Channels

Channels are a typed, thread-safe queue. Channels allow different goroutines to communicate with each other.

Create a channel

Like maps and slices, channels must be created before use. They also use the same make keyword:

ch := make(chan int)

Send data to a channel

ch <- 69

The <- operator is called the channel operator. Data flows in the direction of the arrow. This operation will block until another goroutine is ready to receive the value.

Receive data from a channel

v := <-ch

This reads and removes a value from the channel and saves it into the variable v. This operation will block until there is a value in the channel to be read.

Blocking and deadlocks

A deadlock is when a group of goroutines are all blocking so none of them can continue. This is a common bug that you need to watch out for in concurrent programming.

Assignment

Run the program. You'll see that it deadlocks and never exits. The filterOldEmails function is trying to send on a channel and there is no other goroutine running that can accept the value from the channel.

Fix the deadlock by spawning an anonymous goroutine to write to the isOldChan channel instead of using the same goroutine that's reading from it.