I have a program that basically has, data Expression = Value Value | EVariable Variable | other stuff ... data Value = VNumber Number | other stuff ... data Variable = Variable { variable_name :: String, variable_time :: Expression } data Number = Number { value :: Double, dimension :: Dimension } newtype VariableCount = VariableCount (Variable, Number) The VNumber and EVariable constructors are ugly, though, so I was wondering if I should be using typeclasses - e.g., class Expression a class Expression a => Value a instance Value Number instance Expression Variable ... but I don't see how to define Variable in such a scheme. Maybe I shouldn't be using typeclasses? (Obviously, I actually have lots more type constructors in Expression and Value - dyadic expressions, booleans, etc. - the above with just numbers and variables is somewhat truncated, but should suffice.) -- Mark