Past exam of the year 1 of the computer science course of the University of Oxford / 2017 / Functional programming, design and analysis of algorithms (resit) / 4 / c

The required function composition is

unfold p f g = map f . takeWhile (not . p) . iterate g

iterate g supplies the successive states, takeWhile (not . p) retains precisely those before the first stopping state, and map f produces their outputs. If the initial state satisfies the stopping predicate, both sides return the empty list. Otherwise both emit the same head and repeat from g x; the same argument applies to every demanded finite prefix of an infinite result.