rubyquiz122.hs

import Data.List
import Data.Char
import Control.Monad
import System.Environment

data Card = AMEX String | Discover String | MasterCard String | Visa String | Unknown deriving Show

detectCard :: String -> Card
detectCard cardNumber@(x:y:_)
  | ("34" `isPrefixOf` cardNumber) || ("37" `isPrefixOf` cardNumber) = AMEX cardNumber
  | "6011" `isPrefixOf` cardNumber = Discover cardNumber
  | (x == '5') && (y `elem` map intToDigit [1..5]) = MasterCard cardNumber
  | x == '4' = Visa cardNumber

validLength :: Card -> Bool
validLength (AMEX n) = length n == 15
validLength (Discover n) = length n == 16
validLength (MasterCard n) = length n == 16
validLength (Visa n) = length n == 13 || length n == 16
validLength Unknown = False

validLuhn :: Card -> Bool
validLuhn Unknown = False
validLuhn card = sum splitted_doubled `mod` 10 == 0
  where num = case card of
                AMEX s       -> s
                Discover s   -> s
                MasterCard s -> s
                Visa s       -> s
        doubled = zipWith ($) (cycle [id, (* 2)]) $ map digitToInt $ reverse num
        splitted_doubled = concatMap (map digitToInt . show) doubled

main :: IO ()
main = do s <- liftM (filter (/= ' ') . head) getArgs
          let card = detectCard s
          print $ validLength card && validLuhn card