On 7/13/21, Kiong-Gē Liāu <[email protected]> wrote: > Ben, > > Thanks, changing "stream" to "stream apply" does solve the issue. > > I tried to push it further little bit with the following code to see if > typed racket can support generic like Haskell or Scala: > > #lang typed/racket > > (require pfds/stream) > > > (define-type (OverFoldable A) (-> (Listof A) A)) > > (define-type (FibStreamCons A) (-> (Listof A) (Stream A))) > > (define-type (FibStream A) (-> (OverFoldable A) (Listof A) (Stream A))) > > (: sum (OverFoldable Number)) > (define (sum xs) (apply + xs)) > > (: gfib_2 (All (A) (FibStream A))) > (define (gfib_2 f xs) > (: gfib_t (All (A) (FibStreamCons A))) > (define (gfib_t ys) > (stream-cons (last ys) (gfib_t (append (cdr ys) (list (f ys)))))) > (stream-append (apply stream (drop-right xs 1)) (gfib_t xs))) > > However, I got the following error message: > > ; /home/kiong-ge/Programming/Racket/typed_racket_test.rkt:28:61: Type > Checker: type mismatch > ; expected: (Listof A) > ; given: (Listof A) > ; in: ys > > But, according to this error message, expected and given types are exactly > the same, not sure how to deal with this issue.
I think the problem is 2 different type variables that both print as the letter A. When I remove the "All" from the type for gfib_t (FibStreamCons A), it typechecks. -- You received this message because you are subscribed to the Google Groups "Racket Users" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/racket-users/CAFUu9R5nuj-peuqFRVFRR6fKY4waAwaXNdTbmJzNfVTBBx9GbQ%40mail.gmail.com.
