Types are a purely compile-time construct, so type polymorphism doesn’t make
any sense in a dynamic context. You can create a predicate that checks if some
value is any Foo:
(define-predicate foo? (Foo Any))
Or you can even create a predicate that checks if some value is a specific type
of Foo:
(define-syntax-rule (Foo? A)
(make-predicate (Foo A))
But in the latter case, the type obviously must be known at compile-time.
As Alexander mentions, the snapshot version of Typed Racket exports typed/json,
but it just uses 'null for all null values, mostly to sidestep this problem,
and because custom nulls are relatively pointless, anyway.
> On Jan 17, 2015, at 18:20, Jordan Johnson <j...@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.
>
> (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
>
>
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