PrimUnlifted class

[andrewthad]

I'd like to move this from primitive-unlifted into this primitive:

class PrimUnlifted a where
  type Unlifted a :: UnliftedType
  toUnlifted# :: a -> Unlifted a
  fromUnlifted# :: Unlifted a -> a

instance PrimUnlifted (Array a) where
  type Unlifted (Array a) = Array# a
  toUnlifted# (Array a) = a
  fromUnlifted# x = Array x

instance PrimUnlifted (MutableArray s a) where
  type Unlifted (MutableArray s a) = MutableArray# s a
  toUnlifted# (MutableArray a) = a
  fromUnlifted# x = MutableArray x

instance PrimUnlifted (SmallArray a) where
  type Unlifted (SmallArray a) = SmallArray# a
  toUnlifted# (SmallArray a) = a
  fromUnlifted# x = SmallArray x

instance PrimUnlifted (SmallMutableArray s a) where
  type Unlifted (SmallMutableArray s a) = SmallMutableArray# s a
  toUnlifted# (SmallMutableArray a) = a
  fromUnlifted# x = SmallMutableArray x

instance PrimUnlifted (PrimArray a) where
  type Unlifted (PrimArray a) = ByteArray#
  toUnlifted# (PrimArray x) = x
  fromUnlifted# x = PrimArray x

instance PrimUnlifted ByteArray where
  type Unlifted ByteArray = ByteArray#
  toUnlifted# (ByteArray x) = x
  fromUnlifted# x = ByteArray x

I'd have to give up on the ShortText and ShortByteString instances since primitive is not going to pick up bytestring or text-short as a dependency. Having this typeclass would let us have a much more useful implementation of touch and keepAlive. Any implementation of these targeting a lifted wrapper is dangerous and misleading to the user. Unfortunately, that's basically the only thing we would get from doing this. All the other stuff using this class needs to stay in primitive-unlifted for at least a while longer since the backwards-compatibility story for it is rather bad. I do think that PrimUnlifted is stable at this point though.

Any thoughts on this?

[konsumlamm]

I assume we'd also want instances for MutablePrimArray and MutableByteArray?

[andrewthad]

Yes, those as well. And MutVar. We cannot do PrimVar at the moment though because we would want it to lower to an unlifted newtype PrimVar#, which cannot exist in GHCs before 8.10.

[treeowl]

It sounds to me like delaying this would be better, and just adding adequate documentation in primitive to point over. There just isn't enough benefit yet.

[chessai]

It sounds to me like delaying this would be better, and just adding adequate documentation in primitive to point over. There just isn't enough benefit yet.

Do you specifically mean delaying until we drop GHC 8.10?