On Mon, Jul 05 2010, Noel Welsh wrote:
> This type checks for me:
>
> (define-struct: (E) Arrow
> ([arrow : (E -> (Listof E))]))
>
> (: compose-arrows (All (E) ((Arrow E) (Arrow E) -> (Arrow E))))
> (define (compose-arrows a b)
> (Arrow (lambda: ((e : E)) ((Arrow-arrow a) (car ((Arrow-arrow b) e))))))
Ah, much better! I was under the false impression that the type variable
E wasn't visible inside the `define' body; the reason is that, as a
matter of fact, it doesn't seem to be visible when one's defining a
variable which is not a procedure. For instance, adding to the above the
following struct definition:
(define-struct: (A) C
([compose : (A A -> A)]))
the following procedure type-checks just fine:
(: make-CArrow (All (E) (-> (C (Arrow E)))))
(define (make-CArrow)
(C #{compose-arrows :: ((Arrow E) (Arrow E) -> (Arrow E))}))
but, if we want to define a non-procedural value in an analogous fashion:
(: carrow (All (E) (C (Arrow E))))
(define carrow
(C #{compose-arrows :: ((Arrow E) (Arrow E) -> (Arrow E))}))
the type checker complains that E is an unbound type name, for all
occurrences of E inside the define form.
Perhaps i am missing something again?
Thanks,
jao
--
Children enter school as question marks and leave as periods.
-Neil Postman, professor and author (1931- )
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