main.go

package main

import (
	"fmt"
	"sync"
)

type Callback func() int

func doSomething() {}

func callbackFactory(i int) Callback {
	return func() int {
		fmt.Println("callback", i)
		return i
	}
}

var callbacks []Callback = []Callback{
	callbackFactory(1),
	callbackFactory(2),
	callbackFactory(3),
	callbackFactory(4)}

func combiner(result int) {
	sum += result
}

var sum int

func main() {
	sum = 0
	ch := make(chan int, len(callbacks))
	var wg sync.WaitGroup

	for _, callback := range callbacks {
		wg.Add(1)
		doSomething()
		go func(callback Callback) {
			ch <- callback()
		}(callback)
	}

	go func() {
		for c := range ch {
			combiner(c)
			wg.Done()
		}
	}()
	wg.Wait()

	fmt.Println("sum", sum)
}