[Haskell-cafe] Re: Existentials and SYB [Was: GADTs and Scrap your Boilerplate]

[Oscar Finnsson]

>> enDataI :: (Int -> DataBox)
>> enDataI = DataBox
>>
>> enDataB :: (Bool -> DataBox)
>> enDataB = DataBox
>>
>> instance Data DataBox where
>>   gfoldl k z (DataBox d) = z DataBox `k` d
>>   gunfold k z c = (if True then k (z enDataI) else k (z enDataB))
>

Interesting solution but I'm not smart enough to see how the solution
can be generalized to any data type that's an instance of Data. Do I
have to repeat the "if then else" for every instance of the Data type
class (which I can't) or is there some other way?

Oscar