someone who knows for sure would have to say, but this looks like a bug to me.

Robby

On Thu, May 26, 2011 at 9:59 AM, Stephen Bloch <sbloch at adelphi.edu> wrote:
>
> On May 26, 2011, at 10:08 AM, Robby Findler wrote:
>
>> The #lang racket void is not the same thing as the void that comes out
>> of (say) intermediate's assignments and definitions. I'm not sure what
>> the intended behavior is but if you supply a small, complete example
>> that would probably help someone figure out if you're seeing a bug in
>> a printer or just the expected behavior.
>
> These are both pretty short.
>
> Here's the program in which "(void)" is NOT printed:
>
> ; In Definitions...
> (define-struct my-posn (x y swapper formatter))
>
> ; build-with-swapper-and-formatter : number number -> my-posn
> (define (build-with-swapper-and-formatter xval yval)
>  (local [(define p1
>            (make-my-posn xval yval
>                          (lambda ()
>                            (local ((define temp (my-posn-x p1)))
>                              (begin (set-my-posn-x! p1 (my-posn-y p1))
>                                     (set-my-posn-y! p1 temp))))
>                          (lambda ()
>                            (format "(~a, ~a)" (my-posn-x p1) (my-posn-y p1)))))]
>    p1))
>
> ; In Interactions...
> (define here (build-with-swapper-and-formatter 3 4))
> ((my-posn-formatter here))
> ; prints "(3, 4)"
> ((my-posn-swapper here))
> ; doesn't print anything
> ((my-posn-formatter here))
> ; prints "(4, 3)"
>
>
> And here's the one in which it IS printed:
>
> ; In Definitions...
> ; make-my-posn : number number -> (symbol -> number-or-string-or-nothing)
> (define (make-my-posn xval yval)
>  (local [(define stored-x xval)
>          (define stored-y yval)]
>    (lambda (message)
>      (cond [(symbol=? message 'x)
>             stored-x]
>            [(symbol=? message 'y)
>             stored-y]
>            [(symbol=? message 'swap)
>             (local [(define temp stored-x)]
>               (begin (set! stored-x stored-y)
>                      (set! stored-y temp)))]
>            [(symbol=? message 'format)
>             (format "(~a, ~a)" stored-x stored-y)]
>            [else
>             (error "Unrecognized message")]))))
>
> ; In Interactions...
> (define here (make-my-posn 3 4))
> (here 'format)
> ; prints "(3, 4)"
> (here 'swap)
> ; prints (void)
> (here 'format)
> ; prints "(4, 3)"
>
>
> Both are in Advanced Student Language, DrRacket 5.1.1.5.
>
>
> Stephen Bloch
> sbloch at adelphi.edu
>
>