-- | Optional @String@ argument
optStr :: Mod OptionFields (Maybe String) -> Parser (Maybe String)
optStr m = option (Just <$> str) $ value Nothing <> m
optStr :: Mod OptionFields (Maybe String) -> Parser (Maybe String)
optStr m = option (Just <$> str) $ value Nothing <> m
optStr m = option (Just <$> str) $ value Nothing <> m