[racket] Typed Racket: can this recursive typing become polymorphic?

You can do it if you have `flatten` also accept a predicate for A; i.e. 
an (-> Any Boolean : A). Be aware that such functions can't be exported 
to untyped Racket, though.

Neil ⊥

On 02/03/2015 08:31 PM, J. Ian Johnson wrote:
> What if A is (Listof Integer)?
> Generally a type variable naked in a union is just not going to do what you want. There's no way to keep it disjoint from the other types in the union.
>
> -Ian
>
> ----- Original Message -----
> From: "Matthew Butterick" <mb at mbtype.com>
> To: "users" <users at racket-lang.org>
> Sent: Tuesday, February 3, 2015 7:29:47 PM
> Subject: [racket] Typed Racket: can this recursive typing become polymorphic?
>
> I'm trying to create a polymorphic type for `flatten` that reflects its usual behavior.
>
> This monomorphic version works:
>
> #lang typed/racket/base
> (require/typed rackunit [check-equal? (Any Any . -> . Any)])
> (require/typed racket/list [flatten (All (A) (Rec as (U Integer (Listof as))) -> (Listof Integer))])
>
> (define flatten-integers (inst flatten Integer))
> (check-equal? (flatten-integers 1) '(1))
> (check-equal? (flatten-integers '(1)) '(1))
> (check-equal? (flatten-integers '(1 (2 3) 4)) '(1 2 3 4))
> (check-equal? (flatten-integers '(1 (2 3 (4 5)) 6)) '(1 2 3 4 5 6))
>
>
> But when I try to substitute the polymorphic type `A` for `Integer`, it stops working:
>
> #lang typed/racket/base
> (require/typed rackunit [check-equal? (Any Any . -> . Any)])
> (require/typed racket/list [flatten (All (A) (Rec as (U A (Listof as))) -> (Listof A))])
>
> (define flatten-integers (inst flatten Integer))
> (check-equal? (flatten-integers 1) '(1))
> (check-equal? (flatten-integers '(1)) '(1))
> (check-equal? (flatten-integers '(1 (2 3) 4)) '(1 2 3 4))
> (check-equal? (flatten-integers '(1 (2 3 (4 5)) 6)) '(1 2 3 4 5 6))
>
>> git/racket/racket/collects/racket/contract/private/blame.rkt:143:0: flatten: contract violation; none of the branches of the or/c matched
>
>
>
> Is it possible? If so, what am I missing?
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