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

On Thu, Apr 11, 2013 at 6:47 PM, Eric Dobson <eric.n.dobson at gmail.com> wrote:
> 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.

I just filed pr 13687.



>
> 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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