Options pattern

main.go

package main

import (
	"fmt"
)

type engineOption func(e *Engine)

func WithFuel(fuel float64) engineOption {
	return func(e *Engine) {
		e.fuel = fuel
	}
}

func WithAutomatic() engineOption {
	return func(e *Engine) {
		e.automaticTransmission = true
	}
}

type Engine struct {
	fuel                  float64
	automaticTransmission bool
}

func NewEngine(opts ...engineOption) *Engine {
	e := &Engine{fuel: 0, automaticTransmission: false}

	for _, opt := range opts {
		opt(e)
	}

	return e
}

func main() {
	e := NewEngine()
	fmt.Printf("%+v\n", e)

	e = NewEngine(WithFuel(3.20), WithAutomatic())
	fmt.Printf("%+v\n", e)
}