01-creditCardNumbers.hs

-- Sulution for https://www.seas.upenn.edu/~cis194/spring13/hw/01-intro.pdf
toDigits :: Integer -> [Integer]
toDigits n
         | n < 1 = []
         | otherwise = map (\x -> read [x] :: Integer) $ show n

toDigitsRev :: Integer -> [Integer]
toDigitsRev n = reverse (toDigits n)

-- taken and modified from stackoverflow to be more readable,
-- at first i did not understand it but after messing with it in the repl now i do :)
doubleEveryOther :: [Integer] -> [Integer]
doubleEveryOther xs = fst $ foldr (\x (result, shouldDub) ->
                      (
                      -- add the result to the list
                      (if shouldDub then 2 * x else x) : result,

                      -- next pass we should do the opposite thing
                      not shouldDub
                      ))

                      -- False is used to remember if we should double the next one or not.
                      ([], False) xs

sumDigits :: [Integer] -> Integer
sumDigits = sum . map sum . map (toDigits)

validate :: Integer -> Bool
validate n =
         if (rem (sumDigits $ doubleEveryOther $ toDigits n) 10) == 0
         then True
         else False