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

Vows 🆕

When you call $p.keep(...) yourself, anyone who holds $p could settle it. Sometimes you want the opposite: the code that produces the result should be the only thing allowed to settle the promise, while everyone else may only wait on it. That separation is a vow.

Promise.new gives you a promise; calling .vow on it hands back a Vow — the sole right to keep or break that promise. From then on the promise is meant to be read-only: it is settled through the vow. The usual shape is a producer that keeps the vow private and returns just the promise:

sub after($seconds) {
    my $p = Promise.new;
    my $v = $p.vow;                       # take the settle-right
    start {
        sleep $seconds;
        $v.keep("done after $seconds s"); # settle it through the vow
    }
    return $p;                            # callers get only the read side
}

my $job = after(0.2);
say $job.status;   # Planned
say await $job;    # done after 0.2 s

Taking the vow makes the promise genuinely read-only: from that moment $p.keep and $p.break are refused, because the vow now owns the sole right to settle it.

my $p = Promise.new;
my $v = $p.vow;

$p.keep(1); # dies: Access denied to keep/break this Promise; already vowed

So the caller can await the promise, chain .then onto it, or check its .status, but it cannot settle it — only the holder of the vow can. This is how libraries turn callback- and event-based APIs into ordinary promises: create a promise, hold its vow, and fulfil it from inside the callback when the event finally arrives.

This is also exactly what start does for you behind the scenes — it makes a promise, keeps the vow to itself, and settles the promise with the block’s value (or breaks it if the block dies). Making the promise and its vow by hand simply lets you do the same thing when the result does not come from a single block of code.

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Vows 🆕

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