Practical example using functional options for creating extendable functions in Go(golang).

option-func-example.go

package main

import (
	"flag"
	"fmt"
)

// An ServerOption configures our Server
type ServerOption interface {
	apply(s *Server) // 😈 an option is now ours and ours alone to implement
}

// optionFunc wraps a func so it satisfies the ServerOption interface.
type optionFunc func(*Server)

// apply satisfies the interface and
// invokes the optionFunc
func (f optionFunc) apply(s *Server) {
	f(s)
}

// Use OptionHost to set the host the server will use.
func OptionHost(host string) ServerOption {
	return optionFunc(func(s *Server) {
		s.host = host
	})
}

// Use OptionPort to set the port the server will use.
func OptionPort(port int) ServerOption {
	return optionFunc(func(s *Server) {
		s.port = port
	})
}

// This is the type whose options you're enabling users to set.
type Server struct {
	host string
	port int
}

// This is your creator function that accepts a list of option functions.
func NewServer(opts ...ServerOption) *Server {
	s := &Server{}

	// call option functions on instance to set options on it
	for _, opt := range opts {
		opt.apply(s)
	}

	return s
}

func (s *Server) Addr() string {
	return fmt.Sprintf("%s:%d", s.host, s.port)
}

func main() {
	var host = flag.String("host", "127.0.0.1", "host")
	var port = flag.Int("port", 3000, "port")
	flag.Parse()

	// This is how your user sets the options.
	s := NewServer(
		OptionHost(*host),
		OptionPort(*port),
	)

	fmt.Printf("Server available at %s", s.Addr())
}