On Sun, Jul 11, 2021 at 8:15 PM Kiong-Gē Liāu <gongyi.l...@gmail.com> wrote:
>
> Hi, in non-typed racket, I can define a generalized Fibonacci sequence
>
> X[n+k] = f(X[n], X[n+1], ...., X[n+k-1])
>
> using the following code
>
> #lang racket
>
> (require racket/stream)
>
> (define (gfib f xs)
> (define (gfib_t xs)
> (stream-cons (last xs) (gfib_t (append (cdr xs) (list (f xs))))))
> (stream-append (drop-right xs 1) (gfib_t xs)))
>
> (define (sum xs) (apply + xs))
> ;; Example of a (0, 1) initialized Fibonacci sequence
> (define gfib20 (gfib sum '(0 1 )))
>
> But using typed racket, the following code
>
> #lang typed/racket
>
> (require pfds/stream)
>
> (define (sum [xs : (Listof Number)] ) (apply + xs))
>
> (define (gfib [f : (-> (Listof Number) Number)] [xs : (Listof Number)] )
> (define (gfib_t [ys : (Listof Number)] )
^~~~~~~ You can annotate the codomain type of gfib_t here:
(define (gfib_t [ys : (Listof Number)]) : Your-Codomain-Type
...)
> (stream-cons (last ys) (gfib_t (append (cdr ys) (list (f ys))))))
> (stream-append (stream (drop-right xs 1)) (gfib_t xs)))
>
> leads to error message
>
> ; /home/kiong-ge/Programming/Racket/typed_racket_test.rkt:8:11: Type Checker:
> insufficient type information to typecheck. please add more type annotations
> ; in: gfib_t
>
> How should I set the type signature in the typed racket in order to get the
> same result generated non-typed racket code ?
>
> Thanks,
> Kiong-Ge.
>
>
>
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