credit card validation

Card.hs

module Card where

import Data.Char

produceDigits :: Integer -> [Int]
produceDigits n = [(ord i) - 48| i <- (show n)]

doubleEveryOther :: [Int] -> [Int]
doubleEveryOther l = case (length l) `mod` 2 of
                        0 -> helper0 l
                        1 -> helper1 l

helper0 :: [Int] -> [Int]
helper0 [] = []
helper0 (x : xs) = 2 * x : helper1 xs

helper1 :: [Int] -> [Int]
helper1 (x : xs :xss) = x : 2 * xs : helper1 xss
helper1 l = l

sumDigits :: Int -> Int
sumDigits = sum . produceDigits . fromIntegral

validate :: Integer -> Bool
validate cardNumber = checkSum == last thirdRow
  where thirdRow = map sumDigits $ doubleEveryOther $ produceDigits cardNumber
        checkSum = (10 - ) $ (`mod` 10) $ sum $ init thirdRow

data Issuer = MasterCard | Visa

issuer :: Integer -> Issuer
issuer = undefined

-- validate 79927398713 == True
-- validate 79927398712 == False

I still wanted to add a more compact doubleEveryOther :)

doubleEveryOther :: [Int] -> [Int]
doubleEveryOther [] = []
doubleEveryOther (x:[]) = (x:[])
doubleEveryOther (x:xs:xss) = x : xs * 2 : doubleEveryOther xss