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Promises - An implementation of Promises in Perl


version 1.04


  use AnyEvent::HTTP;
  use JSON::XS qw[ decode_json ];
  use Promises qw[ collect deferred ];

  sub fetch_it {
      my ($uri) = @_;
      my $d = deferred;
      http_get $uri => sub {
          my ($body, $headers) = @_;
          $headers->{Status} == 200
              ? $d->resolve( decode_json( $body ) )
              : $d->reject( $body )

  my $cv = AnyEvent->condvar;

      sub {
          my ($product, $suggestions, $reviews) = @_;
              product     => $product,
              suggestions => $suggestions,
              reviews     => $reviews,
      sub { $cv->croak( 'ERROR' ) }

  my $all_product_info = $cv->recv;


This module is an implementation of the "Promise/A+" pattern for asynchronous programming. Promises are meant to be a way to better deal with the resulting callback spaghetti that can often result in asynchronous programs.


The version of this module is being bumped up to 0.90 as the first step towards 1.0 in which the goal is to have full Promises/A+ spec compatibility. This is a departure to the previous goal of being compatible with the Promises/A spec, this means that behavior may change in subtle ways (we will attempt to document this completely and clearly whenever possible).

It is HIGHLY recommended that you test things very thoroughly before upgrading to this version.


In version up to and including 0.08 there was a bug in how rejected promises were handled. According to the spec, a rejected callback can:

  • Rethrow the exception, in which case the next rejected handler in the chain would be called, or

  • Handle the exception (by not dieing), in which case the next resolved handler in the chain would be called.

In previous versions of Promises, this last step was handled incorrectly: a rejected handler had no way of handling the exception. Once a promise was rejected, only rejected handlers in the chain would be called.

Relation to the various Perl event loops

This module is actually Event Loop agnostic, the SYNOPSIS above uses AnyEvent::HTTP, but that is just an example, it can work with any of the existing event loops out on CPAN. Over the next few releases I will try to add in documentation illustrating each of the different event loops and how best to use Promises with them.

Relation to the Promise/A spec

We are, with some differences, following the API spec called "Promise/A" (and the clarification that is called "Promise/A+") which was created by the Node.JS community. This is, for the most part, the same API that is implemented in the latest jQuery and in the YUI Deferred plug-in (though some purists argue that they both go it wrong, google it if you care). We differ in some respects to this spec, mostly because Perl idioms and best practices are not the same as Javascript idioms and best practices. However, the one important difference that should be noted is that "Promise/A+" strongly suggests that the callbacks given to then should be run asynchronously (meaning in the next turn of the event loop). We do not do this by default, because doing so would bind us to a given event loop implementation, which we very much want to avoid. However we now allow you to specify an event loop "backend" when using Promises, and assuming a Deferred backend has been written it will provide this feature accordingly.

Using a Deferred backend

As mentioned above, the default Promises::Deferred class calls the success or error then() callback synchronously, because it isn't tied to a particular event loop. However, it is recommended that you use the appropriate Deferred backend for whichever event loop you are running.

Typically an application uses a single event loop, so all Promises should use the same event-loop. Module implementers should just use the Promises class directly:

    package MyClass;
    use Promises qw(deferred collect);

End users should specify which Deferred backend they wish to use. For instance if you are using AnyEvent, you can do:

    use Promises backend => ['AnyEvent'];
    use MyClass;

The Promises returned by MyClass will automatically use whichever event loop AnyEvent is using.


Relation to Promises/Futures in Scala

Scala has a notion of Promises and an associated idea of Futures as well. The differences and similarities between this module and the Promises found in Scalar are highlighted in depth in a cookbook entry below.



Read this first! This cookbook provides a step-by-step explanation of how Promises work and how to use them.


This breaks down the example in the SYNOPSIS and walks through much of the details of Promises and how they work.


Promise are just one of many ways to do async programming, this entry takes the Promises SYNOPSIS again and illustrates some counter examples with various modules.


One of the key benefits of Promises is that it retains much of the flow of a synchronous program, this entry illustrates that and compares it with a synchronous (or blocking) version.


This entry explains how to keep the stack under control when using Promises recursively.


This entry takes some examples of Futures in the Scala language and translates them into Promises. This entry also showcases using Promises with Mojo::UserAgent.



This just creates an instance of the Promises::Deferred class it is purely for convenience.

Can take a coderef, which will be dealt with as a then argument.

    my $promise = deferred sub {
        ... do stuff ...

        return $something;

    # equivalent to

    my $dummy = deferred;

    my $promise = $dummy->then(sub {
        ... do stuff ...

        return $something;

resolved( @values )

Creates an instance of Promises::Deferred resolved with the provided @values. Purely a shortcut for

    my $promise = deferred;
rejected( @values )

Creates an instance of Promises::Deferred rejected with the provided @values. Purely a shortcut for

    my $promise = deferred;
collect( @promises )

Accepts a list of Promises::Promise objects and then returns a Promises::Promise object which will be called once all the @promises have completed (either as an error or as a success).

The eventual result of the returned promise object will be an array of all the results of each of the @promises in the order in which they where passed to collect originally, wrapped in arrayrefs, or the first error if at least one of the promises fail.

If collect is passed a value that is not a promise, it'll be wrapped in an arrayref and passed through.

    my $p1 = deferred;
    my $p2 = deferred;

        'not a promise',
        print join ' : ', map { join ', ', @$_ } @_; # => "1 : not a promise : 2, 3"
collect_hash( @promises )

Like collect, but flatten its returned arrayref into a single hash-friendly list.

collect_hash can be useful to a structured hash instead of a long list of promise values.

For example,

  my $id = 12345;

      sub {
          my ($product, $suggestions, $reviews) = @_;
              product     => $product,
              suggestions => $suggestions,
              reviews     => $reviews,
              id          => $id
      sub { $cv->croak( 'ERROR' ) }

could be rewritten as

  my $id = 12345;

      id          => $id,
      product     => fetch_it("$id"),
      suggestions => fetch_it("$id"),
      reviews     => fetch_it("$id"),
      sub {
          my %results = @_;
      sub { $cv->croak( 'ERROR' ) }

Note that all promise values of the key/value pairs passed to collect_hash must return a scalar or nothing, as returning more than one value would mess up the returned hash format. If a promise does return more than one value, collect_hash will consider it as having failed.

If you know that a promise can return more than one value, you can do:

    my $collected = collect_hash(
        this => $promise_returning_scalar,
        that => $promise_returning_list->then(sub{ [ @_ ] } ),


Promises in General

You're Missing the Point of Promises
Systems Programming at Twitter
SIP-14 - Futures and Promises
Promises/A+ spec
Promises/A spec

Perl Alternatives


Part of the Mojolicious package.



Stevan Little <>


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This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.