             then helper (acc * n) (n - 1)
                      else acc
   in helper 1 n'

-- guard
factorial''''    :: Integer -> Integer
factor
factorial''' n' =
   let helper acc n = if n > 1
                      then helper (acc * n) (n - 1)
                      else acc
   in helper 1 n'
factorial''' n' =
   let helper acc n = if n > 1
                      then helper (acc * n) (n - 1)
                      else acc
   in helper 1 n'