# 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](https://metacpan.org/pod/AnyEvent::XSPromises). That module’s interface, a “bare-bones”
subset of that from [Promises](https://metacpan.org/pod/Promises), is retained.

# STATUS

This module is stable, well-tested, and suitable for production use.

# 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"](#avoid-multiples) below.)

See [Promise::ES6](https://metacpan.org/pod/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](https://metacpan.org/pod/Promises) ET AL.

## Empty or uninitialized rejection values

Promise rejections fulfill the same role in asynchronous code that exceptions
do in synchronous code. Perl helpfully warns (under the `warnings` pragma,
anyhow) when you `die(undef)` since an uninitialized value isn’t useful as
an error report and likely indicates a problem.

Promise::XS mimics this behavior by warning if a rejection value list lacks
a defined value. This can happen if the value list is either empty or
contains exclusively uninitialized values.

## `finally()`

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 );

- `$callback` receives _no_ arguments.
- If `$callback` returns anything but a single, rejected promise,
`$new` has the same status as `$p`.
- If `$callback` throws, or if it returns a single, rejected promise,
`$new` is rejected with the relevant value(s).

# EVENT LOOPS

By default this library uses no event loop. This is a generally 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. Following this pattern facilitates use of recursive
promises without exceeding call stack limits.

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

- [AnyEvent](https://metacpan.org/pod/AnyEvent):

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

- [IO::Async](https://metacpan.org/pod/IO::Async) - note the need for an [IO::Async::Loop](https://metacpan.org/pod/IO::Async::Loop) instance
as argument:

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

- [Mojo::IOLoop](https://metacpan.org/pod/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.

# 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](https://metacpan.org/pod/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](https://metacpan.org/pod/Promise::XS::Promise).) This can be useful, e.g., for implementing
mid-flight controls like cancellation.

# TODO

- `all()` and `race()` should ideally be implemented in XS.

# 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](https://metacpan.org/pod/AnyEvent::XSPromises) and [Promises](https://metacpan.org/pod/Promises), you may like [Promise::ES6](https://metacpan.org/pod/Promise::ES6),
which mimics [ECMAScript’s “Promise” class](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise) 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.

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