12 - Now you're thinking with channels

Covered in this module:

channels

channels

Channels are mono-directional pipes that goroutines can use to communicate.

To create a channel use make(chan <type>) where type is the type of the values that will be passed in the channel.

To send a value to a channel you use the syntax channel <- value.

To receive a value from a channel, use <-channel and capture the retrieved value into a variable like variable := <-channel:

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

    go roll1d6(randInts)

    roll := <-randInts // this pulls new rolls from the channel
    fmt.Println("You rolled a", roll)
}

func roll1d6(c chan int) {
    c <- rand.Intn(6) + 1 // this passes new rolls into the channel
}

If you are just using the channel as a ping mechanism (i.e. you don't care about the value in the channel), you can receive with simply <-channel without capturing the retrieved value.

func main() {
    ping := make(chan bool)
    startTime := time.Now()

    go waitForPing(ping)

    <-ping
    fmt.Println("Program took", time.Since(startTime))
}

func waitForPing(c chan bool) {
    time.Sleep(time.Second * 1)
    c <- true
}

prints

Program took 1.00006811s

Send and receive on channels are both blocking. Go will block the enclosing goroutine until both sides of the channel are ready.

You can specify whether a channel is to be used for sending or receiving as part of the function parameter list:

func main() {
    randInts := make(chan int)
    done := make(chan bool)

    go roll1d6(randInts)

    go printRoll(randInts, done)
    <-done
}

func roll1d6(c chan<- int) {
    c <- rand.Intn(6) + 1
}

func printRoll(c <-chan int, done chan<- bool) {
    fmt.Println("You rolled a", <-c)
    done <- true
}

A channel can have any number of senders and receivers. A value going into a channel is guaranteed to be received by only one receiver.

Hands on!

In the repo, open the file ./intermediate/12channels.go
Complete the TODOs
Run make 12 from project root (alternatively, type go run ./12channels.go)
Example implementation available on solutions branch