On Tue, Nov 6, 2012 at 5:19 PM, Ray Racine <ray.rac...@gmail.com> wrote: > To pick up on a previous topic: > > OK given define-predicate doesn't work for mutable datatypes. > > (define-predicate T? (HashTable Symbol String)) ;; goes boom > > Are we also saying in effect that no possible manual predicate construction > will be accepted by TR?
No, this can't work. There's no way to test at one point whether getting data out of that hash table will always produce a string -- someone with a different reference to the data could add an integer. > i,e, is there any form of the below one can construct that TR will accept? > > (: T? (Any -> Boolean : (HashTable Symbol String))) > (define (T? ht) > ...) However, in 5.3.1, you can use the `cast` form: #lang typed/racket (define: t : (HashTable Any Any) (make-hash)) (define: s : (HashTable Symbol String) (cast t (HashTable Symbol String))) > (hash-set! t 'x 6) > (hash-ref s 'x) . . #hash((x . 6) (5 . 6) (5 . 6)): broke its contract promised: String produced: 6 Thanks to Eric Dobson for implementing this. -- sam th sa...@ccs.neu.edu ____________________ Racket Users list: http://lists.racket-lang.org/users
