On Tuesday 04 July 2006 13:20, Simon Peyton-Jones wrote:
Lexically-scoped type variables are undergoing a slight upheaval in GHC 6.6 that has not quite settled, and that is what you are running into.
Thanks for the help. After a lot of trial & error, and reading and stuff I've got past the problems introduced by lexicals. Now I'm hitting another problem. I think there's a missmatch between Maybe (c a) returned by dataCast1 and Maybe (c (t' a)) returned by gcast1. Is this dues to something stupid I have done, or bit-rot between the two libraries? Thanks Matthew Data/Generics2/Instances.hs:290:17: Couldn't match expected type `forall a1. (Data ctx a1) => c (t a1)' against inferred type `c1 (t1 a1)' Expected type: (forall a2. (Data ctx a2) => c (t a2)) -> Maybe (c [a]) Inferred type: c1 (t1 a1) -> Maybe (c1 (t' a1)) In the expression: gcast1 In the definition of `dataCast1': dataCast1 _ = gcast1 The type of gcast is: Data.Typeable. gcast1 :: (Typeable1 t, Typeable1 t') => c (t a) -> Maybe (c (t' a)) And the dataCast1 signature (in Data.Generics2.Basics) is: class (Typeable a, Sat (ctx a)) => Data ctx a where -- | Mediate types and unary type constructors dataCast1 :: Typeable1 t => ctx () -> (forall a. Data ctx a => c (t a)) -> Maybe (c a) ... The implementation (in Data.Generics2.Instances) is: instance (Sat (ctx [a]), Data ctx a) => Data ctx [a] where dataCast1 _ = gcast1 ...