Validate CC checksum

cc_validate.hs

{-
Validate CC's checksum
-}

module HW01 where

lastDigit :: Integer -> Integer
lastDigit x = x `mod` 10

dropLastDigit :: Integer -> Integer
dropLastDigit x = x `div` 10

toDigitsReversed :: Integer -> [Integer]
toDigitsReversed n
  | n <= 0 = []
  | otherwise = lastDigit n : toDigitsReversed (dropLastDigit n)

toDigits :: Integer -> [Integer]
toDigits n = reverse (toDigitsReversed n)

doubleEveryOther :: [Integer] -> [Integer]
doubleEveryOther xs = reverse (doubleOnReversed (reverse xs))

doubleOnReversed :: [Integer] -> [Integer]
doubleOnReversed [] = []
doubleOnReversed (x:[]) = [x]
doubleOnReversed (x:(y:zs)) = x : (2 * y) : doubleOnReversed zs

sumDigits :: [Integer] -> Integer
sumDigits [] = 0
sumDigits (x:xs) = (sum (toDigits x)) + sumDigits xs

validate :: Integer -> Bool
validate n = ((sumDigits (doubleEveryOther (toDigits n))) `mod` 10) == 0