/ 
Future-0.07
River stage three • 127 direct dependents • 319 total dependents
/ Future

NAME

Future - represent an operation awaiting completion

SYNOPSIS

my $future = Future->new;
$future->on_ready( sub {
   say "The operation is complete";
} );

kperform_some_operation( sub {
   $future->done( @_ );
} );

DESCRIPTION

An Future object represents an operation that is currently in progress, or has recently completed. It can be used in a variety of ways to manage the flow of control, and data, through an asynchronous program.

Some futures represent a single operation and are explicitly marked as ready by calling the done or fail methods. These are called "leaf" futures here, and are returned by the new constructor.

Other futures represent a collection sub-tasks, and are implicitly marked as ready depending on the readiness of their component futures as required. These are called "dependent" futures here, and are returned by the various wait_* and need_* constructors.

It is intended that library functions that perform asynchonous operations would use Future objects to represent outstanding operations, and allow their calling programs to control or wait for these operations to complete. The implementation and the user of such an interface would typically make use of different methods on the class. The methods below are documented in two sections; those of interest to each side of the interface.

SUBCLASSING

This class easily supports being subclassed to provide extra behavior, such as giving the get method the ability to block and wait for completion. This may be useful to provide Future subclasses with event systems, or similar.

Each method that returns a new Future object will use the invocant to construct its return value. If the constructor needs to perform per-instance setup it can override the new method, and take context from the given instance.

sub new
{
   my $proto = shift;
   my $self = $proto->SUPER::new;

   if( ret $proto ) {
      # Prototype was an instance
   }
   else {
      # Prototype was a class
   }

   return $self;
}

If an instance provides a method called await, this will be called by the get and failure methods if the instance is pending.

$f->await

The examples directory in the distribution contains some examples of how Futures might be integrated with various event systems.

DEBUGGING

By the time a Future object is destroyed, it ought to have been completed or cancelled. By enabling debug tracing of objects, this fact can be checked. If a Future object is destroyed without having been completed or cancelled, a warning message is printed.

This feature is enabled by setting an environment variable called PERL_FUTURE_DEBUG to some true value.

$ PERL_FUTURE_DEBUG=1 perl -MFuture -E 'my $f = Future->new'
Future=HASH(0xaa61f8) was constructed at -e line 1 and was lost near -e line 0 before it was ready.

Note that due to a limitation of perl's caller function within a DESTROY destructor method, the exact location of the leak cannot be accurately determined. Often the leak will occur due to falling out of scope by returning from a function; in this case the leak location may be reported as being the line following the line calling that function.

$ PERL_FUTURE_DEBUG=1 perl -MFuture
sub foo {
   my $f = Future->new;
}

foo();
print "Finished\n";

Future=HASH(0x14a2220) was constructed at - line 2 and was lost near - line 6 before it was ready.
Finished

CONSTRUCTORS

$future = Future->new

$future = $orig->new

Returns a new Future instance to represent a leaf future. It will be marked as ready by any of the done, fail, or cancel methods. It can be called either as a class method, or as an instance method. Called on an instance it will construct another in the same class, and is useful for subclassing.

This constructor would primarily be used by implementations of asynchronous interfaces.

$future = $f1->followed_by( \&code )

Returns a new Future instance that allows a sequence of operations to be performed. Once $f1 is ready, the code reference will be invoked and is passed one argument, being $f1. It should return a future, $f2. Once $f2 indicates completion the combined future $future will then be marked as complete, with whatever result $f2 gave.

$f2 = $code->( $f1 )

If $future is cancelled before $f1 completes, then $f1 will be cancelled. If it is cancelled after completion then $f2 is cancelled instead.

$future = $f1->and_then( \&code )

A convenient shortcut to followed_by, which invokes the supplied code reference only if the first future completes successfully. If it fails, then the returned future will fail with the same error and the code reference will not be invoked.

$future = $f1->or_else( \&code )

A convenient shortcut to followed_by, which invokes the supplied code reference only if the first future fails. If it completes successfully, then the returned future will complete with the same result and the code reference will not be invoked.

$future = $f1->transform( %args )

Returns a new Future instance that wraps the one given as $f1. With no arguments this will be a trivial wrapper; $future will complete or fail when $f1 does, and $f1 will be cancelled when $future is.

By passing the following named argmuents, the returned $future can be made to behave differently to $f1:

done => CODE

Provides a function to use to modify the result of a successful completion. When $f1 completes successfully, the result of its get method is passed into this function, and whatever it returns is passed to the done method of $future

fail => CODE

Provides a function to use to modify the result of a failure. When $f1 fails, the result of its failure method is passed into this function, and whatever it returns is passed to the fail method of $future.

IMPLEMENTATION METHODS

These methods would primarily be used by implementations of asynchronous interfaces.

$future->done( @result )

Marks that the leaf future is now ready, and provides a list of values as a result. (The empty list is allowed, and still indicates the future as ready). Cannot be called on a dependent future.

Returns the $future.

$code = $future->done_cb

Returns a CODE reference that, when invoked, calls the done method. This makes it simple to pass as a callback function to other code.

The same effect can be achieved using curry:

$code = $future->curry::done;

$future->fail( $exception, @details )

Marks that the leaf future has failed, and provides an exception value. This exception will be thrown by the get method if called. If the exception is a non-reference that does not end in a linefeed, its value will be extended by the file and line number of the caller, similar to the logic that die uses.

The exception must evaluate as a true value; false exceptions are not allowed. Further details may be provided that will be returned by the failure method in list context. These details will not be part of the exception string raised by get.

Returns the $future.

$code = $future->fail_cb

Returns a CODE reference that, when invoked, calls the fail method. This makes it simple to pass as a callback function to other code.

The same effect can be achieved using curry:

$code = $future->curry::fail;

$future->on_cancel( $code )

If the future is not yet ready, adds a callback to be invoked if the future is cancelled by the cancel method. If the future is already ready, throws an exception.

If the future is cancelled, the callbacks will be invoked in the reverse order to that in which they were registered.

$on_cancel->( $future )

$future->on_cancel( $f )

If passed another Future instance, the passed instance will be cancelled when the original future is cancelled.

$cancelled = $future->is_cancelled

Returns true if the future has been cancelled by cancel.

USER METHODS

These methods would primarily be used by users of asynchronous interfaces, on objects returned by such an interface.

$ready = $future->is_ready

Returns true on a leaf future if a result has been provided to the done method, failed using the fail method, or cancelled using the cancel method.

Returns true on a dependent future if it is ready to yield a result, depending on its component futures.

$future->on_ready( $code )

If the future is not yet ready, adds a callback to be invoked when the future is ready. If the future is already ready, invokes it immediately.

In either case, the callback will be passed the future object itself. The invoked code can then obtain the list of results by calling the get method.

$on_ready->( $future )

Returns the $future.

$future->on_ready( $f )

If passed another Future instance, the passed instance will have its done or fail methods invoked when the original future completes successfully or fails respectively.

@result = $future->get

If the future is ready and completed successfully, returns the list of results that had earlier been given to the done method on a leaf future, or the list of component futures it was waiting for on a dependent future.

If the future is ready but failed, this method raises as an exception the failure string or object that was given to the fail method.

If it is not yet ready, or was cancelled, an exception is thrown.

$future->on_done( $code )

If the future is not yet ready, adds a callback to be invoked when the future is ready, if it completes successfully. If the future completed successfully, invokes it immediately. If it failed or was cancelled, it is not invoked at all.

The callback will be passed the result passed to the done method.

$on_done->( @result )

Returns the $future.

$future->on_done( $f )

If passed another Future instance, the passed instance will have its done method invoked when the original future completes successfully.

$exception = $future->failure

$exception, @details = $future->failure

Returns the exception passed to the fail method, undef if the future completed successfully via the done method, or raises an exception if called on a future that is not yet ready.

If called in list context, will additionally yield a list of the details provided to the fail method.

Because the exception value must be true, this can be used in a simple if statement:

if( my $exception = $future->failure ) {
   ...
}
else {
   my @result = $future->get;
   ...
}

$future->on_fail( $code )

If the future is not yet ready, adds a callback to be invoked when the future is ready, if it fails. If the future has already failed, invokes it immediately. If it completed successfully or was cancelled, it is not invoked at all.

The callback will be passed the exception and details passed to the fail method.

$on_fail->( $exception, @details )

Returns the $future.

$future->on_fail( $f )

If passed another Future instance, the passed instance will have its fail method invoked when the original future fails.

To invoke a done method on a future when another one fails, use a CODE reference:

$future->on_fail( sub { $f->done( @_ ) } );

$future->cancel

Requests that the future be cancelled, immediately marking it as ready. This will invoke all of the code blocks registered by on_cancel, in the reverse order. When called on a dependent future, all its component futures are also cancelled.

Returns the $future.

$code = $future->cancel_cb

Returns a CODE reference that, when invoked, calls the cancel method. This makes it simple to pass as a callback function to other code.

The same effect can be achieved using curry:

$code = $future->curry::cancel;

DEPENDENT FUTURES

The following constructors all take a list of component futures, and return a new future whose readiness somehow depends on the readiness of those components. The first component future will be used as the prototype for constructing the return value, so it respects subclassing correctly.

$future = Future->wait_all( @subfutures )

Returns a new Future instance that will indicate it is ready once all of the sub future objects given to it indicate that they are ready. Its result will a list of its component futures.

This constructor would primarily be used by users of asynchronous interfaces.

$future = Future->wait_any( @subfutures )

Returns a new Future instance that will indicate it is ready once any of the sub future objects given to it indicate that they are ready. Any remaining component futures that are not yet ready will be cancelled. Its result will be the result of the first component future that was ready; either success or failure.

This constructor would primarily be used by users of asynchronous interfaces.

$future = Future->needs_all( @subfutures )

Returns a new Future instance that will indicate it is ready once all of the sub future objects given to it indicate that they have completed successfully, or when any of them indicates that they have failed. If any sub future fails, then this will fail immediately, and the remaining subs not yet ready will be cancelled.

If successful, its result will be a concatenated list of the results of all its component futures, in corresponding order. If it fails, its failure will be that of the first component future that failed. To access each component future's results individually, use done_futures.

(NOTE: this result is different from versions of Future before 0.03.)

This constructor would primarily be used by users of asynchronous interfaces.

$future = Future->needs_any( @subfutures )

Returns a new Future instance that will indicate it is ready once any of the sub future objects given to it indicate that they have completed successfully, or when all of them indicate that they have failed. If any sub future succeeds, then this will succeed immediately, and the remaining subs not yet ready will be cancelled.

If successful, its result will be that of the first component future that succeeded. If it fails, its failure will be that of the last component future to fail. To access the other failures, use failed_futures.

(NOTE: this result is different from versions of Future before 0.03.)

Normally when this Future completes successfully, only one of its component futures will be done. If it is constructed with multiple that are already done however, then all of these will be returned from done_futures. Users should be careful to still check all the results from done_futures in that case.

This constructor would primarily be used by users of asynchronous interfaces.

METHODS ON DEPENDENT FUTURES

The following methods apply to dependent (i.e. non-leaf) futures, to access the component futures stored by it.

@f = $future->pending_futures

@f = $future->ready_futures

@f = $future->done_futures

@f = $future->failed_futures

@f = $future->cancelled_futures

Return a list of all the pending, ready, done, failed, or cancelled component futures. In scalar context, each will yield the number of such component futures.

EXAMPLES

The following examples all demonstrate possible uses of a Future object to provide a fictional asynchronous API function called simply koperation.

Providing Results

By returning a new Future object each time the asynchronous function is called, it provides a placeholder for its eventual result, and a way to indicate when it is complete.

sub foperation
{
   my %args = @_;

   my $future = Future->new;

   kdo_something(
      foo => $args{foo},
      on_done => sub { $future->done( @_ ); },
   );

   return $future;
}

In most cases, the done method will simply be invoked with the entire result list as its arguments. In that case, it is simpler to pass the $future object itself as if it was a CODE reference; this will invoke the done method.

my $future = Future->new;

kdo_something(
   foo => $args{foo},
   on_done => $future,
);

The caller may then use this future to wait for a result using the on_ready method, and obtain the result using get.

my $f = foperation( foo => "something" );

$f->on_ready( sub {
   my $f = shift;
   say "The operation returned: ", $f->get;
} );

Indicating Success or Failure

Because the stored exception value of a failed future may not be false, the failure method can be used in a conditional statement to detect success or failure.

my $f = koperation( foo => "something" );

$f->on_ready( sub {
   my $f = shift;
   if( not my $e = $f->failure ) {
      say "The operation succeeded with: ", $f->get;
   }
   else {
      say "The operation failed with: ", $e;
   }
} );

By using not in the condition, the order of the if blocks can be arranged to put the successful case first, similar to a try/catch block.

Because the get method re-raises the passed exception if the future failed, it can be used to control a try/catch block directly. (This is sometimes called Exception Hoisting).

use Try::Tiny;

$f->on_ready( sub {
   my $f = shift;
   try {
      say "The operation succeeded with: ", $f->get;
   }
   catch {
      say "The operation failed with: ", $_;
   };
} );

Immediate Futures

Because the done method returns the future object itself, it can be used to generate a Future that is immediately ready with a result.

my $f = Future->new->done( $value );

Similarly, the fail method can be used to generate a Future that is immediately failed.

my $f = Future->new->fail( "This is never going to work" );

This could be considered similarly to a die call.

Sequencing

The and_then method can be used to create simple chains of dependent tasks, each one executing and returning a Future when the previous operation succeeds.

my $f = do_first()
           ->and_then( sub {
              return do_second();
           })
           ->and_then( sub {
              return do_third();
           });

The result of the $f future itself will be the result of the future returned by the final function, if none of them failed. If any of them fails it will fail with the same value. This can be considered similar to normal exception handling in synchronous code; the first time a function call throws an exception, the subsequent calls are not made.

Catching Errors

The or_else method can be used to create error handling logic, similar to the kind that can be performed synchronously with eval or Try::Tiny.

my $f = try_this()
           ->or_else( sub {
              return handle_failure();
           });

The result of the returned future will be what the first function's future returned, if it was successful, or else whatever the second function's future returned.

As the failure-handling code block is given the failed future, and has to return a future; it can return the failed future itself to apply some clean-up logic similar to catching but re-throwing an exception:

my $f = try_this()
           ->or_else( sub {
              my $failure = shift;
              cleanup();
              return $failure;
           });

The followed_by method can attach a code block that is run whenever a future is ready, regardless of whether it succeeded or failed. This can be used to create an additional finally-like block. If the followed_by block returns the future it was passed, then the entire combination still succeeds or fails according to the result of the first.

my $f = try_this()
           ->followed_by( sub {
              finally_this()
              return $_[0];
           });

A combination of or_else and followed_by can create a structure similar to the full try/catch/finally semantics.

my $f = try_this()
           ->or_else( sub {
              return catch_this();
           })
           ->followed_by( sub {
              finally_this();
              return $_[0];
           });

Merging Control Flow

A wait_all future may be used to resynchronise control flow, while waiting for multiple concurrent operations to finish.

my $f1 = koperation( foo => "something" );
my $f2 = koperation( bar => "something else" );

my $f = Future->wait_all( $f1, $f2 );

$f->on_ready( sub {
   say "Operations are ready:";
   say "  foo: ", $f1->get;
   say "  bar: ", $f2->get;
} );

This provides an ability somewhat similar to CPS::kpar() or Async::MergePoint.

SEE ALSO

  • curry - Create automatic curried method call closures for any class or object

AUTHOR

Paul Evans <leonerd@leonerd.org.uk>