Friends
Just a heads-up. Pedro is working on implementing "Generic Defaults", as described in his Haskell Symposium 2010 paper www.dreixel.net/research/pdf/gdmh_nocolor.pdf
It will replace (and improve on) the "Derivable type classes" stuff in GHC at the moment, which was originally presented in a paper of that title that Ralf and I wrote in the 2000 Haskell workshop http://research.microsoft.com/en-us/um/people/simonpj/papers/derive.htm.
The "Derivable type class" extension is barely used, I believe, and isn't even documented in the manual.