Collatz

ifOne :: Integral a => [a] -> [a]
ifOne xs =
  if lst == 1
    then xs
    else oddOrEven xs
  where
    lst = last xs

oddOrEven :: Integral a => [a] -> [a]
oddOrEven xs =
  if even lst
    then ifOne (xs ++ [div lst 2])
    else ifOne (xs ++ [lst * 3 + 1])
  where
    lst = last xs

collatz :: Integral a => a -> [a]
collatz x =
  if x < 1
    then error "An integer greater than zero is required!"
    else ifOne [x]

main = print $ collatz 104

-- [104,52,26,13,40,20,10,5,16,8,4,2,1]