What type role should NF's parameter have?

[treeowl]

Currently, we let GHC infer

type role NF representational

I think there's a minor potential problem here. Suppose we have

data T = ...
newtype U = U T

instance NFData T where ...
instance NFData U where ... -- Something *different*

We can coerce from NF T (which is in T-normal form) to NF U (which claims to be in U-normal form). It's not quite clear to me whose problem this is. Should whoever defined U be expected to hide its constructor? Or should we have type role NF nominal?

In practice, I suspect this to come up rather rarely, since types are generally expected to have just one notion of normal form, but there could be exceptions out there.

[ezyang]

Hey @treeowl, I added you to the repo. I think nominal is more correct, so feel free to commit this change.

[treeowl]

@ezyang, thanks! I don't really feel strongly about it, but it seems to be the safer choice.

[treeowl]

@ezyang, I opened up a pull request for this. I also added a coercion to work around the role annotation locally. For now, I put it in a separate Data.NF.Coercion module. Does that make sense to you, or should it go in Data.NF?