Hello, I finally solved exercise 1 where I had to write a programm which checks if a creditcard number is valid. I solved it this way : toDigits :: Integer -> [Integer] toDigits n | n < 0 = [] | n < 10 = [n] | otherwise = toDigits (n `div` 10) ++ [n `mod` 10] -- | convert a number to a reversed array where a negative number will be a empty array toDigitsRev :: Integer -> [Integer] toDigitsRev 0 = [0] toDigitsRev n | n < 0 = [] | n < 10 = [n] | otherwise = n `mod` 10 : toDigitsRev (n `div` 10) -- | Doubles every second number from the right. doubleEveryOther :: [Integer] -> [Integer] doubleEveryOther [] = [] doubleEveryOther (x:[]) = [x] doubleEveryOther (x:(y:zs)) | length (x:(y:zs)) `mod` 2 /= 0 = [x] ++ (y * 2) : doubleEveryOther zs | otherwise = [x *2] ++ y : doubleEveryOther zs -- | sum all the digits of a array sumDigits :: [Integer] -> Integer sumDigits [] = 0 sumDigits (x:zs) | x < 10 = x + sumDigits zs | otherwise = x `mod` 10 + x `div` 10 + sumDigits zs -- | validate a number by looking if a number can be divided by 10 validate :: Integer -> Bool validate n = sumDigits(doubleEveryOther(toDigits(n))) `mod` 10 == 0 -- | The main entry point. main :: IO () main = do print $ validate 4012888888881881 Any remarks about this solution. I know there are higher function solutions but that part is not descrived in chapter 1 . Roelof