Builder pattern in Golang

builder.go

package car

import "fmt"

type Speed float64
const (
  MPH Speed = 1
  KPH Speed = 1.60934
)

type Color string
const (
  BlueColor Color = "blue"
  GreenColor = "green"
  RedColor = "red"
)

type Wheels string
const (
  SportsWheels Wheels = "sports"
  SteelWheels = "steel"
)

type VehicleBuilder interface {
  Paint(Color) VehicleBuilder
  Wheels(Wheels) VehicleBuilder
  TopSpeed(Speed) VehicleBuilder
  Build() Car
}

type carBuilder struct {
  wheels Wheels
  color Color
  speed Speed
}

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

func (cb *carBuilder) TopSpeed(s Speed) VehicleBuilder {
  cb.speed = s
  return cb
}

func (cb *carBuilder) Wheels(w Wheels) VehicleBuilder {
  cb.wheels = w
  return cb
}

func (cb carBuilder) Build() Car {
  return Car{
    speed: cb.speed,
    color: cb.color,
    wheels: cb.wheels,
  }
}

func NewBuilder() VehicleBuilder {
  return &carBuilder{}
}

type Car struct {
  wheels Wheels
  color Color
  speed Speed
}

func (c Car) Describe() {
  fmt.Println("WHEELS! ", c.wheels)
  fmt.Println("COLOR! ", c.color)
  fmt.Println("SPEED! ", c.speed)
}

main.go

// directory/
// L main.go
// L package/
//   L builder.go

package main

import (
  "./package"
)

func main() {
  assembly := car.NewBuilder().Paint(car.RedColor)

  familyCar := assembly.Wheels(car.SportsWheels).TopSpeed(50 * car.MPH).Build()
  familyCar.Describe()

  sportsCar := assembly.Wheels(car.SteelWheels).TopSpeed(150 * car.MPH).Build()
  sportsCar.Describe()
}