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 / e

Store two consecutive Fibonacci numbers as the state:

fibs :: [Integer]
fibs = unfold (const False) fst step (0,1)
  where
    step (a,b) = (b,a+b)

The invariant is that the state is $(F_n,F_{n+1})$, with $F_0=0,F_1=1$. Emitting its first component and updating the pair therefore produces $0,1,1,2,3,5,8,\dots$. Integer avoids fixed-width overflow; the bit cost of adding increasingly large integers is separate from the number of unfold steps.