NAME
Future::AsyncAwait - deferred subroutine syntax for futures
SYNOPSIS
use v5.14;
use Future::AsyncAwait;
async sub do_a_thing
{
my $first = await do_first_thing();
my $second = await do_second_thing();
return combine_things( $first, $second );
}
do_a_thing()->get;
DESCRIPTION
This module provides syntax for deferring and resuming subroutines
while waiting for Futures to complete. This syntax aims to make code
that performs asynchronous operations using futures look neater and
more expressive than simply using then chaining and other techniques on
the futures themselves. It is also a similar syntax used by a number of
other languages; notably C# 5, EcmaScript 6, Python 3, Dart, Rust,
C++20.
This module is still under active development. While it now seems
relatively stable enough for most use-cases and has received a lot of
"battle-testing" in a wide variety of scenarios, there may still be the
occasional case of memory leak left in it, especially if still-pending
futures are abandoned.
The new syntax takes the form of two new keywords, async and await.
async
The async keyword should appear just before the sub keyword that
declares a new function. When present, this marks that the function
performs its work in a potentially asynchronous fashion. This has two
effects: it permits the body of the function to use the await
expression, and it wraps the return value of the function in a Future
instance.
async sub myfunc
{
return 123;
}
my $f = myfunc();
my $result = $f->get;
As well as named function declarations it is also supported on
anonymous function expressions.
my $code = async sub { return 456 };
my $f = $code->();
my $result = $f->get;
This async-declared function always returns a Future instance when
invoked. The returned future instance will eventually complete when the
function returns, either by the return keyword or by falling off the
end; the result of the future will be the return value from the
function's code. Alternatively, if the function body throws an
exception, this will cause the returned future to fail.
If the final expression in the body of the function returns a Future,
don't forget to await it rather than simply returning it as it is, or
else this return value will become double-wrapped - almost certainly
not what you wanted.
async sub otherfunc { ... }
async sub myfunc
{
...
return await otherfunc();
}
await
The await keyword forms an expression which takes a Future instance as
an operand and yields the eventual result of it. Superficially it can
be thought of similar to invoking the get method on the future.
my $result = await $f;
my $result = $f->get;
However, the key difference (and indeed the entire reason for being a
new syntax keyword) is the behaviour when the future is still pending
and is not yet complete. Whereas the simple get method would block
until the future is complete, the await keyword causes its entire
containing function to become suspended, making it return a new
(pending) future instance. It waits in this state until the future it
was waiting on completes, at which point it wakes up and resumes
execution from the point of the await expression. When the now-resumed
function eventually finishes (either by returning a value or throwing
an exception), this value is set as the result of the future it had
returned earlier.
await provides scalar context to its controlling expression.
async sub func {
# this function is invoked in scalar context
}
await func();
Because the await keyword may cause its containing function to suspend
early, returning a pending future instance, it is only allowed inside
async-marked subs.
The converse is not true; just because a function is marked as async
does not require it to make use of the await expression. It is still
useful to turn the result of that function into a future, entirely
without awaiting on any itself.
Any function that doesn't actually await anything, and just returns
immediate futures can be neatened by this module too.
Instead of writing
sub imm
{
...
return Future->done( @result );
}
you can now simply write
async sub imm
{
...
return @result;
}
with the added side-benefit that any exceptions thrown by the elided
code will be turned into an immediate-failed Future rather than making
the call itself propagate the exception, which is usually what you
wanted when dealing with futures.
await (toplevel)
Since version 0.47.
An await expression is also permitted directly in the main script at
toplevel, outside of async sub. This is implemented by simply invoking
the get method on the future value. Thus, the following two lines are
directly equivalent:
await afunc();
afunc()->get;
This is provided as a syntax convenience for unit tests, toplevel
scripts, and so on. It allows code to be written in a style that can be
easily moved into an async sub, and avoids encouraging "bad habits" of
invoking the get method directly.
CANCEL
Experimental. Since version 0.44.
The CANCEL keyword declares a block of code which will be run in the
event that the future returned by the async sub is cancelled.
async sub f
{
CANCEL { warn "This task was cancelled"; }
await ...
}
f()->cancel;
A CANCEL block is a self-contained syntax element, similar to perl
constructions like BEGIN, and does not need a terminating semicolon.
When a CANCEL block is encountered during execution of the async sub,
the code in its block is stored for the case that the returned future
is cancelled. Each will take effect as it is executed, possibly
multiple times if it appears inside a loop, or not at all if it appears
conditionally in a branch that was not executed.
async sub g
{
if(0) {
CANCEL { warn "This does not happen"; }
}
foreach my $x ( 1..3 ) {
CANCEL { warn "This happens for x=$x"; }
}
await ...
}
g()->cancel;
CANCEL blocks are only invoked if a still-pending future is cancelled.
They are discarded without being executed if the function finishes;
either successfully or if it throws an exception.
Experimental Features
Some of the features of this module are currently marked as
experimental. They will provoke warnings in the experimental category,
unless silenced.
You can silence this with no warnings 'experimental' but then that will
silence every experimental warning, which may hide others
unintentionally. For a more fine-grained approach you can instead use
the import line for this module to only silence this module's warnings
selectively:
use Future::AsyncAwait qw( :experimental(cancel) );
use Future::AsyncAwait qw( :experimental ); # all of the above
SUPPORTED USES
Most cases involving awaiting on still-pending futures should work
fine:
async sub foo
{
my ( $f ) = @_;
BEFORE();
await $f;
AFTER();
}
async sub bar
{
my ( $f ) = @_;
return 1 + await( $f ) + 3;
}
async sub splot
{
while( COND ) {
await func();
}
}
async sub wibble
{
if( COND ) {
await func();
}
}
async sub wobble
{
foreach my $var ( THINGs ) {
await func();
}
}
async sub quux
{
my $x = do {
await func();
};
}
async sub splat
{
eval {
await func();
};
}
Plain lexical variables are preserved across an await deferral:
async sub quux
{
my $message = "Hello, world\n";
await func();
print $message;
}
On perl versions 5.26 and later async sub syntax supports the
signatures feature if it is enabled:
use v5.26;
use feature 'signatures';
async sub quart($x, $y)
{
...
}
Cancellation
Cancelled futures cause a suspended async sub to simply stop running.
async sub fizz
{
await func();
say "This is never reached";
}
my $f = fizz();
$f->cancel;
Cancellation requests can propagate backwards into the future the async
sub is currently waiting on.
async sub floof
{
...
await $f1;
}
my $f2 = floof();
$f2->cancel; # $f1 will be cancelled too
This behaviour is still more experimental than the rest of the logic.
The following should be noted:
* Cancellation propagation is only implemented on Perl version 5.24
and above. An async sub in an earlier perl version will still stop
executing if cancelled, but will not propagate the request backwards
into the future that the async sub is currently waiting on. See
"TODO".
SUBCLASSING Future
By default when an async sub returns a result or fails immediately
before awaiting, it will return a new completed instance of the Future
class. In order to allow code that wishes to use a different class to
represent futures the module import method can be passed the name of a
class to use instead.
use Future::AsyncAwait future_class => "Subclass::Of::Future";
async sub func { ... }
This has the usual lexically-scoped effect, applying only to async subs
defined within the block; others are unaffected.
use Future::AsyncAwait;
{
use Future::AsyncAwait future_class => "Different::Future";
async sub x { ... }
}
async sub y { ... } # returns a regular Future
This will only affect immediate results. If the await keyword has to
suspend the function and create a new pending future, it will do this
by using the prototype constructor on the future it itself is waiting
on, and the usual subclass-respecting semantics of "new" in Future will
remain in effect there. As such it is not usually necessary to use this
feature just for wrapping event system modules or other similar
situations.
Such an alternative subclass should implement the API documented by
Future::AsyncAwait::Awaitable.
WITH OTHER MODULES
Syntax::Keyword::Try
As of Future::AsyncAwait version 0.10 and Syntax::Keyword::Try version
0.07, cross-module integration tests assert that basic try/catch blocks
inside an async sub work correctly, including those that attempt to
return from inside try.
use Future::AsyncAwait;
use Syntax::Keyword::Try;
async sub attempt
{
try {
await func();
return "success";
}
catch {
return "failed";
}
}
As of Future::AsyncAwait version 0.50, finally blocks are invoked even
during cancellation.
Syntax::Keyword::Dynamically
As of Future::AsyncAwait version 0.32, cross-module integration tests
assert that the dynamically correctly works across an await boundary.
use Future::AsyncAwait;
use Syntax::Keyword::Dynamically;
our $var;
async sub trial
{
dynamically $var = "value";
await func();
say "Var is still $var";
}
Syntax::Keyword::Defer
As of Future::AsyncAwait version 0.50, defer blocks are invoked even
during cancellation.
use Future::AsyncAwait;
use Syntax::Keyword::Defer;
async sub perhaps
{
defer { say "Cleaning up now" }
await $f1;
}
my $fouter = perhaps();
$fouter->cancel;
Object::Pad
As of Future::AsyncAwait version 0.38 and Object::Pad version 0.15,
both modules now use XS::Parse::Sublike to parse blocks of code.
Because of this the two modules can operate together and allow class
methods to be written as async subs which await expressions:
use Future::AsyncAwait;
use Object::Pad;
class Example
{
async method perform($block)
{
say "$self is performing code";
await $block->();
say "code finished";
}
}
SEE ALSO
* "Awaiting The Future" - TPC in Amsterdam 2017
https://www.youtube.com/watch?v=Xf7rStpNaT0 (slides)
<https://docs.google.com/presentation/d/13x5l8Rohv_RjWJ0OTvbsWMXKoNEWREZ4GfKHVykqUvc/edit#slide=id.p>
TODO
* Suspend and resume with some consideration for the savestack; i.e.
the area used to implement local and similar. While in general local
support has awkward questions about semantics, there are certain
situations and cases where internally-implied localisation of
variables would still be useful and can be supported without the
semantic ambiguities of generic local.
our $DEBUG = 0;
async sub quark
{
local $DEBUG = 1;
await func();
}
Since foreach loops on non-lexical iterator variables (usually the $_
global variable) effectively imply a local-like behaviour, these are
also disallowed.
async sub splurt
{
foreach ( LIST ) {
await ...
}
}
Some notes on what makes the problem hard can be found at
https://rt.cpan.org/Ticket/Display.html?id=122793
* Currently this module requires perl version 5.16 or later.
Additionally, threaded builds of perl earlier than 5.22 are not
supported.
https://rt.cpan.org/Ticket/Display.html?id=122252
https://rt.cpan.org/Ticket/Display.html?id=124351
* Implement cancel back-propagation for Perl versions earlier than
5.24. Currently this does not work due to some as-yet-unknown effects
that installing the back-propagation has, causing future instances to
be reclaimed too early.
https://rt.cpan.org/Ticket/Display.html?id=129202
KNOWN BUGS
This is not a complete list of all known issues, but rather a summary
of the most notable ones that currently prevent the module from working
correctly in a variety of situations. For a complete list of known
bugs, see the RT queue at
https://rt.cpan.org/Dist/Display.html?Name=Future-AsyncAwait.
* await inside map or grep blocks does not work. This is due to the
difficulty of detecting the map or grep context from internal perl
state at suspend time, sufficient to be able to restore it again when
resuming.
https://rt.cpan.org/Ticket/Display.html?id=129748
As a workaround, consider converting a map expression to the
equivalent form using push onto an accumulator array with a foreach
loop:
my @results = map { await func($_) } ITEMS;
becomes
my @results;
foreach my $item ( ITEMS ) {
push @results, await func($item);
}
with a similar transformation for grep expressions.
Alternatively, consider using the fmap* family of functions from
Future::Utils to provide a concurrent version of the same code, which
can keep multiple items running concurrently:
use Future::Utils qw( fmap );
my @results = await fmap { func( shift ) }
foreach => [ ITEMS ],
concurrent => 5;
ACKNOWLEDGEMENTS
With thanks to Zefram, ilmari and others from irc.perl.org/#p5p for
assisting with trickier bits of XS logic.
Thanks to genio for project management and actually reminding me to
write some code.
Thanks to The Perl Foundation for sponsoring me to continue working on
the implementation.
AUTHOR
Paul Evans <leonerd@leonerd.org.uk>