For input arguments, it's okay, (Sequenceof a) does accept (Listof a);
but for the return value, (Listof a) does not accept (Sequenceof a).
You can pass a list as the input, but you cannot narrow the type annotation
in you example,
say (list->vector) satisfies the annotation, both the input and output are
sequences, but the output is not a list.
Nonetheless, you can try the intersection type:
(: step (All (a b)
(-> (-> a (∩ (Sequenceof a) b) (∩ (Sequenceof a) b))
a
(∩ (Sequenceof a) b)
(∩ (Sequenceof a) b))))
(define (step fn x seq)
(fn x seq))
On Tue, Mar 21, 2017 at 12:46 PM, Sourav Datta <soura.ja...@gmail.com>
wrote:
> Hello all!
>
> I have a question regarding how sequence abstraction works in TR. In the
> sequence part of the docs it says that a sequence can consist of lists or
> vectors among other things. But does this make a function which ideally
> accepts a (Listof a) to accept something that is declared as a (Sequenceof
> a)? For example, in TR this works:
>
> (: x (Sequenceof Symbol))
> (define x '(a b c d))
>
> However, this shows a type mismatch:
>
> #lang typed/racket
>
> (: step (All (a)
> (-> (-> a (Sequenceof a) (Sequenceof a))
> a
> (Sequenceof a)
> (Sequenceof a))))
> (define (step fn x seq)
> (fn x seq))
>
>
> (step (ann cons (-> Symbol (Listof Symbol) (Listof Symbol)))
> 'a
> '(b c))
>
> Results in:
>
> Type Checker: Polymorphic function `step' could not be applied to
> arguments:
> Argument 1:
> Expected: (-> a (Sequenceof a) (Sequenceof a))
> Given: (-> Symbol (Listof Symbol) (Listof Symbol))
> Argument 2:
> Expected: a
> Given: 'a
> Argument 3:
> Expected: (Sequenceof a)
> Given: (List 'b 'c)
>
> I think, trying to instantiate the polymorphic step function might help
> but I was not able to figure out how.
>
> Thanks in advance.
>
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