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91. Setting timeouts

Do not wait for a slow code block if it takes too long.

Promises are the best way to create timeouts. In the following example, two code blocks are created; they are executed in parallel.

my $timeout = Promise.in(2).then({
    say 'Timeout after 2 seconds';
});

my $code = start {
    sleep 5;

    say 'Done after 5 seconds';
}

🧵 This program uses concurrency (promises, threads). The in-browser engine (Raku.js) is single-threaded and can’t run it yet — try it on your own computer.

Both $timeout and $code are the promises, i. e. objects of the Promise type. Actually, the $timeout variable is a promise that is executed as a result of keeping the anonymous promise created by the Promise.in(2) call. The in method of the Promise class creates a promise that becomes kept after the given number of seconds.

The second promise, stored in the $code variable, is created by the start function. This is a long-running code block, which does not return within five seconds. The $code promise can be kept only after that time.

The $timeout promise is kept earlier than the $code one. To let the program continue earlier, create another promise with the help of the anyof method:

await Promise.anyof($timeout, $code);

say 'All done';

The flow of the whole program is the following: first, the $timeout code block is created and starts running in a separate thread. Then, without waiting, the $code block has been created and launched. Finally, the next line of the main thread is executed; it creates an anonymous thread and waits until it is kept. The await routine blocks the execution of the main program until at least one of its arguments is kept. This program prints ‘All done’ after two seconds, and exits:

$ raku timeout.rk
Timeout after 2 seconds
All done

If the $code thread is completed first (say, if we changed the timeout value to 10 seconds), then the output is different, and the timeout is not triggered:

$ raku timeout.rk
Done after 5 seconds
All done

Keep in mind that in our examples, the program finishes after printing ‘All done’. In case the program continues after that, the longest promise will still be running.

For example, add a simple delay like the one shown below:

await Promise.anyof($timeout, $code);
say 'All done';
sleep 20;

In this case, the program prints all three messages:

$ raku timeout.rk
Timeout after 2 seconds
All done
Done after 5 seconds

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