func main() {
    chA := make(chan int)

    go func() { chA <- 0 }()

    for a := range chA {
        fmt.Println("A:", a)
        time.Sleep(time.Second * 1)
        chA <- a + 1
    }
}

func main() {
    chA := make(chan int)

    go func() { chA <- 0 }()

    for a := range chA { // consume one
        fmt.Println("A:", a)
        time.Sleep(time.Second * 1)
        go func(a int) {
            chA <- a + 1 // produce one
        }(a)
    }
}

func main() {
    chA := make(chan int)

    go func() { ChA <- 0 }()

    for a := range chA {
        fmt.Println("A:", a)
        for i := 0; i < 10; i++ {
            chA <- i
        }
    }
}

func main() {
    chA := make(chan int)

    go func() { ChA <- 0 }()

    for a := range chA { // consume one
        fmt.Println("A:", a)
        for i := 0; i < 10; i++ { // produce many
            go func(i int) {
                chA <- i // no sequence guaranteed
            }(i)
        }
    }
}

func main() {
    chA := make(chan int)
    chB := make(chan int)

    go func() { chB <- 0 }()

    go func() {
        for a := range chA { // consume one
            time.Sleep(time.Second * 1)
            fmt.Println("A:", a)
            chB <- a + 1 // produce one
        }
    }()

    for b := range chB { // consume one
        fmt.Println("B:", b)
        chA <- b + 1 // produce one
    }
}

func main() {
    chA := make(chan int)
    chB := make(chan int)

    go func() { chB <- 0 }()

    go func() {
        for a := range chA {
            time.Sleep(time.Second * 1)
            fmt.Println("A:", a)
            chB <- a
        }
    }()

    for b := range chB {
        fmt.Println("B:", b)
        for i := 0; i < 10; i++ {
            chA <- i
        }
    }
}

func main() {
    chA := make(chan int)
    chB := make(chan int)

    go func() { chB <- 0 }()

    go func() { // worker goroutine
        for a := range chA {
            time.Sleep(time.Second * 1)
            fmt.Println("A:", a)
            go func(a int) { // goroutines accumulated
                chB <- a // no sequence guaranteed
            }(a)
        }
    }()

    for b := range chB {
        fmt.Println("B:", b)
        for i := 0; i < 10; i++ {
            chA <- i // sequence guaranteed
        }
    }
}

func main() {
    chA := make(chan int)
    chB := make(chan int)

    go func() { chB <- 0 }()

    for i := 0; i < 20; i++ {
        go func() {
            for a := range chA {
                time.Sleep(time.Second * 1) // simulate real work
                fmt.Println("A:", a)
                go func(a int) { // goroutine accumulation
                    chB <- a
                }(a)
            }
        }()
    }

    for b := range chB { // consume one
        fmt.Println("B:", b)
        for i := 0; i < 10; i++ { // produce many
            chA <- i
        }
    }
}

func main() {
    worklist := make(chan []string)

    go func() { worklist <- os.Args[1:] }()

    seed := make(map[string]bool)
    for list := range worklist { // consume one
        for _, link := range list {
            if !seen[link] {
                seed[link] = true
                go func(link string) { // goroutine accumulation
                    worklist <- crawl(link) // produce many
                }(link)
            }
        }
    }
}

var tokens = make(chan struct{}, 20)

func crawl(url string) []string {
    tokens <- struct{}{} // acquire a token
    list, err := links.Extract(url)
    <-tokens // release the token
    if err != nil {
        log.Printf(err)
    }
    return list
}