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Felipe Gasper
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Promise::ES6 - ES6-style promises in Perl


    $Promise::ES6::DETECT_MEMORY_LEAKS = 1;

    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 class. 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.


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

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


Right now this doesn’t try for interoperability with other promise classes. If that’s something you want, make a feature request.


As of version 0.05, unhandled rejections prompt a warning only if one of the following is true:

1) The unhandled rejection happens outside of the constructor.
2) The unhandled rejection happens via an uncaught exception (even within the constructor).


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::AnyEvent, or one of the alternatives that that module’s documentation suggests.


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

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

  • Implement the cancellation on the object that creates your promises. 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 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:

  • As of version 0.07, 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.

  • 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.

  • 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::ES6 serves much the same role as Future but exposes a standard, minimal, cross-language API rather than a proprietary (large) one.

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


Copyright 2019 Gasper Software Consulting.

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