haskell

factorial :: Int -> Int
factorial num
    | num == 0   = 1
    | otherwise  = num * factorial (num-1)

-- this is deferred operation or something, answer not evaluated until the recursion starts unwinding?:
factorial2 :: Int -> Int
factorial2 n = help n 1
help :: Int -> Int -> Int
help n m
    | n == 0    = m
    | otherwise = help (n - 1) (m * n)