[racket-dev] A tricky chaperone puzzle

Unless I still have it wrong, the implementation of 2 was
straightforward.

I would have overlooked the need to restrict `chaperone-struct` to
chaperones of accessors and mutators if you hadn't mentioned it.

At Thu, 24 Jul 2014 15:45:18 -0400, Sam Tobin-Hochstadt wrote:
> Consider the following module:
> 
> (module m racket
>   (struct x [a])
>   (define v1 (x 'secret))
>   (define v2 (x 'public))
>   (provide v1 v2)
>   (provide/contract [x-a (-> x? (not/c 'secret))]))
> 
> It appears that this ensures that you can't get 'secret. But, it turns
> out that I can write a function outside of `m` that behaves like `x-a`
> without the contract:
> 
> (require (prefix-in m: 'm))
> 
> (define (x-a v)
>   (define out #f)
>   (with-handlers ([void void])
>     (m:x-a (chaperone-struct v m:x-a (λ (s v) (set! out v) v))))
>   out)
> 
> Now this works:
> 
> (displayln (x-a m:v1)) ;; => 'secret
> 
> The problem is that `m:x-a` is treated as a
> `struct-accessor-procedure?`, which is a capability for accessing the
> a field, even though it's a significantly restricted capability.
> 
> There are a couple possible solutions I've thought of:
> 
> 1. Require a non-chaperoned/impersonated accessor.
> 2. Actually use the chaperoned/impersonatored accessor to get the
> value out instead of the underlying accessor.
> 
> 1 is a little less expressive. But note that 2 means that you have to
> only allow chaperoned procedures with `chaperone-struct`, and imposes
> significant complication on the runtime.
> 
> I favor 1.
> 
> Sam
> 
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