1Future(3) User Contributed Perl Documentation Future(3)
2
6 "Future" - represent an operation awaiting completion
7
9 my $future = Future->new;
10 perform_some_operation(
12 on_complete => sub {
13 $future->done( @_ );
14 }
15 );
16 $future->on_ready( sub {
18 say "The operation is complete";
19 } );
20
22 A "Future" object represents an operation that is currently in
23 progress, or has recently completed. It can be used in a variety of
24 ways to manage the flow of control, and data, through an asynchronous
25 program.
26 Some futures represent a single operation and are explicitly marked as
28 ready by calling the "done" or "fail" methods. These are called "leaf"
29 futures here, and are returned by the "new" constructor.
30 Other futures represent a collection of sub-tasks, and are implicitly
32 marked as ready depending on the readiness of their component futures
33 as required. These are called "convergent" futures here as they
34 converge control and data-flow back into one place. These are the ones
35 returned by the various "wait_*" and "need_*" constructors.
36 It is intended that library functions that perform asynchronous
38 operations would use future objects to represent outstanding
39 operations, and allow their calling programs to control or wait for
40 these operations to complete. The implementation and the user of such
41 an interface would typically make use of different methods on the
42 class. The methods below are documented in two sections; those of
43 interest to each side of the interface.
44 It should be noted however, that this module does not in any way
46 provide an actual mechanism for performing this asynchronous activity;
47 it merely provides a way to create objects that can be used for control
48 and data flow around those operations. It allows such code to be
49 written in a neater, forward-reading manner, and simplifies many common
50 patterns that are often involved in such situations.
51 See also Future::Utils which contains useful loop-constructing
53 functions, to run a future-returning function repeatedly in a loop.
54 Unless otherwise noted, the following methods require at least version
56 0.08.
57 FAILURE CATEGORIES
59 While not directly required by "Future" or its related modules, a
60 growing convention of "Future"-using code is to encode extra semantics
61 in the arguments given to the "fail" method, to represent different
62 kinds of failure.
63 The convention is that after the initial message string as the first
65 required argument (intended for display to humans), the second argument
66 is a short lowercase string that relates in some way to the kind of
67 failure that occurred. Following this is a list of details about that
68 kind of failure, whose exact arrangement or structure are determined by
69 the failure category. For example, IO::Async and Net::Async::HTTP use
70 this convention to indicate at what stage a given HTTP request has
71 failed:
72 ->fail( $message, http => ... ) # an HTTP-level error during protocol
74 ->fail( $message, connect => ... ) # a TCP-level failure to connect a
75 # socket
76 ->fail( $message, resolve => ... ) # a resolver (likely DNS) failure
77 # to resolve a hostname
78 By following this convention, a module remains consistent with other
80 "Future"-based modules, and makes it easy for program logic to
81 gracefully handle and manage failures by use of the "catch" method.
82 SUBCLASSING
84 This class easily supports being subclassed to provide extra behavior,
85 such as giving the "get" method the ability to block and wait for
86 completion. This may be useful to provide "Future" subclasses with
87 event systems, or similar.
88 Each method that returns a new future object will use the invocant to
90 construct its return value. If the constructor needs to perform per-
91 instance setup it can override the "new" method, and take context from
92 the given instance.
93 sub new
95 {
96 my $proto = shift;
97 my $self = $proto->SUPER::new;
98 if( ref $proto ) {
100 # Prototype was an instance
101 }
102 else {
103 # Prototype was a class
104 }
105 return $self;
107 }
108 If an instance provides a method called "await", this will be called by
110 the "get" and "failure" methods if the instance is pending.
111 $f->await
113 In most cases this should allow future-returning modules to be used as
115 if they were blocking call/return-style modules, by simply appending a
116 "get" call to the function or method calls.
117 my ( $results, $here ) = future_returning_function( @args )->get;
119 The examples directory in the distribution contains some examples of
121 how futures might be integrated with various event systems.
122 DEBUGGING
124 By the time a "Future" object is destroyed, it ought to have been
125 completed or cancelled. By enabling debug tracing of objects, this fact
126 can be checked. If a future object is destroyed without having been
127 completed or cancelled, a warning message is printed.
128 $ PERL_FUTURE_DEBUG=1 perl -MFuture -E 'my $f = Future->new'
130 Future=HASH(0xaa61f8) was constructed at -e line 1 and was lost near -e line 0 before it was ready.
131 Note that due to a limitation of perl's "caller" function within a
133 "DESTROY" destructor method, the exact location of the leak cannot be
134 accurately determined. Often the leak will occur due to falling out of
135 scope by returning from a function; in this case the leak location may
136 be reported as being the line following the line calling that function.
137 $ PERL_FUTURE_DEBUG=1 perl -MFuture
139 sub foo {
140 my $f = Future->new;
141 }
142 foo();
144 print "Finished\n";
145 Future=HASH(0x14a2220) was constructed at - line 2 and was lost near - line 6 before it was ready.
147 Finished
148 A warning is also printed in debug mode if a "Future" object is
150 destroyed that completed with a failure, but the object believes that
151 failure has not been reported anywhere.
152 $ PERL_FUTURE_DEBUG=1 perl -Mblib -MFuture -E 'my $f = Future->fail("Oops")'
154 Future=HASH(0xac98f8) was constructed at -e line 1 and was lost near -e line 0 with an unreported failure of: Oops
155 Such a failure is considered reported if the "get" or "failure" methods
157 are called on it, or it had at least one "on_ready" or "on_fail"
158 callback, or its failure is propagated to another "Future" instance (by
159 a sequencing or converging method).
160
162 new
163 $future = Future->new
164 $future = $orig->new
166 Returns a new "Future" instance to represent a leaf future. It will be
168 marked as ready by any of the "done", "fail", or "cancel" methods. It
169 can be called either as a class method, or as an instance method.
170 Called on an instance it will construct another in the same class, and
171 is useful for subclassing.
172 This constructor would primarily be used by implementations of
174 asynchronous interfaces.
175 done (class method)
177 fail (class method)
178 $future = Future->done( @values )
179 $future = Future->fail( $exception, @details )
181 Since version 0.26.
183 Shortcut wrappers around creating a new "Future" then immediately
185 marking it as done or failed.
186 wrap
188 $future = Future->wrap( @values )
189 Since version 0.14.
191 If given a single argument which is already a "Future" reference, this
193 will be returned unmodified. Otherwise, returns a new "Future" instance
194 that is already complete, and will yield the given values.
195 This will ensure that an incoming argument is definitely a "Future",
197 and may be useful in such cases as adapting synchronous code to fit
198 asynchronous libraries driven by "Future".
199 call
201 $future = Future->call( \&code, @args )
202 Since version 0.15.
204 A convenient wrapper for calling a "CODE" reference that is expected to
206 return a future. In normal circumstances is equivalent to
207 $future = $code->( @args )
209 except that if the code throws an exception, it is wrapped in a new
211 immediate fail future. If the return value from the code is not a
212 blessed "Future" reference, an immediate fail future is returned
213 instead to complain about this fact.
214
216 As there are a lare number of methods on this class, they are
217 documented here in several sections.
218
220 The following methods query the internal state of a Future instance
221 without modifying it or otherwise causing side-effects.
222 is_ready
224 $ready = $future->is_ready
225 Returns true on a leaf future if a result has been provided to the
227 "done" method, failed using the "fail" method, or cancelled using the
228 "cancel" method.
229 Returns true on a convergent future if it is ready to yield a result,
231 depending on its component futures.
232 is_done
234 $done = $future->is_done
235 Returns true on a future if it is ready and completed successfully.
237 Returns false if it is still pending, failed, or was cancelled.
238 is_failed
240 $failed = $future->is_failed
241 Since version 0.26.
243 Returns true on a future if it is ready and it failed. Returns false if
245 it is still pending, completed successfully, or was cancelled.
246 is_cancelled
248 $cancelled = $future->is_cancelled
249 Returns true if the future has been cancelled by "cancel".
251 state
253 $str = $future->state
254 Since version 0.36.
256 Returns a string describing the state of the future, as one of the
258 three states named above; namely "done", "failed" or "cancelled", or
259 "pending" if it is none of these.
260
262 These methods would primarily be used by implementations of
263 asynchronous interfaces.
264 done
266 $future->done( @result )
267 Marks that the leaf future is now ready, and provides a list of values
269 as a result. (The empty list is allowed, and still indicates the future
270 as ready). Cannot be called on a convergent future.
271 If the future is already cancelled, this request is ignored. If the
273 future is already complete with a result or a failure, an exception is
274 thrown.
275 fail
277 $future->fail( $exception, @details )
278 Marks that the leaf future has failed, and provides an exception value.
280 This exception will be thrown by the "get" method if called.
281 The exception must evaluate as a true value; false exceptions are not
283 allowed. Further details may be provided that will be returned by the
284 "failure" method in list context. These details will not be part of the
285 exception string raised by "get".
286 If the future is already cancelled, this request is ignored. If the
288 future is already complete with a result or a failure, an exception is
289 thrown.
290 die
292 $future->die( $message, @details )
293 Since version 0.09.
295 A convenient wrapper around "fail". If the exception is a non-reference
297 that does not end in a linefeed, its value will be extended by the file
298 and line number of the caller, similar to the logic that "die" uses.
299 Returns the $future.
301 on_cancel
303 $future->on_cancel( $code )
304 If the future is not yet ready, adds a callback to be invoked if the
306 future is cancelled by the "cancel" method. If the future is already
307 ready, throws an exception.
308 If the future is cancelled, the callbacks will be invoked in the
310 reverse order to that in which they were registered.
311 $on_cancel->( $future )
313 If passed another "Future" instance, the passed instance will be
315 cancelled when the original future is cancelled. This method does
316 nothing if the future is already complete.
317
319 These methods would primarily be used by users of asynchronous
320 interfaces, on objects returned by such an interface.
321 on_ready
323 $future->on_ready( $code )
324 If the future is not yet ready, adds a callback to be invoked when the
326 future is ready. If the future is already ready, invokes it
327 immediately.
328 In either case, the callback will be passed the future object itself.
330 The invoked code can then obtain the list of results by calling the
331 "get" method.
332 $on_ready->( $future )
334 If passed another "Future" instance, the passed instance will have its
336 "done", "fail" or "cancel" methods invoked when the original future
337 completes successfully, fails, or is cancelled respectively.
338 Returns the $future.
340 get
342 @result = $future->get
343 $result = $future->get
345 If the future is ready and completed successfully, returns the list of
347 results that had earlier been given to the "done" method on a leaf
348 future, or the list of component futures it was waiting for on a
349 convergent future. In scalar context it returns just the first result
350 value.
351 If the future is ready but failed, this method raises as an exception
353 the failure string or object that was given to the "fail" method.
354 If the future was cancelled an exception is thrown.
356 If it is not yet ready and is not of a subclass that provides an
358 "await" method an exception is thrown. If it is subclassed to provide
359 an "await" method then this is used to wait for the future to be ready,
360 before returning the result or propagating its failure exception.
361 unwrap
363 @values = Future->unwrap( @values )
364 Since version 0.26.
366 If given a single argument which is a "Future" reference, this method
368 will call "get" on it and return the result. Otherwise, it returns the
369 list of values directly in list context, or the first value in scalar.
370 Since it involves an implicit "await", this method can only be used on
371 immediate futures or subclasses that implement "await".
372 This will ensure that an outgoing argument is definitely not a
374 "Future", and may be useful in such cases as adapting synchronous code
375 to fit asynchronous libraries that return "Future" instances.
376 on_done
378 $future->on_done( $code )
379 If the future is not yet ready, adds a callback to be invoked when the
381 future is ready, if it completes successfully. If the future completed
382 successfully, invokes it immediately. If it failed or was cancelled, it
383 is not invoked at all.
384 The callback will be passed the result passed to the "done" method.
386 $on_done->( @result )
388 If passed another "Future" instance, the passed instance will have its
390 "done" method invoked when the original future completes successfully.
391 Returns the $future.
393 failure
395 $exception = $future->failure
396 $exception, @details = $future->failure
398 Returns the exception passed to the "fail" method, "undef" if the
400 future completed successfully via the "done" method, or raises an
401 exception if called on a future that is not yet ready.
402 If called in list context, will additionally yield a list of the
404 details provided to the "fail" method.
405 Because the exception value must be true, this can be used in a simple
407 "if" statement:
408 if( my $exception = $future->failure ) {
410 ...
411 }
412 else {
413 my @result = $future->get;
414 ...
415 }
416 on_fail
418 $future->on_fail( $code )
419 If the future is not yet ready, adds a callback to be invoked when the
421 future is ready, if it fails. If the future has already failed, invokes
422 it immediately. If it completed successfully or was cancelled, it is
423 not invoked at all.
424 The callback will be passed the exception and details passed to the
426 "fail" method.
427 $on_fail->( $exception, @details )
429 If passed another "Future" instance, the passed instance will have its
431 "fail" method invoked when the original future fails.
432 To invoke a "done" method on a future when another one fails, use a
434 CODE reference:
435 $future->on_fail( sub { $f->done( @_ ) } );
437 Returns the $future.
439 cancel
441 $future->cancel
442 Requests that the future be cancelled, immediately marking it as ready.
444 This will invoke all of the code blocks registered by "on_cancel", in
445 the reverse order. When called on a convergent future, all its
446 component futures are also cancelled. It is not an error to attempt to
447 cancel a future that is already complete or cancelled; it simply has no
448 effect.
449 Returns the $future.
451
453 The following methods all return a new future to represent the
454 combination of its invocant followed by another action given by a code
455 reference. The combined activity waits for the first future to be
456 ready, then may invoke the code depending on the success or failure of
457 the first, or may run it regardless. The returned sequence future
458 represents the entire combination of activity.
459 In some cases the code should return a future; in some it should return
461 an immediate result. If a future is returned, the combined future will
462 then wait for the result of this second one. If the combinined future
463 is cancelled, it will cancel either the first future or the second,
464 depending whether the first had completed. If the code block throws an
465 exception instead of returning a value, the sequence future will fail
466 with that exception as its message and no further values.
467 As it is always a mistake to call these sequencing methods in void
469 context and lose the reference to the returned future (because
470 exception/error handling would be silently dropped), this method warns
471 in void context.
472 then
474 $future = $f1->then( \&done_code )
475 Since version 0.13.
477 Returns a new sequencing "Future" that runs the code if the first
479 succeeds. Once $f1 succeeds the code reference will be invoked and is
480 passed the list of results. It should return a future, $f2. Once $f2
481 completes the sequence future will then be marked as complete with
482 whatever result $f2 gave. If $f1 fails then the sequence future will
483 immediately fail with the same failure and the code will not be
484 invoked.
485 $f2 = $done_code->( @result )
487 else
489 $future = $f1->else( \&fail_code )
490 Since version 0.13.
492 Returns a new sequencing "Future" that runs the code if the first
494 fails. Once $f1 fails the code reference will be invoked and is passed
495 the failure and details. It should return a future, $f2. Once $f2
496 completes the sequence future will then be marked as complete with
497 whatever result $f2 gave. If $f1 succeeds then the sequence future will
498 immediately succeed with the same result and the code will not be
499 invoked.
500 $f2 = $fail_code->( $exception, @details )
502 then (2 arguments)
504 $future = $f1->then( \&done_code, \&fail_code )
505 The "then" method can also be passed the $fail_code block as well,
507 giving a combination of "then" and "else" behaviour.
508 This operation is designed to be compatible with the semantics of other
510 future systems, such as Javascript's Q or Promises/A libraries.
511 catch
513 $future = $f1->catch(
514 name => \&code,
515 name => \&code, ...
516 )
517 Since version 0.33.
519 Returns a new sequencing "Future" that behaves like an "else" call
521 which dispatches to a choice of several alternative handling functions
522 depending on the kind of failure that occurred. If $f1 fails with a
523 category name (i.e. the second argument to the "fail" call) which
524 exactly matches one of the string names given, then the corresponding
525 code is invoked, being passed the same arguments as a plain "else" call
526 would take, and is expected to return a "Future" in the same way.
527 $f2 = $code->( $exception, $name, @other_details )
529 If $f1 does not fail, fails without a category name at all, or fails
531 with a category name that does not match any given to the "catch"
532 method, then the returned sequence future immediately completes with
533 the same result, and no block of code is invoked.
534 If passed an odd-sized list, the final argument gives a function to
536 invoke on failure if no other handler matches.
537 $future = $f1->catch(
539 name => \&code, ...
540 \&fail_code,
541 )
542 This feature is currently still a work-in-progress. It currently can
544 only cope with category names that are literal strings, which are all
545 distinct. A later version may define other kinds of match (e.g.
546 regexp), may specify some sort of ordering on the arguments, or any of
547 several other semantic extensions. For more detail on the ongoing
548 design, see <https://rt.cpan.org/Ticket/Display.html?id=103545>.
549 then (multiple arguments)
551 $future = $f1->then( \&done_code, @catch_list, \&fail_code )
552 Since version 0.33.
554 The "then" method can be passed an even-sized list inbetween the
556 $done_code and the $fail_code, with the same meaning as the "catch"
557 method.
558 transform
560 $future = $f1->transform( %args )
561 Returns a new sequencing "Future" that wraps the one given as $f1. With
563 no arguments this will be a trivial wrapper; $future will complete or
564 fail when $f1 does, and $f1 will be cancelled when $future is.
565 By passing the following named arguments, the returned $future can be
567 made to behave differently to $f1:
568 done => CODE
570 Provides a function to use to modify the result of a successful
571 completion. When $f1 completes successfully, the result of its
572 "get" method is passed into this function, and whatever it
573 returns is passed to the "done" method of $future
574 fail => CODE
576 Provides a function to use to modify the result of a failure.
577 When $f1 fails, the result of its "failure" method is passed
578 into this function, and whatever it returns is passed to the
579 "fail" method of $future.
580 then_with_f
582 $future = $f1->then_with_f( ... )
583 Since version 0.21.
585 Returns a new sequencing "Future" that behaves like "then", but also
587 passes the original future, $f1, to any functions it invokes.
588 $f2 = $done_code->( $f1, @result )
590 $f2 = $catch_code->( $f1, $name, @other_details )
591 $f2 = $fail_code->( $f1, @details )
592 This is useful for conditional execution cases where the code block may
594 just return the same result of the original future. In this case it is
595 more efficient to return the original future itself.
596 then_done
598 then_fail
599 $future = $f->then_done( @result )
600 $future = $f->then_fail( $exception, @details )
602 Since version 0.22.
604 Convenient shortcuts to returning an immediate future from a "then"
606 block, when the result is already known.
607 else_with_f
609 $future = $f1->else_with_f( \&code )
610 Since version 0.21.
612 Returns a new sequencing "Future" that runs the code if the first
614 fails. Identical to "else", except that the code reference will be
615 passed both the original future, $f1, and its exception and details.
616 $f2 = $code->( $f1, $exception, @details )
618 This is useful for conditional execution cases where the code block may
620 just return the same result of the original future. In this case it is
621 more efficient to return the original future itself.
622 else_done
624 else_fail
625 $future = $f->else_done( @result )
626 $future = $f->else_fail( $exception, @details )
628 Since version 0.22.
630 Convenient shortcuts to returning an immediate future from a "else"
632 block, when the result is already known.
633 catch_with_f
635 $future = $f1->catch_with_f( ... )
636 Since version 0.33.
638 Returns a new sequencing "Future" that behaves like "catch", but also
640 passes the original future, $f1, to any functions it invokes.
641 followed_by
643 $future = $f1->followed_by( \&code )
644 Returns a new sequencing "Future" that runs the code regardless of
646 success or failure. Once $f1 is ready the code reference will be
647 invoked and is passed one argument, $f1. It should return a future,
648 $f2. Once $f2 completes the sequence future will then be marked as
649 complete with whatever result $f2 gave.
650 $f2 = $code->( $f1 )
652 without_cancel
654 $future = $f1->without_cancel
655 Since version 0.30.
657 Returns a new sequencing "Future" that will complete with the success
659 or failure of the original future, but if cancelled, will not cancel
660 the original. This may be useful if the original future represents an
661 operation that is being shared among multiple sequences; cancelling one
662 should not prevent the others from running too.
663 retain
665 $f = $f->retain
666 Since version 0.36.
668 Creates a reference cycle which causes the future to remain in memory
670 until it completes. Returns the invocant future.
671 In normal situations, a "Future" instance does not strongly hold a
673 reference to other futures that it is feeding a result into, instead
674 relying on that to be handled by application logic. This is normally
675 fine because some part of the application will retain the top-level
676 Future, which then strongly refers to each of its components down in a
677 tree. However, certain design patterns, such as mixed Future-based and
678 legacy callback-based API styles might end up creating Futures simply
679 to attach callback functions to them. In that situation, without
680 further attention, the Future may get lost due to having no strong
681 references to it. Calling "->retain" on it creates such a reference
682 which ensures it persists until it completes. For example:
683 Future->needs_all( $fA, $fB )
685 ->on_done( $on_done )
686 ->on_fail( $on_fail )
687 ->retain;
688
690 The following constructors all take a list of component futures, and
691 return a new future whose readiness somehow depends on the readiness of
692 those components. The first derived class component future will be used
693 as the prototype for constructing the return value, so it respects
694 subclassing correctly, or failing that a plain "Future".
695 wait_all
697 $future = Future->wait_all( @subfutures )
698 Returns a new "Future" instance that will indicate it is ready once all
700 of the sub future objects given to it indicate that they are ready,
701 either by success, failure or cancellation. Its result will be a list
702 of its component futures.
703 When given an empty list this constructor returns a new immediately-
705 done future.
706 This constructor would primarily be used by users of asynchronous
708 interfaces.
709 wait_any
711 $future = Future->wait_any( @subfutures )
712 Returns a new "Future" instance that will indicate it is ready once any
714 of the sub future objects given to it indicate that they are ready,
715 either by success or failure. Any remaining component futures that are
716 not yet ready will be cancelled. Its result will be the result of the
717 first component future that was ready; either success or failure. Any
718 component futures that are cancelled are ignored, apart from the final
719 component left; at which point the result will be a failure.
720 When given an empty list this constructor returns an immediately-failed
722 future.
723 This constructor would primarily be used by users of asynchronous
725 interfaces.
726 needs_all
728 $future = Future->needs_all( @subfutures )
729 Returns a new "Future" instance that will indicate it is ready once all
731 of the sub future objects given to it indicate that they have completed
732 successfully, or when any of them indicates that they have failed. If
733 any sub future fails, then this will fail immediately, and the
734 remaining subs not yet ready will be cancelled. Any component futures
735 that are cancelled will cause an immediate failure of the result.
736 If successful, its result will be a concatenated list of the results of
738 all its component futures, in corresponding order. If it fails, its
739 failure will be that of the first component future that failed. To
740 access each component future's results individually, use
741 "done_futures".
742 When given an empty list this constructor returns a new immediately-
744 done future.
745 This constructor would primarily be used by users of asynchronous
747 interfaces.
748 needs_any
750 $future = Future->needs_any( @subfutures )
751 Returns a new "Future" instance that will indicate it is ready once any
753 of the sub future objects given to it indicate that they have completed
754 successfully, or when all of them indicate that they have failed. If
755 any sub future succeeds, then this will succeed immediately, and the
756 remaining subs not yet ready will be cancelled. Any component futures
757 that are cancelled are ignored, apart from the final component left; at
758 which point the result will be a failure.
759 If successful, its result will be that of the first component future
761 that succeeded. If it fails, its failure will be that of the last
762 component future to fail. To access the other failures, use
763 "failed_futures".
764 Normally when this future completes successfully, only one of its
766 component futures will be done. If it is constructed with multiple that
767 are already done however, then all of these will be returned from
768 "done_futures". Users should be careful to still check all the results
769 from "done_futures" in that case.
770 When given an empty list this constructor returns an immediately-failed
772 future.
773 This constructor would primarily be used by users of asynchronous
775 interfaces.
776
778 The following methods apply to convergent (i.e. non-leaf) futures, to
779 access the component futures stored by it.
780 pending_futures
782 @f = $future->pending_futures
783 ready_futures
785 @f = $future->ready_futures
786 done_futures
788 @f = $future->done_futures
789 failed_futures
791 @f = $future->failed_futures
792 cancelled_futures
794 @f = $future->cancelled_futures
795 Return a list of all the pending, ready, done, failed, or cancelled
797 component futures. In scalar context, each will yield the number of
798 such component futures.
799
801 set_label
802 label
803 $future = $future->set_label( $label )
804 $label = $future->label
806 Since version 0.28.
808 Chaining mutator and accessor for the label of the "Future". This
810 should be a plain string value, whose value will be stored by the
811 future instance for use in debugging messages or other tooling, or
812 similar purposes.
813 btime
815 rtime
816 [ $sec, $usec ] = $future->btime
817 [ $sec, $usec ] = $future->rtime
819 Since version 0.28.
821 Accessors that return the tracing timestamps from the instance. These
823 give the time the instance was constructed ("birth" time, "btime") and
824 the time the result was determined (the "ready" time, "rtime"). Each
825 result is returned as a two-element ARRAY ref, containing the epoch
826 time in seconds and microseconds, as given by
827 "Time::HiRes::gettimeofday".
828 In order for these times to be captured, they have to be enabled by
830 setting $Future::TIMES to a true value. This is initialised true at the
831 time the module is loaded if either "PERL_FUTURE_DEBUG" or
832 "PERL_FUTURE_TIMES" are set in the environment.
833 elapsed
835 $sec = $future->elapsed
836 Since version 0.28.
838 If both tracing timestamps are defined, returns the number of seconds
840 of elapsed time between them as a floating-point number. If not,
841 returns "undef".
842 wrap_cb
844 $cb = $future->wrap_cb( $operation_name, $cb )
845 Since version 0.31.
847 Note: This method is experimental and may be changed or removed in a
849 later version.
850 This method is invoked internally by various methods that are about to
852 save a callback CODE reference supplied by the user, to be invoked
853 later. The default implementation simply returns the callback argument
854 as-is; the method is provided to allow users to provide extra
855 behaviour. This can be done by applying a method modifier of the
856 "around" kind, so in effect add a chain of wrappers. Each wrapper can
857 then perform its own wrapping logic of the callback. $operation_name is
858 a string giving the reason for which the callback is being saved;
859 currently one of "on_ready", "on_done", "on_fail" or "sequence"; the
860 latter being used for all the sequence-returning methods.
861 This method is intentionally invoked only for CODE references that are
863 being saved on a pending "Future" instance to be invoked at some later
864 point. It does not run for callbacks to be invoked on an already-
865 complete instance. This is for performance reasons, where the intended
866 behaviour is that the wrapper can provide some amount of context save
867 and restore, to return the operating environment for the callback back
868 to what it was at the time it was saved.
869 For example, the following wrapper saves the value of a package
871 variable at the time the callback was saved, and restores that value at
872 invocation time later on. This could be useful for preserving context
873 during logging in a Future-based program.
874 our $LOGGING_CTX;
876 no warnings 'redefine';
878 my $orig = Future->can( "wrap_cb" );
880 *Future::wrap_cb = sub {
881 my $cb = $orig->( @_ );
882 my $saved_logging_ctx = $LOGGING_CTX;
884 return sub {
886 local $LOGGING_CTX = $saved_logging_ctx;
887 $cb->( @_ );
888 };
889 };
890 At this point, any code deferred into a "Future" by any of its
892 callbacks will observe the $LOGGING_CTX variable as having the value it
893 held at the time the callback was saved, even if it is invoked later on
894 when that value is different.
895 Remember when writing such a wrapper, that it still needs to invoke the
897 previous version of the method, so that it plays nicely in combination
898 with others (see the "$orig->( @_ )" part).
899
901 The following examples all demonstrate possible uses of a "Future"
902 object to provide a fictional asynchronous API.
903 For more examples, comparing the use of "Future" with regular
905 call/return style Perl code, see also Future::Phrasebook.
906 Providing Results
908 By returning a new "Future" object each time the asynchronous function
909 is called, it provides a placeholder for its eventual result, and a way
910 to indicate when it is complete.
911 sub foperation
913 {
914 my %args = @_;
915 my $future = Future->new;
917 do_something_async(
919 foo => $args{foo},
920 on_done => sub { $future->done( @_ ); },
921 );
922 return $future;
924 }
925 In most cases, the "done" method will simply be invoked with the entire
927 result list as its arguments. In that case, it is convenient to use the
928 curry module to form a "CODE" reference that would invoke the "done"
929 method.
930 my $future = Future->new;
932 do_something_async(
934 foo => $args{foo},
935 on_done => $future->curry::done,
936 );
937 The caller may then use this future to wait for a result using the
939 "on_ready" method, and obtain the result using "get".
940 my $f = foperation( foo => "something" );
942 $f->on_ready( sub {
944 my $f = shift;
945 say "The operation returned: ", $f->get;
946 } );
947 Indicating Success or Failure
949 Because the stored exception value of a failed future may not be false,
950 the "failure" method can be used in a conditional statement to detect
951 success or failure.
952 my $f = foperation( foo => "something" );
954 $f->on_ready( sub {
956 my $f = shift;
957 if( not my $e = $f->failure ) {
958 say "The operation succeeded with: ", $f->get;
959 }
960 else {
961 say "The operation failed with: ", $e;
962 }
963 } );
964 By using "not" in the condition, the order of the "if" blocks can be
966 arranged to put the successful case first, similar to a "try"/"catch"
967 block.
968 Because the "get" method re-raises the passed exception if the future
970 failed, it can be used to control a "try"/"catch" block directly. (This
971 is sometimes called Exception Hoisting).
972 use Try::Tiny;
974 $f->on_ready( sub {
976 my $f = shift;
977 try {
978 say "The operation succeeded with: ", $f->get;
979 }
980 catch {
981 say "The operation failed with: ", $_;
982 };
983 } );
984 Even neater still may be the separate use of the "on_done" and
986 "on_fail" methods.
987 $f->on_done( sub {
989 my @result = @_;
990 say "The operation succeeded with: ", @result;
991 } );
992 $f->on_fail( sub {
993 my ( $failure ) = @_;
994 say "The operation failed with: $failure";
995 } );
996 Immediate Futures
998 Because the "done" method returns the future object itself, it can be
999 used to generate a "Future" that is immediately ready with a result.
1000 This can also be used as a class method.
1001 my $f = Future->done( $value );
1003 Similarly, the "fail" and "die" methods can be used to generate a
1005 "Future" that is immediately failed.
1006 my $f = Future->die( "This is never going to work" );
1008 This could be considered similarly to a "die" call.
1010 An "eval{}" block can be used to turn a "Future"-returning function
1012 that might throw an exception, into a "Future" that would indicate this
1013 failure.
1014 my $f = eval { function() } || Future->fail( $@ );
1016 This is neater handled by the "call" class method, which wraps the call
1018 in an "eval{}" block and tests the result:
1019 my $f = Future->call( \&function );
1021 Sequencing
1023 The "then" method can be used to create simple chains of dependent
1024 tasks, each one executing and returning a "Future" when the previous
1025 operation succeeds.
1026 my $f = do_first()
1028 ->then( sub {
1029 return do_second();
1030 })
1031 ->then( sub {
1032 return do_third();
1033 });
1034 The result of the $f future itself will be the result of the future
1036 returned by the final function, if none of them failed. If any of them
1037 fails it will fail with the same failure. This can be considered
1038 similar to normal exception handling in synchronous code; the first
1039 time a function call throws an exception, the subsequent calls are not
1040 made.
1041 Merging Control Flow
1043 A "wait_all" future may be used to resynchronise control flow, while
1044 waiting for multiple concurrent operations to finish.
1045 my $f1 = foperation( foo => "something" );
1047 my $f2 = foperation( bar => "something else" );
1048 my $f = Future->wait_all( $f1, $f2 );
1050 $f->on_ready( sub {
1052 say "Operations are ready:";
1053 say " foo: ", $f1->get;
1054 say " bar: ", $f2->get;
1055 } );
1056 This provides an ability somewhat similar to "CPS::kpar()" or
1058 Async::MergePoint.
1059
1061 Cancellation of Non-Final Sequence Futures
1062 The behaviour of future cancellation still has some unanswered
1063 questions regarding how to handle the situation where a future is
1064 cancelled that has a sequence future constructed from it.
1065 In particular, it is unclear in each of the following examples what the
1067 behaviour of $f2 should be, were $f1 to be cancelled:
1068 $f2 = $f1->then( sub { ... } ); # plus related ->then_with_f, ...
1070 $f2 = $f1->else( sub { ... } ); # plus related ->else_with_f, ...
1072 $f2 = $f1->followed_by( sub { ... } );
1074 In the "then"-style case it is likely that this situation should be
1076 treated as if $f1 had failed, perhaps with some special message. The
1077 "else"-style case is more complex, because it may be that the entire
1078 operation should still fail, or it may be that the cancellation of $f1
1079 should again be treated simply as a special kind of failure, and the
1080 "else" logic run as normal.
1081 To be specific; in each case it is unclear what happens if the first
1083 future is cancelled, while the second one is still waiting on it. The
1084 semantics for "normal" top-down cancellation of $f2 and how it affects
1085 $f1 are already clear and defined.
1086 Cancellation of Divergent Flow
1088 A further complication of cancellation comes from the case where a
1089 given future is reused multiple times for multiple sequences or
1090 convergent trees.
1091 In particular, it is in clear in each of the following examples what
1093 the behaviour of $f2 should be, were $f1 to be cancelled:
1094 my $f_initial = Future->new; ...
1096 my $f1 = $f_initial->then( ... );
1097 my $f2 = $f_initial->then( ... );
1098 my $f1 = Future->needs_all( $f_initial );
1100 my $f2 = Future->needs_all( $f_initial );
1101 The point of cancellation propagation is to trace backwards through
1103 stages of some larger sequence of operations that now no longer need to
1104 happen, because the final result is no longer required. But in each of
1105 these cases, just because $f1 has been cancelled, the initial future
1106 $f_initial is still required because there is another future ($f2) that
1107 will still require its result.
1108 Initially it would appear that some kind of reference-counting
1110 mechanism could solve this question, though that itself is further
1111 complicated by the "on_ready" handler and its variants.
1112 It may simply be that a comprehensive useful set of cancellation
1114 semantics can't be universally provided to cover all cases; and that
1115 some use-cases at least would require the application logic to give
1116 extra information to its "Future" objects on how they should wire up
1117 the cancel propagation logic.
1118 Both of these cancellation issues are still under active design
1120 consideration; see the discussion on RT96685 for more information
1121 (<https://rt.cpan.org/Ticket/Display.html?id=96685>).
1122
1124 · Promises - an implementation of the "Promise/A+" pattern for
1125 asynchronous programming
1126 · curry - Create automatic curried method call closures for any class
1128 or object
1129 · "The Past, The Present and The Future" - slides from a talk given
1131 at the London Perl Workshop, 2012.
1132 <https://docs.google.com/presentation/d/1UkV5oLcTOOXBXPh8foyxko4PR28_zU_aVx6gBms7uoo/edit>
1134 · "Futures advent calendar 2013"
1136 <http://leonerds-code.blogspot.co.uk/2013/12/futures-advent-day-1.html>
1138 · "Asynchronous Programming with Futures" - YAPC::EU 2014
1140 <https://www.youtube.com/watch?v=u9dZgFM6FtE>
1142
1144 · Consider the ability to pass the constructor an "await" CODEref,
1145 instead of needing to use a subclass. This might simplify
1146 async/etc.. implementations, and allows the reuse of the idea of
1147 subclassing to extend the abilities of "Future" itself - for
1148 example to allow a kind of Future that can report incremental
1149 progress.
1150
1152 Paul Evans <leonerd@leonerd.org.uk>
1153perl v5.28.0 2018-07-14 Future(3)