Re: [racket-dev] Set Equality with Cyclic Structure is not as Expected

[Matthias Felleisen]

Looks like a bug. Why don't you file a bug report? 


On May 7, 2012, at 6:33 PM, Daniel King wrote:

> Hi,
> 
> The following code snippet is a bit confusing to me. Sets with cyclic 
> structure
> are not equal? even though they meet my intuitive definition of equal. I'm
> curious exactly where my intuition goes wrong.
> 
> I imagine the reason why Racket can conclude that the lists are equal is 
> because
> the ordering of the list provides a necessary level of disambiguation. With a
> set, Racket cannot be certain which element should match between the two
> data, and this means potentially following cycles forever? I'm not entirely
> clear on my reasoning, but I think this must be related to the root cause.
> 
> Note in the following snippet the let* forms differ only in the data 
> constructor
> used: `set' in the first form and `list' in the second form.
> 
> racket@> (let* ((a-box (box #f))
>                (b-box (box #f))
>                (a (set 1 a-box))
>                (b (set 1 b-box)))
>           (set-box! a-box a)
>           (set-box! b-box b)
>           (displayln a)
>           (displayln b)
>           (equal? a b))
> #0=#<set: 1 #&#0#>
> #0=#<set: 1 #&#0#>
> #f
> racket@>  (let* ((a-box (box #f))
>                 (b-box (box #f))
>                 (a (list 1 a-box))
>                 (b (list 1 b-box)))
>            (set-box! a-box a)
>            (set-box! b-box b)
>            (displayln a)
>            (displayln b)
>            (equal? a b))
> #0=(1 #&#0#)
> #0=(1 #&#0#)
> #t
> 
> P.S.
> 
> For future reference, is this more of users@ material or dev@ material?
> 
> --
> Dan King
> College of Computer and Information Science
> Northeastern University
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