 List unfold

Generate a list from a state: a step returning Nothing terminates; Just (x,s') emits x and continues from s'. Thus unfoldr step s = case step s of Nothing -> []; Just (x,s') -> x : unfoldr step s'. A list unfold decomposes the evolving state, whereas a list fold consumes existing constructors. Productivity depends on each demanded constructor becoming available, not merely on using lazy evaluation.