Why does GHC complains on the code below ? (I'll explain in a second a requirement to do just so) I get errors with ghc 6.12.1 and 7.0.2. --------------------------------------------------------------------------------------------------------------------- {-# LANGUAGE GADTs, TypeFamilies #-} class CPU cpu where type CPUFunc cpu data Expr cpu where EVar :: String -> Expr cpu EFunc :: CPU cpu => CPUFunc cpu -> Expr cpu class CPU cpu => FuncVars cpu where funcVars :: CPUFunc cpu -> [String] exprVars :: FuncVars cpu => Expr cpu -> [String] exprVars (EVar v) = [v] -- an offending line: exprVars (EFunc f) = funcVars f --------------------------------------------------------------------------------------------------------------------- I tried to split creation and analysis constraints. CPU required for creation of expressions, FuncVars required for analysis. It all looks nice but didn't work. (In our real code EVar is slightly more complicated, featuring "Var cpu" argument) It looks like GHC cannot relate parameters "inside" and "outside" of GADT constructor. Not that I hesitate to add a method to a CPU class, but I think it is not the right thing to do. So if I can split my task into two classes, I will feel better.