Simple set for Go, could be any type

set.go

var exists = struct{}{}

type int64Set struct {
	m map[int64]struct{}
}

type stringSet struct {
	m map[string]struct{}
}

func newInt64Set(entries ...int64) *int64Set {
	s := &int64Set{}
	s.m = make(map[int64]struct{})
	for _, v := range entries {
		s.m[v] = exists
	}
	return s
}

func (s *int64Set) Len(v int64) int {
	return len(s.m)
}

func (s *int64Set) Add(v ...int64) {
	for _, k := range v {
		s.m[k] = exists
	}
}

func (s *int64Set) Remove(v int64) {
	delete(s.m, v)
}

func (s *int64Set) Contains(v int64) bool {
	_, c := s.m[v]
	return c
}

func (s *int64Set) Union(o *int64Set) *int64Set {
	a := append(s.ToSlice(), o.ToSlice()...)
	us := newInt64Set(a...)
	return us
}

func (s *int64Set) Intersect(o *int64Set) (*int64Set, bool) {
	si := newInt64Set()
	for k := range s.m {
		if _, ok := o.m[k]; ok {
			si.m[k] = exists
		}
	}
	return si, len(si.m) > 0
}

func (s *int64Set) ToSlice() []int64 {
	l := make([]int64, len(s.m))
	i := 0
	for k := range s.m {
		l[i] = k
		i++
	}
	return l
}