Tour of Go 2.0

Range and Close

When a sender is done, it can close the channel to signal “no more values coming.” Receivers can detect this:

v, ok := <-ch

ok is false when the channel is closed and drained. You can also use range to pull values until the channel closes — cleaner and idiomatic:

for i := range c { ... }

A few rules worth knowing:

  • Only the sender should close a channel. Closing from the receiver side is a bug.
  • Sending to a closed channel causes a panic.
  • You don’t always need to close. Only close when the receiver needs to know the stream is finished — like when you’re using range.

Try it: remove the close(c) call and watch the range loop hang forever waiting for values that never come.

main.go ▶ Run in Playground 
package main

import (
	"fmt"
)

func fibonacci(n int, c chan int) {
	x, y := 0, 1
	for i := 0; i < n; i++ {
		c <- x
		x, y = y, x+y
	}
	close(c)
}

func main() {
	c := make(chan int, 10)
	go fibonacci(cap(c), c)
	for i := range c {
		fmt.Println(i)
	}
}