[ 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
