Richard Eisenberg <eir@cis.upenn.edu> writes:

> Yes, I believe that's right. As far as I can figure out, these classes
> really *are* problematic, in that if we allowed
> GeneralizedNewtypeDeriving for them, there would be a way to subvert
> the type system. To make these derivable, we would need to be able to
> restrict various type parameters from taking on values that take a
> nominal argument. Without the ability to restrict the values in this
> way, there could be trouble.
>

I suppose it's unlikely that the roles mechanism will be extended to
allow for such restriction?

Cheers,

- Ben