| GHC 6.8 seems unable to derive (some?) instances for data types with | higher-kinded variables. GHC 6.6 manages these just fine. See below. | data T w = T (w Bool) deriving (Show) | data ID x = ID x deriving (Show) | main = print (T (ID False)) Look at the instance declaration you'd get instance Show (w Bool) => Show (T w) It's not clear that this leads to terminating instance resolution -- and on occasion it didn't! That's bad for a built-in mechanism like 'deriving'. So I tightened up the rules for derived instances, so the (inferred) context must be classes applied to type variable. Nevertheless, if you write the instance decl yourself, GHC lets you: data T w = T (w Bool) deriving instance Show (w Bool) => Show (T w) This uses the new 'standalone deriving' so that you can write the instance decl, including the context; by saying 'deriving' you ask GHC to fill in the methods in the standard way, which it does. Simon