[Haskell] deriving Show for GADT?

[Norman Ramsey]

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