On Tue, Apr 13, 2010 at 10:47 AM, Sam Tobin-Hochstadt <sa...@ccs.neu.edu> wrote: > On Tue, Apr 13, 2010 at 9:08 AM, Robby Findler > <ro...@eecs.northwestern.edu> wrote: >> On Tue, Apr 13, 2010 at 7:15 AM, Sam Tobin-Hochstadt <sa...@ccs.neu.edu> >> wrote: >>> On Tue, Apr 13, 2010 at 4:42 AM, Paulo J. Matos <pocma...@gmail.com> wrote: >>>> >>>> Awesome, this one of my favourite features missing in Scheme. Hopefully >>>> we will see this soon in TS, Sam? >>> >>> Yes, we will hopefully soon see the form Eli describes. But it won't >>> give you exhaustiveness checking in general for `match', that's much >>> harder. >> >> I'm still looking into this, but my initial reading of the paper >> behind the current match implementation [1] seems to suggest that the >> compilation process can detect when it has to insert "else" clauses >> which should be at least a good start on this problem, no? > > `match' always inserts an else clause. The only way to write a > guaranteed exhaustiveness in Scheme in general is to write a catch-all > pattern yourself. I don't think `match' should warn if you don't do > that, since programmers will in general know more about the > exhaustiveness of their patterns than `match' can. For example, > should there be a warning for this code? > > #lang scheme > > (define/contract (f x) > ((or/c number? string?) . -> . number?) > (match x > [(? number?) ...] > [(? string?) ...]))
No. If match can't tell it isn't exhaustive, it should be quiet; so it can just assume that predicates cover everything (and insert an error to be sure) This one, on the other hand: (match x [`(x ,(? number?)) ...] [`(,_ ,(? string?)) ...] should signal an error or warning or something since the non-two-element-list case is not covered. > With type information we can do better, but providing type information > to macro expanders is not something Typed Scheme can do at the moment. > Maybe in the future we'll figure out how to do better, but > exhaustiveness checking is very hard with an expressive pattern > language. Yes, I know. If you reconsider my words under the assumption that I do indeed know that, I'd be grateful. Robby _________________________________________________ For list-related administrative tasks: http://list.cs.brown.edu/mailman/listinfo/plt-dev
