Provided by: libfuture-asyncawait-perl_0.13-2_amd64 
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, and lately even 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 forces the return value of the function to always be 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.
"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.
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.
EARLY-VERSION WARNING
WARNING: The actual semantics in this module are in an early state of implementation. Some
things will randomly break. While it seems stable enough for small-scale development and
experimental testing, don't expect to be able to use this module reliably in production
yet.
Things That Work
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 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;
}
Things That Don't Yet Work
"local" variable assignments inside an "async" function will confuse the suspend
mechanism:
our $DEBUG = 0;
async sub quark
{
local $DEBUG = 1;
await func();
}
Since "foreach" loops on non-lexical iterator variables (usually package variables)
effectively imply a "local"-like behaviour, these are also disallowed.
our $VAR;
async sub splurt
{
foreach $VAR ( LIST ) {
await ...
}
}
Additionally, complications with the savestack appear to be affecting some uses of
package-level "our" variables captured by async functions:
our $VAR;
async sub bork
{
print "VAR is $VAR\n";
await func();
}
See also the "TODO" list for further things.
Async Without Await
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.
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".
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".
Some notes on what makes the problem hard can be found at
<https://rt.cpan.org/Ticket/Display.html?id=122793>
• Clean up the implementation; check for and fix memory leaks.
• Support older versions of perl than 5.18.
<https://rt.cpan.org/Ticket/Display.html?id=122252>
• Support sub signatures in recent perls.
<https://rt.cpan.org/Ticket/Display.html?id=124122>
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.
AUTHOR
Paul Evans <leonerd@leonerd.org.uk>