focal (3) Future::AsyncAwait.3pm.gz
NAME
"Future::AsyncAwait" - deferred subroutine syntax for futures
SYNOPSIS
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 is
considering adding it.
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;
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 "await"ing 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.
STABILITY WARNING
This module is still relatively new and under active development. While it now seems relatively stable
enough for most use-cases, there may still be a number of memory leaks left in it, especially if still-
pending futures are abandoned.
While it seems stable enough for small-scale development and experimental testing, take care when using
this module in production, as some growth in memory over time may be observed. Careful use of monitoring
and periodic restarts of long-running processes may be a wise precaution.
That said, using this module in places like unit-tests and short-term scripts does appear to be quite
stable, so do try experimenting with it in this sort of situation, and let me know what does and doesn't
work.
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 $message;
}
On perl versions 5.26 and later "async sub" syntax supports the "signatures" feature if it is enabled:
use 5.026;
use feature 'signatures';
async sub quart($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:
• There is currently no way to perform the equivalent of "on_cancel" in Future to add a cancellation
callback to a future chain.
• 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 sub"s 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";
}
}
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";
}
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>