Re: [racket-users] Typed Racket: Using (Sequenceof a) in place of (Listof a)

[WarGrey Gyoudmon Ju]

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 
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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[racket-users] Typed Racket: Using (Sequenceof a) in place of (Listof a)

[Sourav Datta]

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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