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.