`StablePtr` in `ST`

[Jon Purdy]

Our use case is unsafeCoercing a mutable reference to use as a key in an
IntMap. Our reasoning is that coercing an IORef/STRef is unsuitable because
the underlying MutVar# may move, invalidating the key (i.e., you cannot
safely coerce back if a GC has happened between insertion and reading). (If
that’s incorrect, do enlighten us!) This is a “very nice to have” for our
purposes—with the understanding that it’s wicked unsafe. ;)

On Tue, Aug 20, 2019, 18:09 David Feuer <david.feuer at gmail.com> wrote:

> You also need to avoid inspecting the StablePtr itself, which is just a
> number, to maintain purity. The whole thing is a bit weird. Why do you want
> this anyway?
>
> On Wed, Aug 21, 2019, 7:39 AM David Feuer <david.feuer at gmail.com> wrote:
>
>> So something like
>>
>> newtype StablePtr a = StablePtr (StablePtrST RealWorld a)?
>>
>> I suppose that could work with some discipline. You have to assume that
>> foreign code doesn't pick its address out of a hat and so something silly,
>> but I guess you pretty much have to assume that anyway.
>>
>> On Wed, Aug 21, 2019, 5:00 AM Matthew Farkas-Dyck <strake888 at gmail.com>
>> wrote:
>>
>>> I have been doing some work where i want `StablePtr`, but also to not
>>> be confined to `IO`. I saw the following comment in
>>> "compiler/prelude/PrimOp.hs":
>>>
>>> Question: Why @RealWorld@ - won't any instance of @_ST@ do the job?
>>> [ADR]
>>>
>>> It has been there for 20 years. What is the answer? If it is safe i'll
>>> send the patch generalizing these operations.
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