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

Sending and receiving

Create a channel with Channel.new. Put values in with .send, and take them out with .receive:

my $c = Channel.new;
$c.send(1);
$c.send(2);
say $c.receive; # 1
say $c.receive; # 2

A channel is a queue: values come out in the same order they went in — first in, first out. So the first .receive returns 1, the value sent first.

The point of a channel is that sending and receiving are safe to do from different threads at the same time. A producer can keep sending while a consumer keeps receiving, and Raku makes sure no value is lost or duplicated:

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

Here a background task sends 1, 2, 3; the main program receives. .receive waits if the channel is momentarily empty, so the consumer never races ahead of the producer.

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