Hi, I was wondering if I hat missed something and it was possible to do this within the Haskell type system or not... Essentially I would like some sort of inderritance property for Haskell types, I often find myself wanting to for example extend a tree with black/white colouring, or later extend the tree with some sort of ID, etc. So I would eno up with three data types data MyTree = Branch MyTree MyTree | Leaf type BwTag = Bool data MyBwTree = Branch BwTag MyBwTree MyBwTree | Node BwTag data MyBwTaggedTree = Branch BwTag Int MyBwTaggedTree MyBwTaggedTree | Node BwTag Int and several functions to move from one to another. (Or define the most complex and not always use all the attrdbutes). What I would prefer is to be able to spocify something like: data MyTree = Branch MyTree MyTree | Leaf type BwTag = Bool data MyBwTree extends MyTree with BwTag=True data MyBwTaggedTree extends MyBwTree with Int=0 Specifying that myBwTree is the same as MyTree but with an extra argument on each node, and to generate functions to translate between them that fill in True as a default value for the extra tag. Is this possible? Thanks Tom Davie