Just fooling around with personnummer-validation.

ssnLab.hs

import Data.Char
import Data.List

type Personnummer = [Int]

mod10 x = mod x 10

cleanAndConvert :: String -> Maybe Personnummer
cleanAndConvert cs = return $ map digitToInt $ filter isDigit cs

validateLength :: Personnummer -> Maybe Personnummer
validateLength xs | (length xs) `elem` [10, 12] = find (\x -> (length x) == 10) $ tails xs
                  | otherwise                   = Nothing

validateLuhn :: Personnummer -> Maybe Personnummer
validateLuhn xs | last xs == result = return xs
                | otherwise         = Nothing
    where 
        xs'    = take 9 xs
        result = mod10 $ 10 - (mod10 $ sum $ map (uncurry (+)) $ zipWith (\a b -> divMod (a*b) 10) (cycle [2,1]) xs')


validate :: String -> Maybe Personnummer
validate s = (cleanAndConvert s) >>= validateLength >>= validateLuhn

main = do
    putStrLn "Vänligen ange ett personnummer: "
    s <- getLine
    case validate s of
        Just _  -> putStrLn "Grattis, ditt personnummer validerar!"
        Nothing -> putStrLn "Tyvärr validerar inte ditt personnummer. Försök igen!"
    main