[Haskell] deriving Show for GADT?

Norman Ramsey nr at cs.tufts.edu
Tue Apr 14 00:28:20 EDT 2009

I've got a fairly large GADT for which I wished to use
   deriving (Show)
but I got a mysterious error message:

    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.)


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