2011/7/22 Gábor Lehel
Yeah, this is pretty much what I ended up doing. As I said, I don't think I lose anything in expressiveness by going the MPTC route, I just think the two separate but linked classes way reads better. So it's just a "would be nice" thing. Do recursive equality superclasses make sense / would they be within the realm of the possible to implement?
Those equality superclasses are not recursive in the same way, as far as I can tell. The specifications for classes require that there is no chain: C ... => D ... => E ... => ... => C ... However, your example just had (~) as a context for C, but C is not required by (~). And the families involved make no reference to C, either. A fully desugared version looks like: type family Frozen a :: * type family Thawed a :: * class (..., Thawed (Frozen t) ~ t) => Mutable t where ... I think this will be handled if you use a version where equality superclasses are allowed. -- Dan