Dear GHC experts, Certain behaviour when using {-# LANGUAGE GADTs, TypeFamilies #-} surprises me. The following is accepted by GHC 6.12.1: data GADT a where BoolGADT :: GADT Bool foo :: GADT a -> a -> Int foo BoolGADT True = 42 But the following is not: data family DataFam a data instance DataFam Bool where BoolDataFam :: DataFam Bool fffuuuu :: DataFam a -> a -> Int fffuuuu BoolDataFam True = 42 GHC 6.12.1 throws the following error (GHC 6.10.4 panics): Couldn't match expected type `a' against inferred type `Bool' `a' is a rigid type variable bound by the type signature for `fffuuuu' at gadtDataFam.hs:13:19 In the pattern: BoolDataFam In the definition of `fffuuuu': fffuuuu BoolDataFam True = 42 I expect that `fffuuuu` should be accepted just like `foo`. Do I expect too much? Cheers, Sebastian -- Underestimating the novelty of the future is a time-honored tradition. (D.G.)