Largest product in a series

gistfile1.txt

digits :: Integral x => x -> [x]
digits 0 = []
digits x = digits (div x 10) ++ [mod x 10]

largestProduct :: Int -> Integer -> Integer
largestProduct digitCount number =
    largestProduct' digitCount (digits number)

largestProduct' :: Int -> [Integer] -> Integer
largestProduct' digitCount list
    | (length list) < digitCount = 0
    | otherwise = max (product (take digitCount list)) (largestProduct' digitCount (tail list))