Haskell

{- Convert positive integer to list of digit -} {- Input : 12345 o/p : [1, 2, 3, 4, 5 -} toList :: Integer -> [Integer] toList 0 = [] toList x = toList (x `div` 10) ++ [x `mod` 10] {- Reverse the integer after converting it to list -} -- toDigitsRev :: Integer -> [Integer] toListReverse :: Integer -> [Integer] toListReverse xs = (reverse . toList) xs {- Double every second number -} doubleSecondEach :: [Integer] -> [Integer] doubleSecondEach [] = [] doubleSecondEach (x:[]) = [x] doubleSecondEach (x:y:xs) = x : (2*y) : doubleSecondEach xs {- Sum the digit in the number we are sure there is one digit number or two digit number -} sum' :: Integer -> Integer sum' x = (x `mod` 10) + (x `div` 10) {- sum all the element in list -} {- when a element is [13,5,6] we have to add like this: 1 + 3 + 5 + 6 -} sumDigits :: [Integer] -> Integer sumDigits [] = 0 sumDigits [x] = sum' x sumDigits (x:xs) = sum' x + sumDigits xs {- Validate the credit card number -} validate :: Integer -> Bool validate x = if (sumDigits $ doubleSecondEach $ toListReverse x ) `mod` 10 == 0 then True else False