Set in Go

set.go

package main

import "fmt"

type SetNone struct{}
type Set map[interface{}]SetNone

func (s Set) Add(el interface{}) {
	s[el] = SetNone{}
}

func (s Set) Contains(el interface{}) (ok bool) {
	_, ok = s[el]
	return
}

func (s Set) Merge(s2 Set) {
	for element, _ := range s2 {
		s.Add(element)
 	}
}

func main() {
	s := make(Set)
	s2 := make(Set)
	s.Add("dog")
	s2.Add("cat")
	fmt.Printf("Dog: %+v\n", s.Contains("dog"))
	fmt.Printf("Cat: %+v\n", s.Contains("cat"))
	s.Merge(s2)
	fmt.Printf("Cat: %+v\n", s.Contains("cat"))
}