[Haskell-cafe] pointer equality

[David Barbour]

On Wed, Jul 20, 2011 at 10:40 AM, Chris Smith <cdsmith at gmail.com> wrote:

> I have looked up crowbar in a number of dictionaries of slang and
> informal usage... and still have no idea what you just said.  Can you
> reword it?
>

Crowbars offer 'leverage'.


>
> The point, I think, is that if pointer equality testing really does what
> it says, then there shouldn't *be* any correct implementation in which
> false positives are possible.  It seems the claim is that the garbage
> collector might be moving things around, have just by chance happened to
> place the second value in the spot formerly occupied by the first, and
> have not updated the first pointer yet.  But if that's the case, and
> it's executing arbitrary user code that may refer to that memory, then
> the garbage collector contains race conditions!


You assume that the GC uses the same primitive as the developer, and is
inherently subject to its own race conditions.

But Bertram has said that false positives are not possible. I can only
assume that the pointer comparison is atomic with respect to the GC.
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