What is the role of the flag when attempting to read from a closed channel in programming?

In Go, when reading from a channel, the operation returns two values: the value itself and a boolean flag indicating whether the channel is open.

If the channel is closed and all values have been read, the read returns the zero value of the type and false.

Example:

value, ok := <-ch
if !ok {
    // The channel is closed, further reading is impossible
}

Thus, the ok flag helps determine whether the data transfer through the channel has finished and whether to stop reading.