Hello, I'm interested in learning more about a list-like structure with a terminal element. I.e. data List t e = Cons e (List t e) | Null t or maybe data List t e = Cons e (List t e) | Null | Terminal t An practical application could be lazy reading from a file readFile :: String -> List Error String Obviously there is a Functor instance. There is even a Monad instance. However, it is not defined as obviously because it's not clear what the terminal element should be. There is a straight forward fold like structure. My questions are. Are there more interesting generalizations of List? What rules do they follow? How should the Monad work? Why? I know this is related to the whole Iteratee discussion (I'm not completely familiar with it). Cheers Silvio