fib' :: (Integral a) => a -> (a, a)
fib' 0 = (0, 0)
fib' 1 = (0, 1)
fib' x = (b, a + b)
  where (a, b) = fib'(x-1)

fib :: (Integral a) => a -> a
fib x = result
  where (_, result) = fib'(x)