Do we have performance measurements that show the importance of such reorderings? Robby On Thursday, October 6, 2011, Neil Toronto <<a href="mailto:neil.toronto@gmail.com">neil.toronto@gmail.com</a>> wrote: > On 10/06/2011 01:20 PM, Eli Barzilay wrote: >> >> Just now, Neil Toronto wrote: >>> >>> On 10/06/2011 12:28 PM, Prabhakar Ragde wrote: >>>> >>>> On 10/6/11 2:12 PM, Eli Barzilay wrote: >>>> >>>>> Sam is talking about building the ASTs *while* matching, which is >>>>> what Jay was trying to do with uses of `app'. I think that a >>>>> teaching context is in particular one where such a thing doesn't >>>>> fit -- it obscures the distinction between the side the sexpr >>>>> goes into, and the side where an AST comes out. >>>> >>>> Okay, I see the distinction, and I apologize for not having fully >>>> understood Jay's example. I agree that this obscurity is >>>> hazardous. I think, though, that I have always assumed >>>> left-to-right matching, though I may never have constructed >>>> anything that would break if it didn't happen. --PR >>> >>> I think most people expect branching constructs like 'match' to make >>> in-order (left-to-right/depth-first), short-cutting decisions. >>> Additionally, the cases themselves do this. So I think the fact that >>> the patterns don't is very surprising. >> >> IIRC, the cases are also reordered to optimize tests -- and that's an >> even more important optimization: >> >> -> (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??) 2 3) 1] >> [(list _ (? list??) _) 2] >> [(list (? one??) 20 30) 3]) >> list-checking (2) >> 2 >> >> and after Jay broke it, you get >> >> one-checking 1 >> list-checking (2) >> 2 >> >> IMO it is perfectly fine to require that stuff used in `match' >> patterns is side-effect-free, and therefore cachable and reorderable. > > Well I'll be darned. > > I suppose this shows just how deeply I hold assumptions about order and shortcutting. > > Neil T > > _________________________________________________ > For list-related administrative tasks: > <a href="http://lists.racket-lang.org/listinfo/dev">http://lists.racket-lang.org/listinfo/dev</a> >