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

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
<greghendershott at 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
> <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
> >
> >
> >
> >
> > _________________________
> >   Racket Developers list:
> >   http://lists.racket-lang.org/dev
> >
>
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