Oh, right! Thanks! I'll push something that explicitly mentions underscore and adjust the elses to use underscores.
Robby On Sat, May 4, 2013 at 9:09 PM, Greg Hendershott <greghendersh...@gmail.com>wrote: > There will be people who see the long BNF for match in the Reference, > and flee to the Guide to try to learn by example instead. Even when > they later do read the Reference carefully, they'll be left with first > impressions of idioms from the Guide. > > (I might be one of those people. Cough.) > > The Guide <http://docs.racket-lang.org/guide/match.html> uses `else` > in a couple examples. > > Also, I somehow got the idea that `(var x)` was the only way to do > what `else` can do by accident. So I don't _think_ I have any code > that's buggy this way, or, which would break if the change were made, > either way. I think. > > Until I saw your email I didn't appreciate why it would make sense to > use `_`. Going forward I will. > > > On Fri, May 3, 2013 at 9:39 AM, 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 > > >
_________________________ Racket Developers list: http://lists.racket-lang.org/dev
