Go Tip: Embedding Example with interfaces

embedding.go

package main

import (
	"fmt"
)

type GreetingsService interface {
	SayHello() string
	SayGoodMorning() string
}

type EnglishService struct {
	Country string
}

func (EnglishService) SayHello() string      { return "hello!" }
func (EnglishService) SayGoodMorning() string { return "good morning!" }

type SpanishService struct{}

func (SpanishService) SayHello() string      { return "hola!" }
func (SpanishService) SayGoodMorning() string { return "buenos dias!" }

type GiveMeYourMoneyService struct {
	GreetingsService
	Country string
}

func (GiveMeYourMoneyService) SayGoodMorning() string { return "$$$" }

func main() {
	e := EnglishService{Country: "Neverland"}
	fmt.Printf("EnglishService: %+v\n", e)
	fmt.Printf("EnglishService.SayHello %v\n", e.SayHello())
	fmt.Printf("EnglishService.GoodMorning %v\n", e.SayGoodMorning())

	o := GiveMeYourMoneyService{GreetingsService: &e, Country: "Sokovia"}
	fmt.Printf("FancierService: %+v\n", o)
	fmt.Printf("FancierService.SayHello: %s\n", o.SayHello())
	fmt.Printf("FancierService.GoodMorning: %s\n", o.SayGoodMorning())

	// This panics because you already assigned it to the specific type,
	// you will need to re-instantiate "o" and specify the value again
	// o.GreetingsService = &SpanishService{}

	var e1 *EnglishService = o.GreetingsService.(*EnglishService)
	fmt.Printf("original.Country: %s\n", e1.Country)

	//-
	o = GiveMeYourMoneyService{GreetingsService: &SpanishService{}, Country: "Pueblo Viejo"}
	fmt.Printf("FancierService: %+v\n", o)
	fmt.Printf("FancierService.SayHello: %s\n", o.SayHello())
	fmt.Printf("FancierService.GoodMorning: %s\n", o.SayGoodMorning())
}