[racket-dev] A proposal for parametric opaque types in Typed Racket

From: Alexander D. Knauth (alexander at knauth.org)
Date: Thu Jan 29 23:20:48 EST 2015

Furthermore, even if the wrappers were shared between functions, untyped code would recieved wrapped values, which would render them quite useless.

If it’s not an opaque type, but something like a list, then this works, and the untyped code receiving wrapped values isn’t a problem here:
#lang typed/racket
; make Posn parametric
(define-type (Posn A) (List A A))
(provide Posn)
 [make-posn (All (A) A A -> (Posn A))]
 [posn-x (All (A) (Posn A) -> A)]
 [posn-y (All (A) (Posn A) -> A)]
 [real-posn? [(Posn Any) -> Boolean]])
> (define p (make-posn 1 2))
(make-posn #<A6> #<A6>) ; a printf that I put in make-posn from “untyped.rkt"
> p
- : (Listof Positive-Byte) [more precisely: (List Positive-Byte Positive-Byte)]
'(1 2) ; unwrapped
> (posn-x p)
- : Integer [more precisely: Positive-Byte]
> (posn-y p)
- : Integer [more precisely: Positive-Byte]
> (real-posn? p)
- : Boolean

Even though for a short time it's wrapped, it’s unwrapped as soon as make-posn returns, and then after that if it flows into untyped code again it’s not wrapped and functions like real-posn? work fine.  

But the problem is that if it’s an opaque type then it can’t unwrap it once the value is returned from make-posn.

And I don’t think parametric opaque types could solve this unless all posns themselves were wrapped with an opaque struct on the typed side, which I guess does make sense now that I think about it.  But then you couldn’t do any operations on it except those that you use import with require/typed, right?  Or not?  And what happens if you use cast on one of these things?

On Jan 29, 2015, at 9:25 PM, Alexis King <lexi.lambda at gmail.com> wrote:

> I recently ran into a problem in which opaque types (types imported from untyped code) cannot by parameterized by Typed Racket. I initially encountered this problem in my attempt to port 2htdp/image to TR.
> After some further consideration, I’m interested in adding support to make something like this possible, which would certainly have additional benefits beyond this specific use-case. I’ve outlined my proposal here:
> http://lexi-lambda.github.io/racket-parametric-opaque-types/
> Any feedback, suggestions, or advice would be appreciated, especially from those who are familiar with Typed Racket’s internals.
> Thank you,
> Alexis
> _________________________
>  Racket Developers list:
>  http://lists.racket-lang.org/dev

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