go_concurrency.say_hello_4.main.go

package main

import (
	"fmt"
	"sync"
)

var wg = sync.WaitGroup{} // 1. Create a WaitGroup

func main() {
	wg.Add(1) // 2. Add 1 to WaitGroup counter

	go sayHello()

	wg.Wait() // 3. Tell main() to wait until all goroutines finished
}

func sayHello() {
	fmt.Println("Hello world!")
  
	wg.Done() // 4. Inform 1 goroutine finished
}