checksum digits
| odd $ length digits = digits
| otherwise = checksum $ map (checksumPair) $ pairs digits
-
-
-checksumPair :: String -> Char
-checksumPair p = if (length $ nub p) == 1 then '1' else '0'
+ where checksumPair p = if (length $ nub p) == 1 then '1' else '0'
pairs :: [a] -> [[a]]