Re: [Haskell-cafe] Can reallyUnsafePtrEquality give false positives?

According to Edwark Kmett it can give false positives as well, or at least could in 2010: https://mail.haskell.org/pipermail/haskell-cafe/2010-June/079532.html Erik On 22 November 2017 at 20:06, Andrew Martin wrote:

It cannot give false positives. If it could, that would make it totally worthless.

Sent from my iPhone

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On Nov 22, 2017, at 12:22 PM, Michael Walker wrote:

Hello,

Can reallyUnsafePtrEquality give false positives? I can see how it can give false negatives (eg, compiler optimisations increasing or decreasing sharing), but I'm not so sure if it can give false positives.

I don't see how in a garbage collected language two live values could compare reference equal. Unless the implementation is something like:

reallyUnsafePtrEquality a b = getptr a == getptr b

...as that then means that if the GC moves things after `getptr a` is evaluated but before `getptr b` is, then you could get a false positive. But that doesn't seem like a sensible implementation to me, because then reallyUnsafePtrEquality would surely be totally useless.

-- Michael Walker (http://www.barrucadu.co.uk) _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

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