I'm trying to define a type called Stream. For now, assume that that the type has 3 fields: uid, x, y. X and y represent a point in space that a stream occupies, and uid is a unique identifier for the stream. The uid should be "auto-generated". It's important that streams have an identity so that they can be referred to and manipulated. Streams will be put together into a list, and they'll eventually need to be able to point to one another. It would be nice to be able to print the uid, just to show that it works. Now it occured to me that what one might want to generate the uids is a monad, let's call it UID. It'll have a function get, which returns another identifier. I assumed that the best solution for the problem would be in terms of monads, because successive calls to get return different results; i.e. there's a bit of state going on inside. The values returned can start from 1, and increment by 1 each time. I had a look at some documentation at: http://www.haskell.org/tutorial/monads.html and I'm afraid my brain just froze. I get the idea that data SM a = SM (S -> (a,S)) maps a state to a result, and a new state. OTOH, looking at instance Monad SM where -- defines state propagation SM c1 >>= fc2 = SM (\s0 -> let (r,s1) = c1 s0 SM c2 = fc2 r in c2 s1) return k = SM (\s -> (k,s)) just confuses me no end. Any pointers, like am I taking completely the wrong approach anyway? I'm also puzzled as to how the initial state would be set. ___________________________________________________________ Yahoo! Messenger - NEW crystal clear PC to PC calling worldwide with voicemail http://uk.messenger.yahoo.com