Simple GADTs Question

Hello, I'm attempting to use GADTs for the first time, and I'm running into an (I think) odd error. My file includes: data DFExpr :: * -> * where Deriv :: (Num t) => t -> Deriv Int (DFExpr t) (DFExpr t) (:+:) :: (Num t) => DFExpr t -> DFExpr t -> DFExpr t -- and so forth, DFFeat :: FExpr t and I'm getting an error "Not in scope: type constructor or class `Deriv'. I've got -fglasgow-exts in the options - what else should I be doing? /g