Scala environment config with typed validation

case class DbConfig(url: String, port: Int, poolSize: Int)

def loadDbConfig(env: String => Option[String] = sys.env.get): Either[List[String], DbConfig] =
  def required(key: String): Either[List[String], String] =
    env(key).filter(_.nonEmpty).toRight(s"$key is required").left.map(List(_))
  def int(key: String, default: Int, valid: Int => Boolean): Either[List[String], Int] =
    env(key).filter(_.nonEmpty) match
      case None => Right(default)
      case Some(raw) =>
        raw.toIntOption.filter(valid).toRight(s"$key has an invalid integer value").left.map(List(_))
  for
    url <- required("DB_URL")
    port <- int("DB_PORT", 5432, p => p > 0 && p < 65536)
    pool <- int("DB_POOL_SIZE", 8, _ > 0)
  yield DbConfig(url, port, pool)