Bug in GHC when compiling with -O ?

[MR K P SCHUPKE]

You will have to give both the errors and the source code...
I have done quite a bit with classes and GHC's constraint
inferance is pretty good. 

The chances are you really do need to add some extra 
constraints...

(by the way if you are working with heterogeneous collections,
you may be interested in out paper, http://www.cwi.nl/~ralf/HList
)

	Keean.