zygomys-update/slides/snoopy2

// Go:

type Flyer interface {
	Fly(w *Weather) (s string, err error)
}

func (p *Snoopy) Fly(w *Weather) (s string, err error) {
	w.Type = "VERY " + w.Type // side-effect, for demo purposes
	s = fmt.Sprintf("Snoopy sees weather '%s', cries '%s'", w.Type, p.Cry)
	fmt.Println(s)
	for _, flyer := range p.Friends {
		flyer.Fly(w)
	}
	return s, err
}