I've got this fizzbuzz project I am using for a blog series, among other things. In this version, the fizzbuzz function is fed from a Fibonacci generator. I'm particularly concerned with the efficiency of the Fibonacci generator, but all scrutiny is welcomed. I'll included a link to the entire project, but below are the parts I think would be sufficient to spot trouble with how I am generating Fibonacci numbers. -- Driver function performs following-- (1) checks that input is proper-- (2) creates integer list for fibonacci generator-- (3) calculates first x in fibonnaci sequence-- (4) generates fizzbuzz output using (3) fizzBuzzFib :: [Text] -> Either FizzError [Text] fizzBuzzFib str = mapM fizzbuzz =<< mapM fibb =<< -- Possible problem here(\x -> Right [1 .. x]) =<< convertToPInt =<< mustHaveOne str fibb :: Integer -> Either FizzError Integer fibb n = Right $ snd . foldl' fib' (1, 0) . map (toEnum . fromIntegral) $ unfoldl divs n where unfoldl f x = case f x of Nothing -> [] Just (u, v) -> unfoldl f v ++ [u] divs 0 = Nothing divs k = Just (uncurry (flip (,)) (k `divMod` 2)) fib' (f, g) p | p = (f*(f+c*g), f^c + g^c) | otherwise = (f^c+g^c, g*(c*f-g)) where c :: Integer -- See codebase for reasons c = 2 The whole project, for your critiquing eye:https://github.com/mlitchard/swiftfizz