Course of Raku / Functional, concurrent, reactive, and web programming / Reactive programming / await

Awaiting a promise

await takes a promise, waits for it to complete, and returns its result:

my $p = start { 7 * 6 };
say await $p; # 42

The program pauses at await only as long as needed, then continues with the value the promise produced.

Given several promises, await waits for all of them and returns their results in order:

my @jobs = (start { 5 }), (start { 10 }), (start { 15 });
say await @jobs;     # (5 10 15)
say [+] await @jobs; # 30

The jobs run concurrently, but await @jobs does not return until every one is done, so the results are complete and in order.

If an awaited promise was broken — its block threw an exception — await rethrows that exception at the point of the await. This means errors in background work surface where you wait for it, so you can handle them with the usual try / CATCH from the part on exceptions. Awaiting is therefore not just about getting a value; it is the moment where concurrent work rejoins the main flow, results and errors alike.

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