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Promise::ES6 - ES6-style promises in Perl


    my $promise = Promise::ES6->new( sub {
        my ($resolve_cr, $reject_cr) = @_;

        # ..
    } );

    my $promise2 = $promise->then( sub { .. }, sub { .. } );

    my $promise3 = $promise->catch( sub { .. } );

    my $promise4 = $promise->finally( sub { .. } );

    my $resolved = Promise::ES6->resolve(5);
    my $rejected = Promise::ES6->reject('nono');

    my $all_promise = Promise::ES6->all( \@promises );

    my $race_promise = Promise::ES6->race( \@promises );


This module provides a Perl implementation of promises, a useful pattern for coordinating asynchronous tasks.

Unlike most other promise implementations on CPAN, this module mimics ECMAScript 6’s Promise interface. As the SYNOPSIS above shows, you can thus use patterns from JavaScript in Perl with only minimal changes needed to accommodate language syntax.

This is a rewrite of an earlier module, Promise::Tiny. It fixes several bugs and superfluous dependencies in the original.


This module is in use in production and, backed by a pretty extensive set of regression tests, may be considered stable.


  • Promise resolutions and rejections accept exactly one argument, not a list.

  • Unhandled rejections are reported via warn(). (See below for details.)

  • The Promises/A+ test suite avoids testing the case where an “executor” function’s resolve callback itself receives another promise, e.g.:

        my $p = Promise::ES6->new( sub ($res) {
            $res->( Promise::ES6->resolve(123) );
        } );

    What will $p’s resolution value be? 123, or the promise that wraps it?

    This module favors conformity with the ES6 standard, which indicates intent that $p’s resolution value be 123.


This module considers any object that has a then() method to be a promise. Note that, in the case of Future, this will yield a “false-positive”, as Future is not compatible with promises.

(See Promise::ES6::Future for more tools to interact with Future.)


This module’s handling of unhandled rejections has changed over time. The current behavior is: if any rejected promise is DESTROYed without first having received a catch callback, a warning is thrown.


In JavaScript, the following …

    Promise.resolve().then( () => console.log(1) );

… will log 2 then 1 because JavaScript’s then() defers execution of its callbacks until the end of the current iteration through JavaScript’s event loop.

Perl, of course, has no built-in event loop. This module’s then() method, thus, when called on a promise that is already “settled” (i.e., not pending), will run the appropriate callback immediately. That means that this:

    Promise::ES6->resolve(0)->then( sub { print 1 } );
    print 2;

… will print 12 instead of 21.

This is an intentional divergence from the Promises/A+ specification. A key advantage of this design is that Promise::ES6 instances can abstract over whether a given function works synchronously or asynchronously.

If you want a Promises/A+-compliant implementation, look at Promise::ES6::IOAsync, Promise::ES6::Mojo, or Promise::ES6::AnyEvent in this distribution. CPAN provides other alternatives.


Promises have never provided a standardized solution for cancellation—i.e., aborting an in-process operation. If you need this functionality, then, you’ll have to implement it yourself. Two ways of doing this are:

  • Subclass Promise::ES6 and provide cancellation logic in that subclass. See DNS::Unbound::AsyncQuery’s implementation for an example of this.

  • Implement the cancellation on a request object that your “promise-creator” also consumes. This is probably the more straightforward approach but requires that there be some object or ID besides the promise that uniquely identifies the action to be canceled. See Net::Curl::Promiser for an example of this approach.

You’ll need to decide if it makes more sense for your application to leave a canceled query in the “pending” state or to “settle” (i.e., resolve or reject) it. All things being equal, I feel the first approach is the most intuitive.


It’s easy to create inadvertent memory leaks using promises in Perl. Here are a few “pointers” (heh) to bear in mind:

  • Any Promise::ES6 instances that are created while $Promise::ES6::DETECT_MEMORY_LEAKS is set to a truthy value are “leak-detect-enabled”, which means that if they survive until their original process’s global destruction, a warning is triggered. You should normally enable this flag in a development environment.

  • If your application needs recursive promises (e.g., to poll iteratively for completion of a task), the current_sub feature (i.e., __SUB__) may help you avoid memory leaks. In Perl versions that don’t support this feature (i.e., anything pre-5.16) you can imitate it thus:

        use constant _has_current_sub => eval "use feature 'current_sub'";
        use if _has_current_sub(), feature => 'current_sub';
        my $cb;
        $cb = sub {
            my $current_sub = do {
                no strict 'subs';
                _has_current_sub() ? __SUB__ : eval '$cb';

    Of course, it’s better if you can avoid doing that. :)

  • Garbage collection before Perl 5.18 seems to have been buggy. If you work with such versions and end up chasing leaks, try manually deleting as many references/closures as possible. See t/race_success.t for a notated example.

    You may also (counterintuitively, IMO) find that this:

        my ($resolve, $reject);
        my $promise = Promise::ES6->new( sub { ($resolve, $reject) = @_ } );
        # … etc.

    … works better than:

        my $promise = Promise::ES6->new( sub {
            my ($resolve, $reject) = @_;
            # … etc.
        } );


If you’re not sure of what promises are, there are several good introductions to the topic. You might start with this one.

Promise::XS is a lot like this library but implemented mostly in XS for speed. It derives from AnyEvent::XSPromises.

Promises is another pure-Perl Promise implementation.

Future fills a role similar to that of promises.

CPAN contains a number of other modules that implement promises. I think mine are the nicest :), but YMMV. Enjoy!


Copyright 2019-2020 Gasper Software Consulting.

This library is licensed under the same terms as Perl itself.