sum_simple.go

package main

import (
	"fmt"
	"sync/atomic"
)

func main() {
	const num = 1000

	var nRoutines int32 = 0
	ch := make(chan int, 1)

	for i := 0; i < num; i++ {
		// sender
		atomic.AddInt32(&nRoutines, 1)
		go func(v int, c chan<- int) {
			c <- v
			atomic.AddInt32(&nRoutines, -1)
		} (i, ch)
	}

	for atomic.LoadInt32(&nRoutines) > 1 {
		augend := <-ch
		addend := <-ch
		// adder
		atomic.AddInt32(&nRoutines, 1)
		go func(augend, addend int, c chan<- int) {
			c <- (augend + addend)
			atomic.AddInt32(&nRoutines, -1)
		} (augend, addend, ch)
	}

	fmt.Println(<-ch)
}