[racket] Typed Racket: Parametricity foils equality checking?

From: Matthias Felleisen (matthias at ccs.neu.edu)
Date: Thu Nov 1 18:45:47 EDT 2012

Thank you, thank you, thank you. 

[This validates that my objection to our parametricity is real.]

On Nov 1, 2012, at 4:56 PM, John Clements wrote:

> I can't tell whether this is a bug.  In the program below, remove-duplicates doesn't remove duplicates. This looks like an attempt to preserve parametricity a la theorems for free, but I don't think that TR actually tries to guarantee parametricity in this sense.  I'm guessing that the values in the list are being wrapped in contracts that prevent 'eq?'-checking; I thought that chaperones were supposed to... no, I shouldn't even finish that sentence.
> Anyhow, I'm just writing to make sure that this isn't a bug. It's certainly not the behavior that I would naively expect. I can certainly work around the bug by substituting a more specific type for remove-duplicates, as in the commented-out version.
> John
> #lang typed/racket
> (require/typed racket
>               [remove-duplicates (All (T) (Listof T) -> (Listof T))]
>               #;[remove-duplicates ((Listof Symbol) -> (Listof Symbol))])
> (remove-duplicates '(a b c a d)) ;; => produces '(a b c a d)--that is, a list with duplicates
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