Go Standard Library Cookbook

How to do it...

1. Open the console and create the folder chapter10/recipe03.
2. Navigate to the directory.
3. Create the file once.go with the following content:

        package main        import (          "fmt"          "sync"        )        var names = []interface{}{"Alan", "Joe", "Jack", "Ben",                                  "Ellen", "Lisa", "Carl", "Steve",                                  "Anton", "Yo"}        type Source struct {          m *sync.Mutex          o *sync.Once          data []interface{}        }        func (s *Source) Pop() (interface{}, error) {          s.m.Lock()          defer s.m.Unlock()          s.o.Do(func() {            s.data = names            fmt.Println("Data has been loaded.")          })          if len(s.data) > 0 {            res := s.data[0]            s.data = s.data[1:]            return res, nil          }          return nil, fmt.Errorf("No data available")        }        func main() {          s := &Source{&sync.Mutex{}, &sync.Once{}, nil}          wg := &sync.WaitGroup{}          wg.Add(10) for i := 0; i < ...