[racket-dev] else clauses: possible change to match?

That one is twisted indeed, if you forget about how `else' must be written
in `match' (which I had, not using match too much).
I was thinking about issuing a warning when a syntax-parameter gets
shadowed (if that is even catchable).
Then suddenly I realized Racket doesn't do warnings. (Is there a reason for
that?)

Laurent


On Fri, May 3, 2013 at 3:39 PM, Robby Findler
<robby at eecs.northwestern.edu>wrote:

> [ for those that just want to see what I'm getting at, scroll to the end ]
>
> While the docs are clear (enough) on this point, I think it can be quite
> confusing. See if you spot the bug in this program:
>
> #lang racket
> (define (find-literals e)
>   (define literals '())
>   (let loop ([e e])
>     (match e
>       [`(λ (,x) ,e)
>        (loop e)]
>       [`(,e1 ,e2)
>        (loop e1)
>        (loop e2)]
>       [(? symbol?) (void)]
>       [else
>        (cond
>          [(member e literals)
>           (void)]
>          [else
>           (set! literals (cons e literals))])]))
>   literals)
>
> (module+ test
>   (require rackunit)
>   (check-equal? (find-literals '(λ (x) x)) '())
>   (check-equal? (find-literals '((λ (x) x) 1)) '(1))
>   (check-equal? (find-literals '((λ (x) x) #f)) '(#f)))
>
>
> Hint: the last test case fails. Second hint: if you switch it to use sets,
> it starts working. Third hint: this isn't about lists. :)
>
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> The bug is that 'else' is treated as a variable in match, so it gets bound
> to #f, which shadows the else in the cond which .... is confusing.
>
>  So, how about making match treat else specially?
>
>
> I don't ask this completely from outer space; there was a bug in Redex's
> random generation that was caused exactly by this shadowing of else.
>
> Robby
>
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>   Racket Developers list:
>   http://lists.racket-lang.org/dev
>
>
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