Negatively recursive data types

Hi... I'm currently looking at the semantics of recursive data types. One thing that Haskell allows, but the semantics for it is very hairy, is *negatively* recursive data types. That is, data types where the recursion occurs to the left of a function arrow. For example: data Neg a b = MkNeg (a -> Neg a b -> b) Here (Neg a b) occurs to the left of the function arrow. Some members of this data type might be: n1, n2 :: Neg Int [Int] n1 = MkNeg (\ x _ -> [x]) n2 = MkNeg (\ x (MkNeg f) -> x : f (x+1) n1) This example is not a very good one. Does anyone have an example of a useful data type involving negative recursion? Thanks. --KW 8-)