Hi Chris What does the Stream class *do* though? class Stream a b c d where (->>) :: a -> (b -> c) -> d Even with Neil's change its still quite unusual: class Stream a b c where (->>) :: a -> (b -> c) -> c In the first formulation there is an input of type a, a function (b -> c) and a result of a completely different type d. In Neil's class the function relates to the type of the answer but not to the input. The "difficult" type classes in Haskell - Applicative, Monad, and Arrows / Category - are related to some degree to fairly standard combinators on functions. But they generalize the combinators to operate on other types than the function type (->). As there isn't a relation between input and output, I don't quite see how the Stream type could start as a combinator. Best wishes Stephen