[Haskell-beginners] Haskell data generics

Hello everyone, Here is what I am trying to do: using Data and Typeable generate a representation of any datatype. After looking at some examples this was my attempt: http://lpaste.net/132669. But I can't seem to understand how extQ should work. For instance https://github.com/faylang/fay/blob/master/src/Fay/Convert.hs#L54 https://github.com/lymar/hastache/blob/master/Text/Hastache/Context.hs#L321 All seem to use extQ to "choose" the right way to encode the values(I imagine it's because of the cast inside extQ?). So what am I missing here?