Re: `StablePtr` in `ST`

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Re: `StablePtr` in `ST`

David Feuer
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 <[hidden email]> 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 <[hidden email]> 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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Re: `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 <[hidden email]> 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 <[hidden email]> 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 <[hidden email]> 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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[hidden email]
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