From type/Promise

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my class Vow {
    has Promise $.promise;
    method keep() { ... }
    method break() { ... }
}
method vow(Promise:D: --> Vow:D)

Returns an object that holds the sole authority over keeping or breaking a promise. Calling keep or break on a promise that has vow taken throws an exception of type X::Promise::Vowed.

my $p   = Promise.new;
my $vow = $p.vow;
$vow.keep($p);
say $p.status;          # OUTPUT: «Kept␤»