number :: String -> Maybe String
number xs
  | isValid cleanNumber = Just cleanNumber
  | otherwise = Nothing
  where
    cleanNumber = (stripCountryCode . filter isDigit) xs
number :: String -> Maybe String
number xs
  | isValid cleanNumber = Just cleanNumber
  | otherwise = Nothing
  where
    cleanNumber = (stripCountryCode . filter isDigit) xs
number xs
  | isValid cleanNumber = Just cleanNumber
  | otherwise = Nothing
  where
    cleanNumber = (stripCountryCode . filter isDigit) xs