Add UnliftedRef

[treeowl]

I believe we should be able to unsafeCoerce# our way into references containing TYPE 'UnliftedRep. This is horrible, of course, but also useful. We can support all the usual MutVar operations except atomicModifyMutVar* ones.

[andrewthad]

I don’t quite understand what you mean. Would this just be backed by an UnliftedArray of length 1?

[treeowl]

I mean backed by a MutVar# for now. But what I really want to do is fix RuntimeRep so we won't need UnliftedArray#. I want

data RuntimeRep
  = PtrRep Levity
  | ...
data Levity = Lifted | Unlifted
type Type = TYPE ('PtrRep 'Lifted)
data MutableArray# :: Type -> TYPE ('PtrRep levity) -> TYPE ('PtrRep 'Unlifted)
data MutVar# :: Type -> TYPE ('PtrRep levity) -> TYPE ('PtrRep 'Unlifted)
...

In that regime, most MutVar# operations would be levity-polymorphic, but atomicModifyMutVar2# and atomicModifyMutVar_# would only be available for lifted types.

[andrewthad]

Ah, thanks for clarifying. Now I get it. This seems reasonable, and it should work. It would be nice to fix RuntimeRep as you suggest. The documentation of mkWeak# even hints at this shortcoming of the current system.

[treeowl]

OTOH, maybe that doesn't help much. We still couldn't write user functions that didn't care about levity....

[andrewthad]

True. We could unify the types but we would still need an extra copy of all the functions. Unless someone finds a good way to allow levity polymorphism in binding positions.

[konsumlamm]

Since GHC 9.4, MutVar# is levity polymorphic, so this can be now implemented by just making MutVar levity polymorphic too - no unsafeCoerce# needed. See #356.