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 <ro...@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. :) > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > 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 > > > > > _________________________ > Racket Developers list: > http://lists.racket-lang.org/dev > >
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