[racket-dev] racket/match is broken

On Thu, Oct 6, 2011 at 6:10 PM, Jay McCarthy <jay.mccarthy at gmail.com> wrote:
> In Eli's example, only the second pattern could match
>
> But if we wrote it this way:
>
>  (define (list?? x) (printf "list-checking ~s\n" x) (list? x))
>  (define (one?? x) (printf "one-checking ~s\n" x) (eq? 1 x))
>  (match '(1 (2) 3)
>         [(list (? one??) (list 2) 3) 1]
>         [(list _ (? list??) _) 2]
>         [(list (? one??) (list 20) 30) 3])
>
> Are you saying that match is allowed to return 1 or 2 depending on
> these reorderings and this is in some sense an "illegal" match
> expression?

No, definitely not!

`match' might test components in whatever order it wants, but the
semantics of when a match is found is just top-to-bottom, and `match'
always takes the first successful match.
-- 
sam th
samth at ccs.neu.edu