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

Chaining and combining

Promises become 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.

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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