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Paul Evans

NAME

AnyEvent::Future - use Future with AnyEvent

SYNOPSIS

 use AnyEvent;
 use AnyEvent::Future;

 my $future = AnyEvent::Future->new;

 some_async_function( ..., cb => sub { $future->done( @_ ) } );

 print Future->await_any(
    $future,
    AnyEvent::Future->new_timeout( after => 10 ),
 )->get;

Or

 use AnyEvent::Future qw( as_future_cb );

 print Future->await_any(
    as_future_cb {
       some_async_function( ..., cb => shift )
    },
    AnyEvent::Future->new_timeout( after => 10 ),
 )->get;

DESCRIPTION

This subclass of Future integrates with AnyEvent, allowing the await method to block until the future is ready. It allows AnyEvent-using code to be written that returns Future instances, so that it can make full use of Future's abilities, including Future::Utils, and also that modules using it can provide a Future-based asynchronous interface of their own.

For a full description on how to use Futures, see the Future documentation.

CONSTRUCTORS

$f = AnyEvent::Future->new

Returns a new leaf future instance, which will allow waiting for its result to be made available, using the await method.

$f = AnyEvent::Future->new_delay( @args )

$f = AnyEvent::Future->new_timeout( @args )

Returns a new leaf future instance that will become ready at the time given by the arguments, which will be passed to the AnyEvent->timer method.

new_delay returns a future that will complete successfully at the alotted time, whereas new_timeout returns a future that will fail with the message Timeout.

$f = AnyEvent::Future->from_cv( $cv )

Returns a new leaf future instance that will become ready when the given AnyEvent::CondVar instance is ready. The success or failure result of the future will be the result passed to the condvar's send or croak method.

METHODS

$cv = $f->as_cv

Returns a new AnyEvent::CondVar instance that wraps the given future; it will complete with success or failure when the future does.

Note that because AnyEvent::CondVar->croak takes only a single string message for the argument, any subsequent failure semantics are lost from the Future. To capture these as well, you may wish to use an on_fail callback or the failure method, to obtain them.

UTILITY FUNCTIONS

The following utility functions are exported as a convenience.

$f = as_future { CODE }

Returns a new leaf future instance, which is also passed in to the block of code. The code is called in scalar context, and its return value is stored on the future. This will be deleted if the future is cancelled.

 $w = CODE->( $f )

This utility is provided for the common case of wanting to wrap an AnyEvent function which will want to receive a callback function to inform of completion, and which will return a watcher object reference that needs to be stored somewhere.

$f = as_future_cb { CODE }

A futher shortcut to as_future, where the code is passed two callback functions for done and fail directly, avoiding boilerplate in the common case for creating these closures capturing the future variable. In many cases this can reduce the code block to a single line.

 $w = CODE->( $done_cb, $fail_cb )

EXAMPLES

Wrapping watcher-style AnyEvent functions

The as_future_cb utility provides an excellent wrapper to take the common style of AnyEvent function that returns a watcher object and takes a completion callback, and turn it into a Future that can be used or combined with other Future-based code. For example, the AnyEvent::HTTP function called http_get performs in this style.

 use AnyEvent::Future qw( as_future_cb );
 use AnyEvent::HTTP qw( http_get );

 my $url = ...;

 my $f = as_future_cb {
    my ( $done_cb ) = @_;

    http_get $url, $done_cb;
 };

This could of course be easily wrapped by a convenient function to return futures:

 sub http_get_future
 {
    my @args = @_;

    as_future_cb {
       my ( $done_cb ) = @_;

       http_get @args, $done_cb;
    }
 }

Using Futures as enhanced CondVars

While at first glance it may appear that a Future instance is much like an AnyEvent::CondVar, the greater set of convergence methods (such as needs_all or needs_any), and the various utility functions (in Future::Utils) makes it possible to write the same style of code in a more concise or powerful way.

For example, rather than using the CondVar begin and end methods, a set of CondVar-returning functions can be converted into Futures, combined using needs_all, and converted back to a CondVar again:

 my $cv = Future->needs_all(
    Future::AnyEvent->from_cv( FUNC1() ),
    Future::AnyEvent->from_cv( FUNC2() ),
    ...
 )->as_cv;

 my @results = $cv->recv;

This would become yet more useful if, instead of functions that return CondVars, we were operating on functions that return Futures directly. Because the needs_all will cancel any still-pending futures the moment one of them failed, we get a nice neat cancellation of outstanding work if one of them fails, in a way that would be much harder without the Futures. For example, using the http_get_future function from above:

 my $cv = Future->needs_all(
    http_get_future( "http://url-1" ),
    http_get_future( "http://url-2" ),
    http_get_future( "https://url-third/secret" ),
 )->as_cv;

 my @results = $cv->recv;

In this case, the moment any of the HTTP GET functions fails, the ones that are still pending are all cancelled (by dropping their cancellation watcher object) and the overall recv call throws an exception.

Of course, there is no need to convert the outermost Future into a CondVar; the full set of waiting semantics are implemented on these instances, so instead you may simply call get on it to achieve the same effect:

 my $f = Future->needs_all(
    http_get_future( "http://url-1" ),
    ...
 );

 my @results = $f->get;

This has other side advantages, such as the list-valued semantics of failures that can provide additional information besides just the error message, and propagation of cancellation requests.

TODO

  • Consider whether or not it would be considered "evil" to inject a new method into AnyEvent::CondVar; namely by doing

     sub AnyEvent::CondVar::as_future { AnyEvent::Future->from_cv( shift ) }

AUTHOR

Paul Evans <leonerd@leonerd.org.uk>