Haskell — bidirectional class instance type implications OR GADT existential type qualifications?

Add the Show constraint to the existential data D t where C :: t -> D t R :: Show be => D be -> D (Either a b) and you're in business Prelude> :r 1 of 1 Compiling A ( A. Hs, interpreted ) Ok, modules loaded: A. *A> R (C 7) R (C 7).

Add the Show constraint to the existential, data D t where C :: t -> D t R :: Show be => D be -> D (Either a b) and you're in business. Prelude> :r 1 of 1 Compiling A ( A. Hs, interpreted ) Ok, modules loaded: A.

*A> R (C 7) R (C 7).

I already tried that, and it won't work form my case, since this class forms the basis of an arrow value type, which needs to work on all b, whether they derive Show or not. I want to write code that says, any "D" value whose arguments (existential types) derive Show should be showable. – gatoatigrado May 1 at 1:37 1 @gatoatigrado: I don't think it's possible to make GHC derive this, at least in part because the deriving mechanism doesn't work well with GADT's.

However, it's very easy to manually write the Show instance you want. – John L May 1 at 18:23.

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