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CPS-0.17
River stage two • 3 direct dependents • 10 total dependents
2 ++
/ CPS::Future

NAME

CPS::Future - represent an operation awaiting completion

SYNOPSIS

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

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

DESCRIPTION

An CPS::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 (returned by the new constructor), and are explicitly marked as ready by calling the done method. Others represent a tree of sub-tasks (returned by the wait_all or needs_all constructors), and are implicitly marked as ready when all of their component futures are ready.

It is intended that library functions that perform asynchonous operations would use CPS::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.

CONSTRUCTORS

$future = CPS::Future->new

Returns a new CPS::Future instance to represent a leaf future. It will be marked as ready by any of the done, fail, or cancel methods.

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

$future = CPS::Future->wait_all( @subfutures )

Returns a new CPS::Future instance that will indicate it is ready once all of the sub future objects given to it indicate that they are ready.

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

$future = CPS::Future->needs_all( @subfutures )

Returns a new CPS::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.

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

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

Returns a new CPS::Future instance that allows a sequence of dependent operations to be performed. Once $f1 indicates a successful completion, the code reference will be invoked and is passed one argument, being $f1. It should return a new future, $f2. Once $f2 indicates completion the combined future $future will then be marked as complete. The result of calling get on the combined future will return whatever was passed to the done method of $f2.

 $f2 = $code->( $f1 )

If $f1 fails then $future will indicate this failure immediately and the block of code will not be invoked.

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->transform( %args )

Returns a new CPS::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 non-leaf future.

Returns the $future.

$future->( @result )

This method is used to overload the calling operator, so simply invoking the future object itself as if it were a CODE reference is equivalent to calling the done method. This makes it simple to pass as a callback function to other code.

It turns out however, that this behaviour is too subtle and can lead to bugs when futures are accidentally used as plain CODE references. See the done_cb method instead. This overload behaviour will be removed in a later version.

$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.

$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.

$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 CPS::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 wait_all future if all the sub-tasks are ready.

Returns true on a needs_all future if all the sub-tasks have completed successfully or if any of them have failed.

$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 CPS::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, returns the list of results that had earlier been given to the done method. If not, will raise an exception.

If called on a wait_all or needs_all future, it will return a list of the futures it was waiting on, in the order they were passed to the constructor.

$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 CPS::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 CPS::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 non-leaf future, all its sub-tasks are also cancelled.

$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.

EXAMPLES

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

Providing Results

By returning a new CPS::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 = CPS::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 = CPS::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 failued CPS::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: ", $_;
    };
 } );

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 = CPS::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.

TODO

Lots of things still need adding. API or semantics is somewhat unclear in places.

  • CPS::Future->needs_first, which succeeds on the first success of dependent futures and cancels the outstanding ones, only fails if all the dependents do.

  • Some way to do deferred futures that don't even start their operation until invoked somehow. Ability to chain these together in a sequence, like CPS::kseq().

AUTHOR

Paul Evans <leonerd@leonerd.org.uk>