On Thu, Aug 12, 2010 at 4:11 PM, Neil Toronto <[email protected]> wrote: > Are there any plans for it? Would it be difficult to do? I ask because it > seems it would make using polymorphic struct subtypes easier. Here's an > example of the problem: > > > #lang typed/racket > > (struct: (T) datum ([val : T])) > (struct: (T) data datum ([vals : (Listof T)])) > > (: get-vals (All (T) ((datum T) -> (Listof T)))) > ;; Returns the values in the struct as a list > (define (get-vals d) > ; type error here: > (cond [(data? d) (cons (datum-val d) (data-vals d))] > [(datum? d) (list (datum-val d))])) > > (get-vals (data 1 '(2 3 4))) > > #| > Type Checker: Polymorphic function cons could not be applied to arguments: > Types: a (Listof a) -> (Listof a) > a b -> (Pairof a b) > Arguments: Nothing (Listof Any) > Expected result: (Listof T) > in: (cons (datum-val d) (data-vals d)) > |# > > > The problem is the type of data?, which is (Any -> Boolean : (data Any)), so > in the first branch above, d has type (data Any) instead of (data T). But if > data? : (All (S) (Any -> Boolean : (data S))), it seems I could make this > function work, even if I had to annotate data? with (Any -> Boolean : (data > T)) to use it.
Unfortunately, that isn't the semantics of the underlying struct predicate - there's no way to ask Racket to "specialize" struct predicates to a particular type. The Typed Racket type corresponds to what the `data?' predicate that the `struct' form defines. -- sam th [email protected] _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/users
