Future(3) User Contributed Perl Documentation Future(3)NAME
"Future" - represent an operation awaiting completion
SYNOPSIS
my $future = Future->new;
perform_some_operation(
on_complete => sub {
$future->done( @_ );
}
);
$future->on_ready( sub {
say "The operation is complete";
} );
DESCRIPTION
A "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 asynchronous
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.
See also Future::Utils which contains useful loop-constructing
functions, to run a future-returning function repeatedly in a loop.
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( ref $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
In most cases this should allow future-returning modules to be used as
if they were blocking call/return-style modules, by simply appending a
"get" call to the function or method calls.
my ( $results, $here ) = future_returning_function( @args )->get;
The examples directory in the distribution contains some examples of
how futures might be integrated with various event systems.
MODULE DOCUMENTATION
Modules that provide future-returning functions or methods may wish to
adopt the following styles in some way, to document the eventual return
values from these futures.
func( ARGS, HERE... ) ==> ( RETURN, VALUES... )
OBJ->method( ARGS, HERE... ) ==> ( RETURN, VALUES... )
Code returning a future that yields no values on success can use empty
parentheses.
func( ... ) ==> ()
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 = Future->wrap( @values )
If given a single argument which is already a "Future" reference, this
will be returned unmodified. Otherwise, returns a new "Future" instance
that is already complete, and will yield the given values.
$future = Future->call( \&code, @args )
A convenient wrapper for calling a "CODE" reference that is expected to
return a future. In normal circumstances is equivalent to
$future = $code->( @args )
except that if the code throws an exception, it is wrapped in a new
immediate fail future. If the return value from the code is not a
blessed "Future" reference, an immediate fail future is returned
instead to complain about this fact.
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 to allow easy chaining to create an immediate
future by
return Future->new->done( ... )
If the future is already cancelled, this request is ignored. If the
future is already complete with a result or a failure, an exception is
thrown.
$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.
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 to allow easy chaining to create an immediate
failed future by
return Future->new->fail( ... )
If the future is already cancelled, this request is ignored. If the
future is already complete with a result or a failure, an exception is
thrown.
$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->die( $message, @details )
A convenient wrapper around "fail". 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.
Returns the $future.
$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. This method does
nothing if the future is already complete.
$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", "fail" or "cancel" methods invoked when the original future
completes successfully, fails, or is cancelled respectively.
$done = $future->is_done
Returns true on a future if it is ready and completed successfully.
Returns false if it is still pending, failed, or was cancelled.
@result = $future->get
$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. In scalar context it returns just the first result
value.
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 the future was cancelled an exception is thrown.
If it is not yet ready and is not of a subclass that provides an
"await" method an exception is thrown. If it is subclassed to provide
an "await" method then this is used to wait for the future to be ready,
before returning the result or propagating its failure exception.
$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. It is not an error to attempt to cancel a
future that is already complete or cancelled; it simply has no effect.
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;
SEQUENCING METHODS
The following methods all return a new future to represent the
combination of its invocant followed by another action given by a code
reference. The combined activity waits for the first future to be
ready, then may invoke the code depending on the success or failure of
the first, or may run it regardless. The returned sequence future
represents the entire combination of activity.
In some cases the code should return a future; in some it should return
an immediate result. If a future is returned, the combined future will
then wait for the result of this second one. If the combinined future
is cancelled, it will cancel either the first future or the second,
depending whether the first had completed. If the code block throws an
exception instead of returning a value, the sequence future will fail
with that exception as its message and no further values.
As it is always a mistake to call these sequencing methods in void
context and lose the reference to the returned future (because
exception/error handling would be silently dropped), this method warns
in void context.
$future = $f1->then( \&done_code )
Returns a new sequencing "Future" that runs the code if the first
succeeds. Once $f1 succeeds the code reference will be invoked and is
passed the list of results. It should return a future, $f2. Once $f2
completes the sequence future will then be marked as complete with
whatever result $f2 gave. If $f1 fails then the sequence future will
immediately fail with the same failure and the code will not be
invoked.
$f2 = $done_code->( @result )
$future = $f1->else( \&fail_code )
Returns a new sequencing "Future" that runs the code if the first
fails. Once $f1 fails the code reference will be invoked and is passed
the failure and details. It should return a future, $f2. Once $f2
completes the sequence future will then be marked as complete with
whatever result $f2 gave. If $f1 succeeds then the sequence future will
immediately succeed with the same result and the code will not be
invoked.
$f2 = $fail_code->( $exception, @details )
$future = $f1->then( \&done_code, \&fail_code )
The "then" method can also be passed the $fail_code block as well,
giving a combination of "then" and "else" behaviour.
This operation is designed to be compatible with the semantics of other
future systems, such as Javascript's Q or Promises/A libraries.
$future = $f1->transform( %args )
Returns a new sequencing "Future" 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 arguments, 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.
$future = $f1->then_with_f( \&code )
Returns a new sequencing "Future" that runs the code if the first
succeeds. Identical to "then", except that the code reference will be
passed both the original future, $f1, and its result.
$f2 = $code->( $f1, @result )
This is useful for conditional execution cases where the code block may
just return the same result of the original future. In this case it is
more efficient to return the original future itself.
$future = $f->then_done( @result )
$future = $f->then_fail( $exception, @details )
Convenient shortcuts to returning an immediate future from a "then"
block, when the result is already known.
$future = $f1->else_with_f( \&code )
Returns a new sequencing "Future" that runs the code if the first
fails. Identical to "else", except that the code reference will be
passed both the original future, $f1, and its exception and details.
$f2 = $code->( $f1, $exception, @details )
This is useful for conditional execution cases where the code block may
just return the same result of the original future. In this case it is
more efficient to return the original future itself.
$future = $f->else_done( @result )
$future = $f->else_fail( $exception, @details )
Convenient shortcuts to returning an immediate future from a "else"
block, when the result is already known.
$future = $f1->followed_by( \&code )
Returns a new sequencing "Future" that runs the code regardless of
success or failure. Once $f1 is ready the code reference will be
invoked and is passed one argument, $f1. It should return a future,
$f2. Once $f2 completes the sequence future will then be marked as
complete with whatever result $f2 gave.
$f2 = $code->( $f1 )
$future = $f1->and_then( \&code )
An older form of "then_with_f"; this method passes only the original
future itself to the code, not its result. The code would have to call
"get" on the future to obtain the result.
$f2 = $code->( $f1 )
This method may be removed in a later version; use "then_with_f" in new
code.
$future = $f1->or_else( \&code )
An older form of "else_with_f"; this method passes only the original
future itself to the code, not its failure and details. The code would
have to call "failure" on the future to obtain the result.
$f2 = $code->( $f1 )
This method may be removed in a later version; use "else_with_f" in new
code.
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 derived class component future will be used
as the prototype for constructing the return value, so it respects
subclassing correctly, or failing that a plain "Future".
$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,
either by success or failure. Its result will a list of its component
futures.
When given an empty list this constructor returns a new immediately-
done future.
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,
either by success or failure. 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.
When given an empty list this constructor returns an immediately-failed
future.
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".
When given an empty list this constructor returns a new immediately-
done future.
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".
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.
When given an empty list this constructor returns an immediately-failed
future.
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.
For more examples, comparing the use of "Future" with regular
call/return style Perl code, see also Future::Phrasebook.
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;
do_something_async(
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 use the
"done_cb" wrapper method to create the "CODE" reference.
my $future = Future->new;
do_something_async(
foo => $args{foo},
on_done => $future->done_cb,
);
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 = foperation( 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: ", $_;
};
} );
Even neater still may be the separate use of the "on_done" and
"on_fail" methods.
$f->on_done( sub {
my @result = @_;
say "The operation succeeded with: ", @result;
} );
$f->on_fail( sub {
my ( $failure ) = @_;
say "The operation failed with: $failure";
} );
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 );
This is neater handled by the "wrap" class method, which encapsulates
its arguments in a new immediate "Future", except if it is given a
single argument that is already a "Future":
my $f = Future->wrap( $value );
Similarly, the "fail" and "die" methods can be used to generate a
"Future" that is immediately failed.
my $f = Future->new->die( "This is never going to work" );
This could be considered similarly to a "die" call.
An "eval{}" block can be used to turn a "Future"-returning function
that might throw an exception, into a "Future" that would indicate this
failure.
my $f = eval { function() } || Future->new->fail( $@ );
This is neater handled by the "call" class method, which wraps the call
in an "eval{}" block and tests the result:
my $f = Future->call( \&function );
Sequencing
The "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()
->then( sub {
return do_second();
})
->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 failure. 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.
Merging Control Flow
A "wait_all" future may be used to resynchronise control flow, while
waiting for multiple concurrent operations to finish.
my $f1 = foperation( foo => "something" );
my $f2 = foperation( 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
· "The Past, The Present and The Future" - slides from a talk given
at the London Perl Workshop, 2012.
<https://docs.google.com/presentation/d/1UkV5oLcTOOXBXPh8foyxko4PR28_zU_aVx6gBms7uoo/edit>
· "Futures advent calendar 2013"
<http://leonerds-code.blogspot.co.uk/2013/12/futures-advent-day-1.html>
AUTHOR
Paul Evans <leonerd@leonerd.org.uk>
perl v5.18.2 2014-05-14 Future(3)
