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

Chaining and combining

Promises become even more powerful when you combine them. The most common need is to wait for several at once. await accepts a list of promises and returns their results in order:

my @jobs = (start { 1 }), (start { 2 }), (start { 3 });
say await @jobs;     # (1 2 3)
say [+] await @jobs; # 6

The three jobs run concurrently; await @jobs waits for all of them and hands back (1, 2, 3). Because the results come back in the same order as the promises, you can reduce them as usual.

You can also chain work onto a promise with .then, which runs a follow-up block once the promise is done. Inside it, .result is the value of the original promise:

my $p = start { 10 };
my $q = $p.then({ .result * 2 });
say await $q; # 20

$q is a new promise that doubles the result of $p. Awaiting it gives 20.

The block has no explicit parameter, so it receives the finished promise as the topic variable $_ — which is why a bare .result works: it is really $_.result. If you would rather give that promise a name, use a pointy block:

my $q = $p.then(-> $prev { $prev.result * 2 });

This behaves exactly the same way; $prev is the original promise $p, and $prev.result is its value.

With start, await, and .then, you can describe whole pipelines of concurrent work: launch many tasks, combine their results, and build new tasks from old ones — all without touching a thread directly.

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