[Haskell] deriving Show for GADT?

Ben Moseley ben_moseley at mac.com
Tue Apr 14 01:43:48 EDT 2009

Unfortunately, this isn't supported at present.


On 14 Apr 2009, at 05:28, Norman Ramsey wrote:

> 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
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