Functional Options in Go

functionaloptions.go

// Functional Options
// Written to help me wrapping my brains around Functional Options - see this
// blog post about a speech by Dave Cheney:
// http://dotgo.sourcegraph.com/post/99643162983/dave-cheney-functional-options
//
// The idea behind Functional Options:
// How to add options to an API later without breaking the API?
// Solution:

package main

import (
	"errors"
	"fmt"
	"time"
)

// Message contains two options, Level and Timestamp.
// Below we will define functions for setting these two options.
type Message struct {
	msg   string
	level int
	useTs bool
}

// Set sets the message sting and any options passed in.
func (m *Message) Set(msg string, options ...func(*Message) error) {
	m.msg = msg
	m.level = 0
	m.useTs = false
	m.setOptions(options...)
}

// Send sends the message.
func (m *Message) Send() {
	if m.useTs {
		t := time.Now()
		fmt.Printf("%v ", t.UTC().Format("2006-01-02 15:04:05.000"))
	}
	levels := [5]string{"Fatal", "Error", "Warning", "Info", "Debug"}
	fmt.Printf("(%v) %v\n", levels[m.level], m.msg)
}

// SetOptions takes one or more option functions and calls each of them.
// Each option function can then apply its option to the receiver.
func (m *Message) setOptions(options ...func(*Message) error) error {
	for _, option := range options {
		err := option(m)
		if err != nil {
			return err
		}
	}
	return nil
}

// Level is an option function for Message. It returns a function that sets
// the severity level of the message.
func Level(lvl int) func(m *Message) error {
	return func(m *Message) error {
		if lvl < 0 || lvl > 4 {
			return errors.New("Level must be between 0 and 5.")
		}
		m.level = lvl
		return nil
	}
}

// UseTimestamp is an option function for Message. Unlike Level(), it does
// not create and return a function. Rather, because it represents a
// boolean option, it has no parameters and thus already has the signature
// of an option function. Just pass it to the initializer function.
// UseTimestamp sets useTs to true.
// If useTs is true, Send() adds a timestamp to the message.
func UseTimestamp(m *Message) error {
	m.useTs = true
	return nil
}

// Add your option function here.
// Add a private option member to the Message type.
// No need to change the Set() function in any way. Message's API remains stable!

// func MyOption(p MyParam) func(m *Message) error {
//    ...
//}

func main() {
	msg := &Message{}
	msg.Set("Test message", Level(3), UseTimestamp)
	msg.Send()
	msg.Set("Another test message", Level(4))
	msg.Send()
	msg.Set("Panic!")
	msg.Send()
}

The best way to understand a programming concept is to write a pice of code that demonstrate that concept. I stumbled over Functional Option via the blog post mentioned in the package comment and wrote this to understand how Functional Options work.
This code is "go runable".

License:

Copyright (c) 2015, Christoph Berger code@christophberger.com

Redistribution and use in source and binary forms, with or without modification,
are permitted provided that the following conditions are met:

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   list of conditions and the following disclaimer.
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   this list of conditions and the following disclaimer in the documentation
   and/or other materials provided with the distribution.
3. Neither the name of the copyright holder nor the names of its contributors
   may be used to endorse or promote products derived from this software without
   specific prior written permission.

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