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Promise-XS-0.08_06
River stage one • 5 direct dependents • 6 total dependents
/ Promise::XS

NAME

Promise::XS - Fast promises in Perl

SYNOPSIS

use Promise::XS ();

my $deferred = Promise::XS::deferred();

# Do one of these once you have the result of your operation:
$deferred->resolve( 'foo', 'bar' );
$deferred->reject( 'oh', 'no!' );

# Give this to your caller:
my $promise = $deferred->promise();

The following aggregator functions are exposed:

# Resolves with a list of arrayrefs, one per promise.
# Rejects with the results from the first rejected promise.
my $all_p = Promise::XS::all( $promise1, $promise2, .. );

# Resolves/rejects with the results from the first
# resolved or rejected promise.
my $race_p = Promise::XS::race( $promise3, $promise4, .. );

For compatibility with preexisting libraries, all() may also be called as collect().

The following also exist:

my $pre_resolved_promise = Promise::XS::resolved('already', 'done');

my $pre_rejected_promise = Promise::XS::rejected('it’s', 'bad');

All of Promise::XS’s static functions may be exported at load time, e.g., use Promise::XS qw(deferred).

DESCRIPTION

This module exposes a Promise interface with its major parts implemented in XS for speed. It is a fork and refactor of AnyEvent::XSPromises. That module’s interface, a “bare-bones” subset of that from Promises, is retained.

STATUS

Breaking changes in this interface are unlikely; however, the implementation is relatively untested since the fork. Your mileage may vary.

DIFFERENCES FROM ECMASCRIPT PROMISES

This library is built for compatibility with pre-existing Perl promise libraries. It thus exhibits some salient differences from how ECMAScript promises work:

  • Neither the resolve() method of deferred objects nor the resolved() convenience function define behavior when given a promise object.

  • The all() and race() functions accept a list of promises, not a “scalar-array-thing” (ECMAScript “arrays” being what in Perl we call “array references”). So whereas in ECMAScript you do:

    Promise.all( [ promise1, promise2 ] );

    … in this library it’s:

    Promise::XS::all( $promise1, $promise2 );

  • Promise resolutions and rejections may contain multiple values. (But see "AVOID MULTIPLES" below.)

See Promise::ES6 for an interface that imitates ECMAScript promises more closely.

AVOID MULTIPLES

For compatibility with preexisting Perl promise libraries, Promise::XS allows a promise to resolve or reject with multiple values. This behavior, while eminently “perlish”, allows for some weird cases where the relevant standards don’t apply: for example, what happens if multiple promises are returned from a promise callback? Or even just a single promise plus extra returns?

Promise::XS tries to help you catch such cases by throwing a warning if multiple return values from a callback contain a promise as the first member. For best results, though—and consistency with promise implementations outside Perl—resolve/reject all promises with single values.

DIFFERENCES FROM Promises ET AL.

This module implements ECMAScript’s finally() interface, which differs from that in some other Perl promise implementations.

Given the following …

my $new = $p->finally( $callback );

  • $callback is given no arguments.

  • If $callback returns anything but a single, rejected promise, $new has the same status as $p.

  • If $callback throws or returns a single, rejected promise, $new is rejected with $callback’s exception.

EVENT LOOPS

By default this library uses no event loop. This is a perfectly usable configuration; however, it’ll be a bit different from how promises usually work in evented contexts (e.g., JavaScript) because callbacks will execute immediately rather than at the end of the event loop as the Promises/A+ specification requires.

To achieve full Promises/A+ compliance it’s necessary to integrate with an event loop interface. This library supports three such interfaces:

  • AnyEvent:

    Promise::XS::use_event('AnyEvent');

  • IO::Async - note the need for an IO::Async::Loop instance as argument:

    Promise::XS::use_event('IO::Async', $loop_object);

  • Mojo::IOLoop:

    Promise::XS::use_event('Mojo::IOLoop');

Note that all three of the above are event loop interfaces. They aren’t event loops themselves, but abstractions over various event loops. See each one’s documentation for details about supported event loops.

REMINDER: There’s no reason why promises need an event loop; it just satisfies the Promises/A+ convention.

MEMORY LEAK DETECTION

Any promise created while $Promise::XS::DETECT_MEMORY_LEAKS is truthy will throw a warning if it survives until global destruction.

SUBCLASSING

You can re-bless a Promise::XS::Promise instance into a different class, and then(), catch(), and finally() will assign their newly-created promise into that other class. (It follows that the other class must subclass Promise::XS::Promise.) This can be useful, e.g., for implementing mid-flight controls like cancellation.

TODO

  • all() and race() should be implemented in XS, as should resolved() and rejected().

KNOWN ISSUES

  • Interpreter-based threads may or may not work.

  • This module interacts badly with Perl’s fork() implementation on Windows. There may be a workaround possible, but none is implemented for now.

SEE ALSO

Besides AnyEvent::XSPromises and Promises, you may like Promise::ES6, which mimics ECMAScript’s “Promise” class as much as possible. It can even (experimentally) use this module as a backend, which helps but is still significantly slower than using this module directly.