[racket-dev] Catching the undefined value

On 2014-04-15 21:19:16 -0600, Neil Toronto wrote:
> Does TR not having to worry about #<undefined> mean that the following
> program will typecheck?
>
>   #lang typed/racket
>
>   (define-type (Lazy-Listof A) (Promise (Pair A (Lazy-Listof A))))
>
>   (let ()
>     (define: lazy-zeros : (Lazy-Listof Zero)
>       (delay (cons 0 lazy-zeros)))
>     lazy-zeros)

Yes, that should be the case. Here's an interaction from my pull request branch
for removing undefined handling in TR:

  Welcome to Racket v6.0.1.3.
  -> (define-type (Lazy-Listof A) (Promise (Pair A (Lazy-Listof A))))
  -> (let ()
       (define: lazy-zeros : (Lazy-Listof Zero)
         (delay (cons 0 lazy-zeros)))
       lazy-zeros)
  - : (Lazy-Listof Zero)
  #<promise:lazy-zeros>

That's assuming there aren't any design problems with that PR that come
up in code review.

> BTW, I was happy to see that the type `Lazy-Listof' could be defined like
> that when coming up with the example.

:D

Cheers,
Asumu