[racket-dev] how to define a custom "null" in typed racket

You shouldn't be able to send the TR into an inifinite loop.
Typechecking is supposed to terminate. Please try to reduce it to the
minimum testcase and file a bug.

On Thu, Apr 11, 2013 at 3:32 PM, Danny Yoo <dyoo at hashcollision.org> wrote:
> Perhaps something like this?
>
>
> ;;;;;;;;;;;;;
> #lang typed/racket/base
>
> (provide mynull mycar mycdr)
>
> (struct: MyNull ())
> (define mynull (MyNull))
>
> (define-type MyListof (All (A) (Rec X (U MyNull (Pair A X)))))
>
> (: mycar : (All (A) ((MyListof A) -> A)))
> (define (mycar lst)
>   (if (MyNull? lst)
>       (error 'mycar "given list is empty")
>       (car lst)))
>
> (: mycdr : (All (A) ((MyListof A) -> (MyListof A))))
> (define (mycdr lst)
>   (if (MyNull? lst)
>       (error 'mycdr "given list is empty")
>       (cdr lst)))
> ;;;;;;;;;;;;;
>
> where mynull is the singleton instance of the opaque structure type MyNull.
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