Course of Raku / Functional, concurrent, reactive, and web programming / Concurrent programming / Channels

Receiving past the end

What happens if you call .receive more times than there are values to take? The answer depends on whether the channel has been closed.

Take the producer–consumer example from earlier: a background task sends three values and closes the channel. What if the consumer asks for a fourth?

my $c = Channel.new;
start {
    $c.send($_) for 1..3;
    $c.close;
}

say $c.receive; # 1
say $c.receive; # 2
say $c.receive; # 3
say $c.receive; # one receive too many

On a closed channel, an extra receive throws

Closing does not throw away the values already queued — the three .receive calls above still return 1, 2, and 3. But once the channel is closed and empty, no more values can ever arrive, so rather than let you wait forever, Raku raises an exception:

Cannot receive a message on a closed channel

The exception is X::Channel::ReceiveOnClosed. This is helpful rather than annoying: it tells the consumer, unmistakably, that it has drained the channel. In practice you avoid ever hitting it by consuming with .list (which stops cleanly at the close) or by looping only while values remain — both shown on the previous page.

On an open channel, an extra receive blocks

If the producer never closes the channel, .receive has no way to know whether another value is still coming, so it simply waits:

my $c = Channel.new;
$c.send(1);
say $c.receive; # 1
say $c.receive; # waits here — the channel is open but empty

The program hangs at the second .receive, because a value might still be sent. This is the danger the previous page warned about: forget to .close, and a normal “wait for the next value” becomes a wait that never ends.

So the two outcomes are opposite, in a useful way. A closed channel fails fast when it is empty — you get an exception you can see and catch. An open channel keeps waiting — exactly right while a producer is still at work, but a silent hang if the producer has quietly stopped without closing. Closing the channel is precisely what turns that silent hang into a clear signal that the stream has ended.

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