Hi, I was experimenting a bit with type families recently and ran into a bit of an issue. Given that I don't know type families that well yet, I was wondering if I made an error somewhere. One thing is that I can't find any relevant changes in the GHC release notes for 7.8.1, .2 or .3. Maybe this code contains an error which 7.6.3 simply wasn't able to find? Thanks. -------- -- this code compiles in 7.6.3, but breaks in 7.8.3 with the following message: -- TypeFamilies.hs:14:31: -- ‘End’ of kind ‘*’ is not promotable -- In the kind ‘End’ -- In 7.6.3, using :kind!, I can see that the type synonyms contained in the family do work the way I intend them to. {-# Language GADTs , TypeFamilies , DataKinds #-} module TypeFamilies where data End = Least | Spot Float | Most deriving (Eq, Show) data Interval = IntervalCons { left :: End, right :: End } deriving (Eq, Show) type family Interval2 (a :: End) (b :: End) :: Interval type instance Interval2 Least Most = IntervalCons Least Most type instance Interval2 (Spot l) Most = IntervalCons (Spot l) Most type instance Interval2 Least (Spot r) = IntervalCons Least (Spot r) type instance Interval2 (Spot l) (Spot r) = IntervalCons (Spot l) (Spot r)