Functional Variadic Arguments as Options

function-options.go

package main

import (
	"fmt"
	"strings"
)

// consoleOptions is a private structure that contains 
// the possible options for a Console. It's private so 
// that consumers do not rely on its members. This allows 
// the code to be able to be extended without breaking how
// consumers use it. 
type consoleOptions struct {
	lastNameOption string
	shoutOption bool
}

// ConsoleOption is a functional option that can be passed 
// to a constructor of a Console. Again, this allows the 
// constructor of a Console to be extended without
// breaking the constructor function's signature. 
type ConsoleOption func(*consoleOptions)

// LastNameConsoleOption can be passed to the Console's 
// constructor function. It adds the last name to the console 
// messages. 
func LastNameConsoleOption(lastName string) ConsoleOption {
	return func(options *consoleOptions) {
		options.lastNameOption = lastName
	}
}

// ShoutConsoleOption can be passed to the Console's 
// constructor function.  It makes each console 
// message "loud" by making each message upper case. 
func ShoutConsoleOption() ConsoleOption {
	return func(options *consoleOptions) {
		options.shoutOption = true
	}
}

// NewConsole returns a pointer to a Console. Note that we
// can add an arbitrary number of options in the future
// without changing the signature of this constructor
// function. 
func NewConsole(
	firstName string, 
	optionFunctions ...ConsoleOption,
) *Console {

	// load the options passed into the constructor
	options := new(consoleOptions)
	for _, function := range optionFunctions {
		function(options)
	}
	
	// Return a pointer to Console, furnished with all 
	// of the required and optional configuration. 
	return &Console{
		firstName: firstName,
		lastNameOption: options.lastNameOption,
		shoutOption: options.shoutOption,
	}
}


// Console prints messages to STDOUT with some additional information
type Console struct {
	firstName string       // required
	lastNameOption string  // optional
	shoutOption bool       // optional
}

// Print the message to STDOUT
func (c *Console) Print(message string) {
	b := new(strings.Builder)
	b.WriteString(c.firstName)
	b.WriteRune(' ')
	
	// If the lastNameOption is an empty string, then it was not set
	if c.lastNameOption != "" {
		b.WriteString(c.lastNameOption)
		b.WriteRune(' ')
	}
	
	// If shoutOption is set, then upcase the message
	b.WriteString(": ")
	if c.shoutOption {
		b.WriteString(strings.ToUpper(message))
	} else {
		b.WriteString(message)
	}
	fmt.Println(b.String())
}


func main() {
	console := NewConsole(
		"Pragmatic",                        // required argument
		LastNameConsoleOption("Coder"), // optional configuration
		ShoutConsoleOption(),           // optional configuration
	)
	console.Print("This message has been customized.")
}