[racket] A type that can't be converted to a contract

This has come up enough times that maybe TR should convert contracts
of the form:

 (case->  (-> <flat> ...) (-> <flat> ...))

into a dependent contract that checks the two 'flat' things? (I guess
you'd have to have an ordering on types in case they overlap, but I
presume you have this already.)

On Sat, Nov 17, 2012 at 1:18 PM, Jens Axel Søgaard
<jensaxel at soegaard.net> wrote:
> I have the following contract on next-prime :
>
>     (: next-prime : (case-> (N -> N) (Z -> Z)) )
>
> It says that for all primes p, positive or negative, (next-prime p)
> will be an integer.
> Furthermore if p is a natural number, then (next-prime p) will also be
> a natural number.
>
> This type can't be converted to a contract:
>    Type Checker: The type of next-prime cannot be converted to a
> contract in: (next-prime 4)
>
> My understanding is that a since N is a subset of Z a predicate can't
> determine whether
> which case to use. Is there an alternative construct, that I can use
> in order to get
> a contract?
>
> My temporary solution is to provide untyped-next-prime
>
>    (: untyped-next-prime : Z -> Z)
>    (define (untyped-next-prime z)
>      (next-prime z))
>
> whose type can be converted to a contract.
>
>
> See details in:
> https://github.com/plt/racket/blob/master/collects/math/private/number-theory/number-theory.rkt
>
> --
> Jens Axel Søgaard
>
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