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

I got the GADT

data DataBox where   DataBox :: (Show d, Eq d, Data d) => d -> DataBox

[snip]

but I can't figure out how to implement gunfold for DataBox.

The error message is

Text/XML/Generic.hs:274:23:     Ambiguous type variable `b' in the constraints:

I had a similar difficultly in Atom making a GADT a member of Eq. At one point I had my head wrapped around the reason for the problem, but now it escapes me. However, I remember the solution: I created a function to convert the GADT into another, unGADT type, which was then used to compute (==).