gistfile1.hs

import Data.Digits
import Data.Maybe

data Isbn = Isbn { isbn10 :: Maybe Isbn10
                 , isbn13 :: Maybe Isbn13 }
            deriving (Eq, Show)

data Isbn10 = Isbn10 Int deriving (Eq, Show)
data Isbn13 = Isbn13 Int deriving (Eq, Show)

mkValidIsbn10 :: Int -> Maybe Isbn10
mkValidIsbn10 n
    | d == 10 = Just $ Isbn10 n
    | otherwise = Nothing
    where d = length $ digits 10 n

mkValidIsbn13 :: Int -> Maybe Isbn13
mkValidIsbn13 n
    | d == 13 = Just $ Isbn13 n
    | otherwise = Nothing
    where d = length $ digits 10 n

getIsbn10 :: Maybe Isbn10 -> Int
getIsbn10 (Just (Isbn10 a)) = a
getIsbn13 :: Maybe Isbn13 -> Int
getIsbn13 (Just (Isbn13 a)) = a

notValidIntError :: Int -> String
notValidIntError n = "Cant create a valid ISBN from " ++ show n

mkValidIsbn :: Int -> Isbn
mkValidIsbn n | isbn10 a == Nothing && isbn13 a == Nothing = error $ notValidIntError n
              | otherwise = a
              where a = Isbn (mkValidIsbn10 n) (mkValidIsbn13 n)