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
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