https://blog.web-apps.tech/go-functional-option-pattern/

FunctionalOptionParttern.go

package main

import (
	"fmt"
	"log"
	"net"
	"time"
)

// Server is struct
type Server struct {
	listener net.Listener
	Timeout  time.Duration
}

// Config is struct
type Config struct {
	Timeout time.Duration
}

// NewServer is func
func NewServer(addr string, options ...func(*Server)) (*Server, error) {
	l, err := net.Listen("tcp", addr)
	if err != nil {
		return nil, err
	}

	srv := Server{listener: l}
	// ここまでは同じ

	for _, option := range options {
		option(&srv)
	}
	return &srv, nil
}

// Timeout is func
func Timeout(t int) func(*Server) {
	return func(s *Server) {
		s.Timeout = time.Duration(t) * time.Second
	}
}
func main() {
	srv, err := NewServer("localhost", Timeout(30))
	if err != nil {
		log.Fatal(err)
	}
	fmt.Println(srv.Timeout)
}