Lukas Atkinson


Async::Trampoline - Trampolining functions with async/await syntax



    use Async::Trampoline qw(
        async async_value async_error async_cancel

    use Async::Trampoline ':all';

Creating Asyncs

    $async = async_value 1, 2, 3;
    $async = async_error "oops";
    $async = async_cancel;
    $async = async { ...; return $new_async };

Running Asyncs

    @result = $async->run_until_completion;

Combining Asyncs

    $async = $other_async->await(sub {
        my (@values) = @_;
        # ...
        return $new_async;

    $async = await [$x, $y] => sub {
        my (@x_and_y_values) = @_;
        # ...
        return $new_async;

    $async = $x->complete_then($y);
    $async = $x->resolved_or($y);
    $async = $x->resolved_then($y);
    $async = $x->value_or($y);
    $async = $x->value_then($y);

    $async = $x->concat($y);


    $gen = async_yield async_value(1, 2, 3) => sub {
        # ...
        return $next_generator;

    $gen = $gen->gen_map(sub {
        my (@values) = @_;
        # ...
        return $new_async;

    $async = $gen->gen_foreach(sub {
        my (@values) = @_;
        return async_cancel if not @values;  # like "last" in Perl
        # ...
        return async_value;  # like "next" in Perl

    $async = $gen->gen_collect;

Misc. accessors

    $str = $async->to_string;

    $bool = $async->is_complete;
    $bool = $async->is_cancelled;
    $bool = $async->is_error;
    $bool = $async->is_value;


Trampolines are a functional programming technique to implement complex control flow: Instead of returning a result from a function, we can return another function that will at some point return a result. The trampoline keeps invoking the returned function until a result is returned. Importantly, such trampolines eliminate tail calls.

This programming style is powerful but inconvenient because you tend to get callback hell. This module implements simple Futures with an async/await syntax. Instead of nesting the callbacks, we can now chain callbacks more easily.

This module was initially created in order to write recursive algorithms around compiler construction: recursive-descent parsers and recursive tree traversal. However, it is certainly applicable to other problems as well. The module is written in C++ to keep runtime overhead minimal.

Example: loop


    my @items;

    my $i = 5;
    while ($i) {
        push @items, $i--;


    sub loop {
        my ($items, $i) = @_;
        return $items if not $i;
        push @$items, $i--;
        return loop($items, $i);  # may lead to deep recursion!

    my $items = loop([], 5);


    sub loop_async {
        my ($items, $i) = @_;
        return async_value $items if not $i;
        push @$items, $i--;
        return async { loop_async($items, $i) };

    my $items = loop_async([], 5)->run_until_completion;


    sub loop_gen {
        my ($i) = @_;
        return async_cancel if not $i;
        return async_yield async_value($i) => sub {
            return loop_gen($i - 1);

    my $items = loop_gen(5)->gen_collect->run_until_completion;


Each Async exists in one of these states:

    +-- Incomplete
        +-- ... (internal)
    +-- Complete
        +-- Cancelled
        +-- Resolved
            +-- Error
            +-- Value

In Incomplete states, the Async will be processed in the future. At some point, the Async will transition to a completed state.

In async and await callbacks, the Async will be updated to the state of the return value of that callback.

Completed states are terminal. The Asyncs are not subject to further processing.

A Cancelled Async represents an aborted computation. They have no value. Cancellation is not an error, but run_until_completion() will die when the Async was cancelled. You can cancel a computation via the async_cancel constructor. Cancellation is useful to abort loops, or to fall back to an alternative with $may_cancel->resolved_or($alternative).

Resolved Asyncs are Completed Asyncs that finished their computation and have a value, either an Error or a Value upon success.

An Error Async indicates that a runtime error occurred. Error Asyncs can be created with the async_error constructor, or when a callback throws. The exception will be rethrown by run_until_completion().

A Value Async contains a list of Perl values. They can be created with the async_value constructor. The values will be returned by run_until_completion(). To access the values of an Async, you can await it.



    $async = async { ... };

Create an Incomplete Async with a code block. The callback must return an Async. When the Async is evaluated, this Async is updated to the state of the returned Async.


    $async = async_value @values;

Create a Value Async containing a list of values. Use this to return values from an Async callback.


    $async = async_error $error;

Create an Error Async with the specified error. The error may be a string or error object. Use this to fail an Async. Alternatively, you can die() inside the Async callback.


    $async = async_cancel;

Create a Cancelled Async. Use this to abort an Async without using an error.



    $async = $dependency->await(sub {
        my (@result) = @_;
        # ...
        return $new_async;

    $async = await $dependency => sub {
        my (@result) = @_;
        # ...
        return $new_async;

    $async = await [@dependencies] => sub {
        my (@results) = @_;
        # ...
        return $new_async;

Wait until the $dependency or @dependencies Asyncs have a value, then call the callback with the values as arguments. If a dependency was cancelled or has an error, the async is updated to that state. The callback must return an Async. Use this to chain Asyncs. It does not directly return the values.



    $async = $first_async->resolved_or($alternative_async);
    $async = $first_async->value_or($alternative_async);

Evaluate the $first_async. Upon success, the Async is updated to the state of the $first_async. On failure, the $second_async is evaluated instead. This creates a new Async that will be updated when the dependencies become available.

resolved_or succeeds on Value or Error, and fails on Cancelled. Use this as a fallback against cancellation.

value_or only succeeds on Value, and fails on Cancelled or Error. Use this as a try-catch to provide default values upon errors.




    $async = $first_async->complete_then($second_async);
    $async = $first_async->resolved_then($second_async);
    $async = $first_async->value_then($second_async);

Evaluate the $first_async. Upon success, the $second_async is evaluated. On failure, the Async is updated to the state of the $first_async. This creates a new Async that will be updated when the dependencies become available.

complete_then always succeeds (Cancelled, Error, Value).

resolved_then succeeds on Error or Value, and fails on Cancelled.

value_then succeeds on Value, and fails on Cancelled or Error. With regards to error propagation, $x->value_then($y) behaves just like $x->await(sub { return $y }.

Use these functions to sequence actions on success and discarding their value. They are like a semicolon ; in Perl, but with different levels of error propagation. You may want to sequence Asyncs if any Async causes side effects.


    $async = $first_async->concat($second_async);

If both asyncs evaluate to Values, concatenate the values.


    $async = (async_value 1, 2, 3)->concat(async_value 4, 5);
    #=> async_value 1, 2, 3, 4, 5


A Generator describes an Async that has a continuation Async as its first value. This continuation can be awaited to get the next continuation + value. If the generator is Cancelled, no further items are available. Errors are propagated.

Generators are useful for yielding a stream of values.

You can use async_yield() to conveniently return a value with a continuation. The gen_*() Async methods can process generator streams. They will fail at runtime when the Async is not a valid generator.

The most flexible way to handle generators is to await() them. However, many use cases are better served by more specialized functions.

Example: a count down generator:

    sub count_down_generator {
        my ($i) = @_;
        return async_cancel if $i < 0;
        return async_yield async_value($i) => sub {
            return count_down_generator($i - 1);

    my $countdown_gen = count_down_generator(10);

Example: transforming a stream:

    $countdown_gen = $countdown_gen->gen_map(sub {
        my ($i) = @_;
        return async_value "ignition" if $i == 3;
        return async_value "liftoff"  if $i == 0;
        return async_value $i;

Example: consuming a stream:

    my $finished_async = $countdown_gen->gen_foreach(sub {
        my ($i) = @_;
        say $i;
        return async_value;  # request next item

Example: repeating each element:

    sub repeat_gen {
        my ($gen) = @_;
        return $gen->await(sub {
            my ($continuation, $x) = @_;
            return async_yield async_value($x) => sub {
                return async_yield async_value($x) => sub {


    $generator = async_yield $async => sub { return $next_generator }

Yield a value from a generator function. The $async contains the value or state you want to yield. The callback will be executed to yield the next value. It receives no arguments. It must return a valid generator.


    $generator = $generator->gen_map(sub {
        my (@values) = @_;
        # ...
        return $new_async;

Transform the values yielded by a generator. The callback receives the values of the current item as parameters. The callback must return an Async, usually a value. It may also return async_cancel to terminate the Generator, or async_error.

You cannot return multiple Asyncs (at most a multi-value Async). Returning a Generator Async is not meaningful, and it will be treated as an ordinary value.


    $async = $generator->gen_foreach(sub {
        my (@values) = @_;
        # ...
        return async_value;

Consume a generator. The callback will be invoked with each item's values. The callback may return an Async Value to receive the next value, or may return an Async Error or Async Cancel to abort the loop.

The returned Async is an empty Value when the loop completes successfully or was aborted, and an Error when there was an error in the loop body or in the generator.


    $async = $generator->gen_collect;

Collects all items in an array ref. This will consume the whole stream, so only works for finite streams.



    @result = $async->run_until_completion;

Creates and event loop and blocks until the $async is completed. If it was cancelled, throws an exception. If it was an error, rethrows that error. If it was a value, the values are returned as a list.

This call should be used sparingly, usually once per program. Sharing Asyncs between multiple event loops may lead to unexpected results.

If you want to use the results of an Async to continue within an Async context, you usually want to await() the Async instead.


    $str = $async->to_string;
    $str = "$async";

Low-level debugging stringification that displays Async identity and type.






    $bool = $async->is_complete;
    $bool = $async->is_cancelled;
    $bool = $async->is_resolved;
    $bool = $async->is_error;
    $bool = $async->is_value;

Inspect the state of an Async (see "Async States").


This module is not very well tested and battle-proven. There are certainly still some bugs lurking around.

This module does not provide first-class corountines or async/await keywords. It is just a library. Check out the Future::AsyncAwait module instead.

This module does not provide first-class Future objects. While Asyncs are Future-like, you cannot resolve an Async explicitly. Check out the Future module instead.

This module does not implement an event loop. The run_until_completion() function does run a dispatch loop, but there is no concept of events, I/O, or timers. Check out the IO::Async module instead.

This module is not thread-aware. Handling the same Async on multiple threads is undefined behaviour.

This module does not detect infinite loops. It is your responsibility to ensure that Async dependencies don't form cycles.

This module does not guarantee any particular evaluation order. If you need a specific sequence, you must encode it explicitly (see Combining Asyncs). Note that the combinators do not declare a partial order between one or more Asyncs, but specify in which order the dependencies of the combinator Async are evaluated. E.g. in $x->complete_then($y), $y may be evaluated first if some other Async depends on $y as well.

This module is not pure-Perl. You will need a C++11 compiler to install it.



Bug Tracker:


amon - Lukas Atkinson (cpan: AMON) <>


Copyright 2017 Lukas Atkinson

This library is free software and may be distributed under the same terms as perl itself. See