go-functional-options.go

// Design Pattern: functional options
//
// Go playground URL: https://play.golang.org/p/FiUmbyqEZgd
// 
// References:
// Rob Pike, Self-referential functions and the design of options: https://commandcenter.blogspot.com/2014/01/self-referential-functions-and-design.html
// Dave Cheney; Functional options for friendly APIs: https://dave.cheney.net/2014/10/17/functional-options-for-friendly-apis
// Functional options on steroids, Márk Sági-Kazár: https://sagikazarmark.hu/blog/functional-options-on-steroids/

package main

import (
	"errors"
	"fmt"
	"os"
	"strconv"
)

type Client struct {
	ID   int
	Name string
}

type ClientOptionFunc func(*Client) error

func NewClient(options ...ClientOptionFunc) (*Client, error) {
	c := &Client{}
	for _, option := range options {
		if err := option(c); err != nil {
			return nil, err
		}
	}
	return c, nil
}

func SetID(id int) ClientOptionFunc {
	return func(c *Client) error {
		if id < 1024 {
			return errors.New("the ID is too small, less than 1024")
		}
		c.ID = id
		return nil
	}
}

func SetName(name string) ClientOptionFunc {
	return func(c *Client) error {
		c.Name = name
		return nil
	}
}

func (c *Client) String() string {
	return strconv.Itoa(c.ID) + "/" + c.Name
}

func main() {
	c, err := NewClient(
		SetID(10024),
		SetName("myClient"))
	if err != nil {
		fmt.Printf("failed to create a client: %v", err)
		os.Exit(1)
	}
	fmt.Println(c)
}