Hello, What's the way to express the following: a compound object is generally made up of two components with identical type. For this common case I'd like to provide some default methods, which take the object apart, operate on the parts and put the results back together. In other cases this default method should be overridden, eg. when the compound object isn't made of two identical components. My first try (below) doesn't work, can you recommend a solution? Thanks: Feri. \begin{code} module Test where class Component b where property :: b -> Int class Compound a where decompose :: Component b => a -> (b,b) additive :: a -> Int additive x = property l + property r where (l,r) = decompose x \end{code} ghci: test.hs:9: Ambiguous type variable(s) `b' in the constraint `Component b' arising from use of `property' at test.hs:9 hugs: ERROR test.hs:9 - Cannot justify constraints in default member binding *** Expression : additive *** Type : Compound a => a -> Int *** Given context : Compound a *** Constraints : Component b