main.go

// https://golang.cafe/blog/golang-functional-options-pattern.html

package main

import (
	"log"
	"time"

	"github.com/acme/pkg/server"
)

func main() {
	svr := server.New(
		server.WithHost("localhost"),
		server.WithPort(8080),
		server.WithTimeout(time.Minute),
		server.WithMaxConn(120),
	)
	if err := svr.Start(); err != nil {
		log.Fatal(err)
	}
}

server.go

package server

type Server {
    host string
    port int
    timeout time.Duration
    maxConn int
}

func New(options...func( * Server)) * Server {
    svr: = & Server {}
    for _,
    o: = range options {
        o(svr)
    }
    return svr
}

func(s * Server) Start() error {
    // todo
}

func WithHost(host string) func( * Server) {
    return func(s * Server) {
        s.host = host
    }
}

func WithPort(port int) func( * Server) {
    return func(s * Server) {
        s.port = port
    }
}

func WithTimeout(timeout time.Duration) func( * Server) {
    return func(s * Server) {
        s.timeout = timeout
    }
}

func WithMaxConn(maxConn int) func( * Server) {
    return func(s * Server) {
        s.maxConn = maxConn
    }
}