[Haskell] deriving Show for GADT?

I've got a fairly large GADT for which I wished to use deriving (Show) but I got a mysterious error message: Exp.hs:13:11: Can't make a derived instance of `Show (Exp a)' (`Exp' has non-Haskell-98 constructor(s)) In the data type declaration for `Exp' This is from GHC. Does anybody know a compiler option or other trick that will coax the compiler into producing a Show instance. (I know I can write one by hand, but I'd rather not bother.) Norman