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

Making your own promise 🆕

Every promise so far came from start. But you can also create one directly with Promise.new — a promise that has not finished yet — and settle it yourself when the result (or the news) is ready. .keep fulfils it with a value:

my $p = Promise.new;   # a fresh, unsettled promise

$p.keep(42);           # fulfil it with a value
say await $p;          # 42

.break is the failure case. Awaiting a broken promise re-throws the error, just as it does for a start block that died — and if nothing catches it, that exception stops the program:

my $p = Promise.new;
$p.break('disk full');

say await $p;             # dies here with 'disk full'
say 'never reached';      # this line does not run

To keep going, catch the failure with try, exactly as you would around any code that might die:

my $p = Promise.new;
$p.break('disk full');

my $result = try await $p;
say $result // "the job failed: $!"; # the job failed: disk full

If the outcome is already known, two shortcuts make a promise that starts out settled: Promise.kept($value) and Promise.broken($error).

say await Promise.kept(7); # 7

Why make a promise by hand? Because not every result comes from a start block. An event that fires later, a message that arrives, a callback from another library — Promise.new lets you turn any of these into a promise that the rest of the program can await and combine like any other.

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