What does this print? (receiving from a closed buffered channel)

package main

import "fmt"

func main() {
	ch := make(chan int, 3)
	ch <- 1
	ch <- 2
	close(ch)

	for i := range 4 {
		v, ok := <-ch
		fmt.Println(i, v, ok)
	}
}

Output:

0 1 true
1 2 true
2 0 false
3 0 false

Closing a channel does not discard buffered values. Receivers drain the buffer with ok == true. Once it is empty, every receive returns immediately with the zero value and ok == false. This is why the comma-ok idiom (or for range) is the only reliable way to detect closure. Checking v == 0 is ambiguous because 0 might be a real value.