After reasonable success with an FFI exercise, I have run into a typing issue that has me stuck. The foreign software is a C plotting library that my colleagues use, really simple and mostly quite amenable to Haskell-ization. The one fly in the ointment is the graph coordinate type, which is a union of double & int64. The graph object has switches for the actual type of x and y coordinates, set at creation time. So plot :: GraphIntInt -> [CLLong] -> [CLLong] -> IO () plot :: GraphIntFloat -> [CLLong] -> [CDouble] -> IO () plot :: GraphFloatInt -> [CDouble] -> [CLLong] -> IO () plot :: GraphFloatFloat -> [CDouble] -> [CDouble] -> IO () That wasn't hard to do with a different name for each plot function - "plotIntInt", "plotIntFloat", etc., and this has the desired effect, that the type enforces the use of the correct coordinate types for the graph type. But for a typeclass function "plot" ... is there some way to express the coordinate types in the graph type? class Coords (g x y) where plot :: g -> [x] -> [y] -> IO () instance Coords (GraphIntInt CLLong CLLong) where ... instance Coords (GraphIntFloat CLLong CDouble) where ... Of course this isn't a major problem, it just seemed like I might be missing something. Thanks, Donn Cave, donn@drizzle.com