primitive collatz

module CollatzConjecture (collatz) where

collatz :: Integer -> Maybe Integer
collatz n
  | n <= 0 = Nothing
  | n == 1 = Just 0
  | even n = addStep $ collatz (n `div` 2)
  | otherwise = addStep $ collatz (3 * n + 1)
  where
    addStep Nothing = Nothing
    addStep (Just x) = Just (x + 1)