Golang Builder pattern

builder.go

package main

import "strconv"
import "fmt"

type Color string
type Make string
type Model string

const (
	BLUE Color = "blue"
	RED        = "red"
)

type Car interface {
	Drive() string
	Stop() string
}

type CarBuilder interface {
	TopSpeed(int) CarBuilder
	Paint(Color) CarBuilder
	Build() Car
}

type carBuilder struct {
	speedOption int
	color       Color
}

func (cb *carBuilder) TopSpeed(speed int) CarBuilder {
	cb.speedOption = speed
	return cb
}

func (cb *carBuilder) Paint(color Color) CarBuilder {
	cb.color = color
	return cb
}

func (cb *carBuilder) Build() Car {
	return &car{
		topSpeed: cb.speedOption,
		color:    cb.color,
	}
}

func New() CarBuilder {
	return &carBuilder{}
}

type car struct {
	topSpeed int
	color    Color
}

func (c *car) Drive() string {
	return "Driving at speed: " + strconv.Itoa(c.topSpeed)
}

func (c *car) Stop() string {
	return "Stopping a " + string(c.color) + " car"
}

func main() {
	builder := New()
	car := builder.TopSpeed(50).Paint(BLUE).Build()

	fmt.Println(car.Drive())
	fmt.Println(car.Stop())
}

Why do you have two similar carBuilder and car structs? Is this an unesseseary overhead?

The Car struct is in effectively immutable where the builder is not. One of the major reasons of the builder pattern of to simplify construction of complex types while all having those types be immutable.

This is one of the simplest code snippets to understand builder pattern.

how do you format long builders?

gofmt doesn't allow oop-style multiline builder

@gocs since each CarBuilder method except Build returns a CarBuilder type, the most straightforward (yet verbose) approach that comes to mind for multiline builders would be re-assigning to the variable builder at the desired line length.

Example:

builder := New()
builder = builder.TopSpeed(50).Paint(BLUE)
builder = builder.TheOtherThing().AndSoOn()
// ... more builder lines here if necessary
car := builder.Build() // actually build a type Car

Best small example for explain builder design pattern in Go, good job. By the way! what is the purpose of Make in line 7?

Make is extra, probably left over from some previous refactor. Its not needed in the code atm.

@gocs @vaskoz

mycar := New().
    TopSpeed(97).
    Paint(BLUE).
    Build()

Playground

multilines are possible:

	builder := New().TopSpeed(50).
		Paint(BLUE)
	car := builder.Build()