Simple GADTs Question

[J. Garrett Morris]

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