Maps / Use map as a set / Essential Go

package main

import "fmt"

// StringSet is a set of unique strings
type StringSet struct {
	m map[string]struct{}
}

func NewStringSet(strings ...string) *StringSet {
	res := &StringSet{
		m: map[string]struct{}{},
	}
	for _, s := range strings {
		res.Add(s)
	}
	return res
}

// Add adds a string to the set. If string is already in the set, it has no effect
func (s *StringSet) Add(str string) {
	s.m[str] = struct{}{}
}

// Exists checks if string exists in the set
func (s *StringSet) Exists(str string) bool {
	_, exists := s.m[str]
	return exists
}

// Delete removes a string from the set
func (s *StringSet) Delete(str string) {
	delete(s.m, str)
}

// Strings returns strings in the set
func (s *StringSet) Strings() []string {
	n := len(s.m)
	if n == 0 {
		return nil
	}
	// for efficiency, pre-allocate the array with known, final capacity
	// this avoids re-allocating underlying array in append
	res := make([]string, 0, n)
	for str := range s.m {
		res = append(res, str)
	}
	return res
}

func printExists(set *StringSet, str string) {
	if set.Exists(str) {
		fmt.Printf("'%s' exists in set\n", str)
	} else {
		fmt.Printf("'%s' doesn't exist in set\n", str)
	}
}

func main() {
	set := NewStringSet("hi", "hello")
	fmt.Printf("Original set: %#v\n", set.Strings())

	set.Delete("hi")
	fmt.Printf("After delete: %#v\n", set.Strings())

	set.Add("hey")
	fmt.Printf("After add   : %#v\n", set.Strings())

	printExists(set, "hello")
	printExists(set, "ola")
}