code: func ListenHostPort(listenAddr string) (string, error) {
  	hp := listenAddr
  	if strings.HasPrefix(hp, ":") {
  		hp = "localhost" + hp
  	} else if strings.HasPrefix(hp, "0.0.0.0:") {
  		hp = "localhost:" + hp[len("0.0.0.0:"):]
  	}
  	if _, _, err := net.SplitHostPort(hp); err != nil {
  		return "", err
  	}
  	return hp, nil
  }
docs: // ListenHostPort maps a listen address into a host:port string.
  // If the host part in listenAddr is empty or 0.0.0.0, localhost
  // is used instead.