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

Closing a channel

When a producer has no more values to send, it calls .close on the channel. Closing signals to the consumer that the stream has ended.

The most convenient way to consume a closed channel is .list, which gives you every remaining value and finishes once the channel is closed:

my $c = Channel.new;
$c.send(1);
$c.send(2);
$c.send(3);
$c.close;

say $c.list; # (1 2 3)

Because .list waits for the channel to be closed, it is perfect for “collect everything the producer sends”. You can treat the result like any other list — sum it, count it, and so on:

my $c = Channel.new;
$c.send($_) for 1..5;
$c.close;

say [+] $c.list; # 15

Closing is how a consumer knows when to stop. Without it, .list (or a loop of .receive) would wait forever for a value that never comes. Always close a channel when the sending is done.

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