[racket] TR: Polymorphic types

On Sat, Jan 17, 2015 at 9:20 PM, Jordan Johnson <jmj at fellowhuman.com> wrote:
> Hi all,
>
> In playing with TR this evening, I found
>
>> (define-type (Foo A)
>>   (U A
>>      (List 'foo (Foo A))))
>>
>> (define-predicate foo? Foo)
>
> generates the error “Type Foo could not be converted to a contract: cannot generate contract for non-function polymorphic type”. I’ve read the page with caveats about inference and polymorphic types, but since the example is about using a function rather than defining a predicate, I’m not clear on how (if at all) the technique could be adapted for this.

No, it can't be adapted. That error message is an instance of this
caveat: http://docs.racket-lang.org/ts-guide/caveats.html#%28part._.Typed-untyped_interaction_and_contract_generation%29
which applies to `define-predicate` as well. There's no way to check
if something is a `Foo`, because Typed Racket wouldn't know what you
meant for `A` to be.

Sam