One way to think about Reactive's notion of "Future" is as a single element of an event stream--something that might happen (yielding a value) some time in the future. 'mempty' on futures is a future that never happens, and 'mappend' says to pick the first of two futures to happen. m >>= k waits for m's future to happen, then passes the result onto k. So my definition of stateMachine says: - Wait for either the future returned by 'run s0' or the first event in the input event stream. - If the future returned by 'run s0' happens first, we have an event instance--return an event whose 'rest of the event stream' is the next state of the state machine - If the future from the input event stream happens first, update the state and continue without spawning an event. - If they happen at the same time, 'run s0' wins (mappend is left-biased) Good luck! -- ryan