Learning haskell

isCreditCardNumberValid.hs

import Data.Char


-- Double the value of every second digit beginning from the right.
-- Add the digits of the doubled values and the undoubled digits from the original number
-- Calculate the remainder when the sum is divided by 10
-- If the result equals 0, then the number is valid.


-- String -> [Int]
let convertStringOfDigitsToIntList str = [digitToInt (str !! i) | i <- [0..(length str)-1]]


-- String -> Bool
let isCreditCardNumberValid cardNumber = sum[if i `mod` 2 == 1 then (cardNumber !! i) * 2 else (cardNumber !! i) | i <- [0..(length cardNumber)-1]] `mod` 10  == 0 


let cardNumberString = "4242424242424242";
let cardNumber = convertStringOfDigitsToIntList cardNumberString
let revertedCardNumber = reverse cardNumber
isCreditCardNumberValid revertedCardNumber