/ 
Future-AsyncAwait-0.06
River stage three • 63 direct dependents • 116 total dependents
/ Future::AsyncAwait

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.

EARLY-VERSION WARNING

WARNING: The actual semantics in this module are in an early state of implementation. Some things work but most do not. Don't expect to be able to use this module for much real code yet.

Things That Work Already

Any function that doesn't actually await anything, and just returns immediate futures is already working fine with this module.

Instead of writing

sub foo
{
   ...
   return Future->done( @result );
}

you can now simply write

async sub foo
{
   ...
   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.

In addition, some simple cases involving awaiting on still-pending futures should be working:

async sub bar
{
   my ( $f ) = @_;

   return 1 + await( $f ) + 3;
}

async sub splot
{
   while( COND ) {
      await func();
   }
}

async sub wibble
{
   if( COND ) {
      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

Any code that attempts to await from inside any sort of foreach loop does not currently work:

async sub wobble
{
   foreach ( THINGs ) {
      await func();
   }
}

local variable assignments inside an async function will confuse the suspend mechanism:

our $DEBUG = 0;

async sub quark
{
   local $DEBUG = 1;
   await func();
}

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.

TODO

  • Suspend and resume over foreach loops, in all their various flavours.

  • Suspend and resume with some consideration for the savestack; i.e. the area used to implement local and similar:

  • Clean up the implementation; check for and fix memory leaks.

  • Support older versions of perl than 5.24.

    https://rt.cpan.org/Ticket/Display.html?id=122252

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>