yafib (yet another fib)

code:haskell

fib0 = fst . foldr phi (0,1) . binary

phi k (a,b) = bool (d, c + d) (c, d) (k == 0)

where

c = a * (2 * b - a)

d = a * a + b * b

binary = unfoldr psi

psi = \ case

0 -> Nothing

n -> Just (swap (divMod n 2))

fib = fst . hylo phi (0,1) psi