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

Joining threads

To wait for a thread to complete, call .finish on it (also spelled .join). The main program pauses there until the thread is done:

my $t = Thread.start({ say 'in the thread' });
$t.finish;
say 'done';

This prints:

in the thread
done

Because .finish waits, the thread’s message is guaranteed to appear before done. Without the .finish, the order would be unpredictable, and the program might even end before the thread had a chance to print.

When you start several threads, you join each one to be sure they have all completed:

my $a = Thread.start({ 1 + 1 });
my $b = Thread.start({ 2 + 2 });
$a.finish;
$b.finish;
say 'both finished';

The two threads run concurrently, and joining both before the final say guarantees that both finished is printed only once they are truly done. Joining is how you bring concurrent work back together into a predictable point in your program.

Since a method call on a junction spreads to every value it holds, you can join several threads in a single expression. Combine them with the all operator & and call .finish on the junction:

my $a = Thread.start({ 1 + 1 });
my $b = Thread.start({ 2 + 2 });

($a & $b).finish;
say 'both finished';

Calling .finish on $a & $b runs it on both threads, so the program waits until neither is still running — the same result as joining them one by one, but it reads as “finish both of these” and works for any number of threads joined with &.

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Joining threads

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