[Haskell-cafe] Can reallyUnsafePtrEquality give false positives?

[Andrew Martin]

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

Sent from my iPhone

> On Nov 22, 2017, at 12:22 PM, Michael Walker <mike at barrucadu.co.uk> 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)
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