The Math Trick Hidden in Your Credit Card Number

    import Data.Char

    card = "4024 6072 3695 0748"

    cardDigits = map digitToInt . filter (not . isSpace)

    doubleDigitSum d = 2 * d - if d<5 then 0 else 9

    doubleEveryOther = reverse . zipWith ($) (cycle [doubleDigitSum,id]) . reverse
   
    main = let
        cd = cardDigits $ card
        sd = sum . doubleEveryOther . init $ cd
        checkDigit = (- (sd + last cd)) `mod` 10
      in print checkDigit