Future::AsyncAwait - deferred subroutine syntax for futures


       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 );


    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,

    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.


    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

       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();


    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();

    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.


    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 ...

    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 ...

    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

       use Future::AsyncAwait qw( :experimental(cancel) );
       use Future::AsyncAwait qw( :experimental );  # all of the above


    Most cases involving awaiting on still-pending futures should work

       async sub foo
          my ( $f ) = @_;
          await $f;
       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 $var ( THINGs ) {
             await func();
       async sub wubble
          # on perl 5.35.5 and above
          foreach my ($k, $v) ( KVTHINGs ) {
             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)

    Since version 0.55 any exceptions thrown by signature validation
    (because of too few or too many arguments being passed) are thrown
    synchronously, and do not result in a failed Future instance.


    Cancelled futures cause a suspended async sub to simply stop running.

       async sub fizz
          await func();
          say "This is never reached";
       my $f = fizz();

    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


    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

    Such an alternative subclass should implement the API documented by



    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.


    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";


    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();


    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";


    As of Future::AsyncAwait version 0.55 and Syntax::Keyword::MultiSub
    version 0.02 a cross-module integration test asserts that the multi
    modifier can be applied to async sub.

       use Future::AsyncAwait;
       use Syntax::Keyword::MultiSub;
       async multi sub f () { return "nothing"; }
       async multi sub f ($key) { return await get_thing($key); }


      * "Awaiting The Future" - TPC in Amsterdam 2017 (slides)


      * 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

      * Currently this module requires perl version 5.16 or later.
      Additionally, threaded builds of perl earlier than 5.22 are not

      * 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.


    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

      * 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

      As a workaround, consider converting a map expression to the
      equivalent form using push onto an accumulator array with a foreach

         my @results = map { await func($_) } ITEMS;


         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;


    With thanks to Zefram, ilmari and others from 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.


    Paul Evans <>