[racket] TR: Polymorphic types

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.

(What I was actually doing, by the way, was seeing if I could port the JSON library to TR. I ran into the problem of the null element being of arbitrary type.)

Best,
jmj