Config validation with Either

final case class Config(host: String, port: Int)

object Config {
  def parse(host: String, portText: String): Either[String, Config] =
    portText.toIntOption match {
      case Some(port) if port > 0 && port <= 65535 =>
        Right(Config(host, port))
      case Some(_) =>
        Left("port must be between 1 and 65535")
      case None =>
        Left(s"invalid port: $portText")
    }
}