On Thu, Oct 6, 2011 at 6:23 PM, Jay McCarthy <[email protected]> wrote: > On Thu, Oct 6, 2011 at 4:17 PM, Sam Tobin-Hochstadt <[email protected]> wrote: >> On Thu, Oct 6, 2011 at 6:10 PM, Jay McCarthy <[email protected]> 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. > > So in this case, match has to call one?? because all the other > components match on the first case?
Exactly. -- sam th [email protected] _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/dev
