NAME

Promise::Me - Fork Based Promise with Asynchronous Execution, Async, Await and Shared Data

SYNOPSIS

use Promise::Me; # exports async, await and share
my $p = Promise::Me->new(sub
{
    # Some regular code here
})->then(sub
{
    my $res = shift( @_ ); # return value from the code executed above
    # more processing...
})->then(sub
{
    my $more = shift( @_ ); # return value from the previous then
    # more processing...
})->catch(sub
{
    my $exception = shift( @_ ); # error that occured is caught here
})->finally(sub
{
    # final processing
})->then(sub
{
    # A last then may be added after finally
};

# You can share data among processes for all systems, including Windows
my $data : shared = {};
my( $name, %attributes, @options );
share( $name, %attributes, @options );

my $p1 = Promise::Me->new( $code_ref )->then(sub
{
    my $res = shift( @_ );
    # more processing...
})->catch(sub
{
    my $err = shift( @_ );
    # Do something with the exception
});

my $p2 = Promise::Me->new( $code_ref )->then(sub
{
    my $res = shift( @_ );
    # more processing...
})->catch(sub
{
    my $err = shift( @_ );
    # Do something with the exception
});

my @results = await( $p1, $p2 );

# Wait for all promise to resolve. If one is rejected, this super promise is rejected
my @results = Promise::Me->all( $p1, $p2 );

# First promise that is resolved or rejected makes this super promise resolved and 
# return the result
my @results = Promise::Me->race( $p1, $p2 );

# Automatically turns this subroutine into one that runs asynchronously and returns 
# a promise
async sub fetch_remote
{
    # Do some http request that will run asynchronously thanks to 'async'
}

sub do_something
{
    # some code here
    my $p = Promise::Me->new(sub
    {
        # some work that needs to run asynchronously
    })->then(sub
    {
        # More processing here
    })->catch(sub
    {
        # Oops something went wrong
        my $exception = shift( @_ );
    });
    # No need for this subroutine 'do_something' to be prefixed with 'async'.
    # This is not JavaScript you know
    await $p;
}

sub do_something
{
    # some code here
    my $p = Promise::Me->new(sub
    {
        # some work that needs to run asynchronously
    })->then(sub
    {
        # More processing here
    })->catch(sub
    {
        # Oops something went wrong
        my $exception = shift( @_ );
    })->wait;
    # Always returns a reference
    my $result = $p->result;
}

VERSION

v0.2.1

DESCRIPTION

Promise::Me is an implementation of the JavaScript promise using fork for asynchronous tasks. Fork is great, because it is well supported by all operating systems (except AmigaOS, RISC OS and VMS) and effectively allows for asynchronous execution.

While JavaScript has asynchronous execution at its core, which means that two consecutive lines of code will execute simultaneously, under perl, those two lines would be executed one after the other. For example:

# Assuming the function getRemote makes an http query of a remote resource that takes time
let response = getRemote('https://example.com/api');
console.log(response);

Under JavaScript, this would yield: undefined, but in perl

my $resp = $ua->get('https://example.com/api');
say( $resp );

Would correctly return the response object, but it will hang until it gets the returned object whereas in JavaScript, it would not wait.

In JavaScript, because of this asynchronous execution, before people were using callback hooks, which resulted in "callback from hell", i.e. something like this[1]:

getData(function(x){
    getMoreData(x, function(y){
        getMoreData(y, function(z){ 
            ...
        });
    });
});

[1] Taken from this StackOverflow discussion

And then, they came up with Promise, so that instead of wrapping your code in a callback function you get instead a promise object that gets called when certain events get triggered, like so[2]:

const myPromise = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve('foo');
  }, 300);
});

myPromise
  .then(handleResolvedA, handleRejectedA)
  .then(handleResolvedB, handleRejectedB)
  .then(handleResolvedC, handleRejectedC);

[2] Taken from Mozilla documentation

Chaining is easy to implement in perl and Promise::Me does it too. Where it gets more tricky is returning a promise immediately without waiting for further execution, i.e. a deferred promise, like the following in JavaScript:

function getRemote(url)
{
    let promise = new Promise((resolve, reject) => 
    {
        setTimeout(() => reject(new Error("Whoops!")), 1000);
    });
    // Maybe do some other stuff here
    return( promise );
}

In this example, under JavaScript, the promise will be returned immediately. However, under perl, the equivalent code would be executed sequentially. For example, using the excellent module Promise::ES6:

sub get_remote
{
    my $url = shift( @_ );
    my $p = Promise::ES6->new(sub($res)
    {
        $res->( Promise::ES6->resolve(123) );
    });
    # Do some more work that would take some time
    return( $p );
}

In the example above, the promise $p would not be returned until all the tasks are completed before the return statement, contrary to JavaScript where it would be returned immediately.

So, in perl people have started to use loop such as AnyEvent or IO::Async with "conditional variable" to get that asynchronous execution, but you need to use loops. For example (taken from Promise::AsyncAwait):

use Promise::AsyncAwait;
use Promise::XS;

sub delay {
    my $secs = shift;

    my $d = Promise::XS::deferred();

    my $timer; $timer = AnyEvent->timer(
        after => $secs,
        cb => sub {
            undef $timer;
            $d->resolve($secs);
        },
    );

    return $d->promise();
}

async sub wait_plus_1 {
    my $num = await delay(0.01);

    return 1 + $num;
}

my $cv = AnyEvent->condvar();
wait_plus_1()->then($cv, sub { $cv->croak(@_) });

my ($got) = $cv->recv();

So, in the midst of this, I have tried to provide something without event loop by using fork instead as exemplified in the "SYNOPSIS"

For a framework to do asynchronous tasks, you might also be interested in Coro, from Marc A. Lehmann original author of AnyEvent event loop.

METHODS

new

my $p = Promise::Me->new(sub
{
    # some code to run asynchronously
});

# or
my $p = Promise::Me->new(sub
{
    # some code to run asynchronously
}, { debug => 4, result_shared_mem_size => 2097152, timeout => 2 });

Instantiate a new Promise::Me object.

It takes a code reference such as an anonymous subroutine or a reference to a subroutine, and optionally an hash reference of options.

The options supported are:

debug integer

Sets the debug level. This can be quite verbose and will slow down the process, so use with caution.

result_shared_mem_size integer

Sets the shared memory segment to store the asynchronous process results. This default to the value of the constant Module::Generic::SharedMem::SHM_BUFSIZ, which is 64K bytes.

timeout integer

Currently unused.

use_cache_file

Boolean. If true, Promise::Me will use a cache file instead of shared memory block. If you are on system that do not support shared memory, Promise::Me will automatically revert to Module::Generic::File::Cache to handle data shared among processes.

You can use the global package variable $SHARE_MEDIUM to set the default value for all object instantiation.

$SHARE_MEDIUM value can be either memory for shared memory or file for shared cache file.

catch

This takes a code reference as its unique argument and is added to the chain of handlers.

It will be called upon an exception being met or if "reject" is called.

reject

This takes one or more arguments that will be passed to the next "catch" handler, if any.

It will mark the promise as rejected and will go no further in the chain.

rejected

Takes a boolean value and sets or gets the rejected status of the promise.

This is typically set by "reject" and you should not call this directly, but use instead "reject".

resolve

This takes one or more arguments that will be passed to the next "then" handler, if any.

It will mark the promise as resolved and will the next "then" handler.

resolved

Takes a boolean value and sets or gets the resolved status of the promise.

This is typically set by "resolve" and you should not call this directly, but use instead "resolve".

result

This sets or gets the result returned by the asynchronous process. The data is exchanged through shared memory.

This method is used internally in combination with "await", "all" and "race"

The value returned is always a reference, such as array, hash or scalar reference.

If the asynchronous process returns a simple string for example, result will be an array reference containing that string.

Thus, unless the value returned is 1 element and it is a reference, it will be made of an array reference.

timeout

Sets gets a timeout. This is currently not used. There is no timeout for the asynchronous process.

If you want to set a timeout, you can use "wait", or "await"

wait

This is a chain method whose purpose is to indicate that we must wait for the asynchronous process to complete.

Promise::Me->new(sub
{
    # Some operation to be run asynchronously
})->then(sub
{
    # Do some processing of the result
})->catch(sub
{
    # Cath any exceptions
})->wait;

CLASS FUNCTIONS

all

Provided with one or more Promise::Me objects, and this will wait for all of them to be resolved.

It returns an array equal in size to the number of promises provided initially.

However, if one promise is rejected, "all" stops and returns it immediately.

my @results = Promise::Me->all( $p1, $p2, $p3 );

Contrary to its JavaScript equivalent, you do not need to pass an array reference of promises, although you could.

# Works too, but not mandatory
my @results = Promise::Me->all( [ $p1, $p2, $p3 ] );

See also Mozilla documentation for more information.

race

Provided with one or more Promise::Me objects, and this will return the result of the first promise that resolves or is rejected.

Contrary to its JavaScript equivalent, you do not need to pass an array reference of promises, although you could.

# Works too, but not mandatory
my @results = Promise::Me->race( [ $p1, $p2, $p3 ] );

See also Mozilla documentation for more information.

EXPORTED FUNCTIONS

async

This is a static function exported by default and that wrap the subroutine thus prefixed into one that returns a promise and return its code asynchronously.

For example:

async sub fetch
{
    my $ua = LWP::UserAgent->new;
    my $res = $ua->get( 'https://example.com' );
}

This would be equivalent to:

Promise::Me->new(sub
{
    my $ua = LWP::UserAgent->new;
    my $res = $ua->get( 'https://example.com' );
});

Of course, since, in our example above, fetch would return a promise, you could chain "then", "catch" and "finally", such as:

async sub fetch
{
    my $ua = LWP::UserAgent->new;
    my $res = $ua->get( 'https://example.com' );
}->then(sub
{
    my $res = shift( @_ );
    if( !$resp->is_success )
    {
        die( My::Exception->new( "Unable to fetch remote content." ) );
    }
})->catch(sub
{
    my $exception = shift( @_ );
    $logger->warn( $exception );
})->finally(sub
{
    $dbi->disconnect;
});

See Mozilla documentation for more information on async

await

Provided with one or more promises and "await" will wait until each one of them is completed and return an array of their result with one entry per promise. Each promise result is a reference (array, hash, or scalar, or object for example)

my @results = await( $p1, $p2, $p3 );

lock

This locks a shared variable.

my $data : shared = {};
lock( $data );
$data->{location} = 'Tokyo';
unlock( $data );

See "SHARED VARIABLES" for more information about shared variables.

Provided with one or more variables and this will enable them to be shared with the asynchronous processes.

Currently supported variable types are: array, hash and scalar (string) reference.

my( $name, @first_names, %preferences );
share( $name, @first_names, %preferences );
$name = 'Momo Taro';

Promise::Me->new(sub
{
    $preferences{name} = $name = 'Mr. ' . $name;
    print( "Hello $name\n" );
    $preferences{location} = 'Okayama';
    $preferences{lang} = 'ja_JP';
    $preferences{locale} = '桃太郎'; # Momo Taro
    my $rv = $tbl->insert( \%$preferences )->exec || die( My::Exception->new( $tbl->error ) );
    $rv;
})->then(sub
{
    my $mail = My::Mailer->new(
        to => $preferences{email},
        name => $preferences{name},
        body => $welcome_ja_file,
    );
    $mail->send || die( $mail->error );
})->catch(sub
{
    my $exception = shift( @_ );
    $logger->write( $exception );
})->finally(sub
{
    $dbh->disconnect;
});

It will try to use shared memory or shared cache file depending on the value of the global package variable $SHARE_MEDIUM

unlock

This unlocks a shared variable. It has no effect on variable that have not already been shared.

See "SHARED VARIABLES" for more information about shared variables.

unshare

Unshare a variable. It has no effect on variable that have not already been shared.

This should only be called before the promise is created.

INTERNAL METHODS

add_final_handler

This is called each time a "finally" method is called and will add to the chain the code reference provided.

add_reject_handler

This is called each time a "catch" method is called and will add to the chain the code reference provided.

add_resolve_handler

This is called each time a "then" method is called and will add to the chain the code reference provided.

args

This method is called upon promise object instantiation when initially called by "async".

It is used to capture arguments so they can be passed to the code executed asynchronously.

exec

This method is called at the end of the chain. It will prepare shared variable for the child process, launch a child process using "fork" in perlfunc and will call the next "then" handler if the code executed successfully, or "reject" if there was an error.

exit_bit

This corresponds to $?. After the child process exited, "_set_exit_values" is called and sets the value for this.

exit_signal

This corresponds to the integer value of the signal, if any, used to interrupt the asynchronous process.

exit_status

This is the integer value of the exit for the asynchronous process. If a process exited normally, this value should be 0.

filter

This is called by the import method to filter the code using perl filter with XS module Filter::Util::Call and enables data sharing, and implementation of async subroutine prefix. It relies on XS module PPI for parsing perl code.

get_finally_handler

This is called when all chaining is complete to get the "finally" handler, if any.

get_next_by_type

Get the next handler by type, i.e. then, catch or finally

get_next_reject_handler

This is called to get the next "catch" handler when a promise has been rejected, such as when an error has occurred.

get_next_resolve_handler

This is called to get the next "then" handler and execute its code passing it the return value from previous block in the chain.

has_coredump

Returns true if the asynchronous process last exited with a core dump, false otherwise.

is_child

Returns true if we are called from within the asynchronous process.

is_parent

Returns true if we are called from within the main parent process.

no_more_chaining

This is set to true automatically when the end of the method chain has been reached.

pid

Returns the pid of the asynchronous process.

share_auto_destroy

This is a promise instantiation option. When set to true, the shared variables will be automatically removed from memory upon end of the main process.

This is true by default. If you want to set it to false, you can do:

Promise::Me->new(sub
{
    # some code here
}, {share_auto_destroy => 0})->then(sub
{
    # some more work here, etc.
});

shared_mem

This returns the Module::Generic::SharedMem object used for sharing data and result between the main parent process and the asynchronous child process.

shared_space_destroy

Boolean. Default to true. If true, the shared space used by the parent and child processes will be destroy automatically. Disable this if you want to debug or take a sneak peek into the data. The shared space will be either shared memory of cache file depending on the value of $SHARE_MEDIUM

use_async

This is a boolean value which is set automatically when a promise is instantiated from "async".

It enables subroutine arguments to be passed to the code being run asynchronously.

PRIVATE METHODS

_browse

Used for debugging purpose only, this will print out the PPI structure of the code filtered and parsed.

_parse

After the code has been collected, this method will quickly parse it and make changes to enable "async"

_reject_resolve

This is a common code called by either "resolve" or "reject"

_set_exit_values

This is called upon the exit of the asynchronous process to set some general value about how the process exited.

See "exit_bit", "exit_signal" and "exit_status"

_set_shared_space

This is called in "exec" to share data including result between main parent process and asynchronous process.

SHARED VARIABLES

It is important to be able to share variables between processes in a seamless way.

When the asynchronous process is executed, the main process first fork and from this point on all data is being duplicated in an impermeable way so that if a variable is modified, it would have no effect on its alter ego in the other process; thus the need for shareable variables.

You can enable shared variables in two ways:

1. declaring the variable as shared

my $name : shared;
# Initiate a value
my $location : shared = 'Tokyo';
# you can also use 'pshared'
my $favorite_programming_language : pshared = 'perl';
# You can share array, hash and scalar
my %preferences : shared;
my @names : shared;

2. calling "share"

my( $name, %prefs, @middle_names );
share( $name, %prefs, @middle_names );

Once shared, you can use those variables normally and their values will be shared between the parent process and the asynchronous process.

For example:

my( $name, @first_names, %preferences );
share( $name, @first_names, %preferences );
$name = 'Momo Taro';

Promise::Me->new(sub
{
    $preferences{name} = $name = 'Mr. ' . $name;
    print( "Hello $name\n" );
    $preferences{location} = 'Okayama';
    $preferences{lang} = 'ja_JP';
    $preferences{locale} = '桃太郎';
    my $rv = $tbl->insert( \%$preferences )->exec || die( My::Exception->new( $tbl->error ) );
    $rv;
})->then(sub
{
    my $mail = My::Mailer->new(
        to => $preferences{email},
        name => $preferences{name},
        body => $welcome_ja_file,
    );
    $mail->send || die( $mail->error );
})->catch(sub
{
    my $exception = shift( @_ );
    $logger->write( $exception );
})->finally(sub
{
    $dbh->disconnect;
});

If you want to mix this feature and the usage of threads' shared feature, use the keyword pshared instead of shared, such as:

my $name : pshared;

Otherwise the two keywords would conflict.

SHARED MEMORY

This module uses shared memory using perl core functions, or shared cache file using Module::Generic::File::Cache if shared memory is not supported, or if the value of the global package variable $SHARE_MEDIUM is set to file instead of memory

Shared memory is used for:

1. shared variables
2. storing results returned by asynchronous processes

You can control how much shared memory is allocated for each by:

1. setting the global variable $SHARED_MEMORY_SIZE, which default to 64K bytes.
2. setting the option result_shared_mem_size when instantiating a new Promise::Me object. If not set, this will default to Module::Generic::SharedMem::SHM_BUFSIZ constant value which is 64K bytes.: If you use shared cache file, then not setting a size is ok. It will use the space on the filesystem as needed and obviously return an error if there is no space left.

CONCURRENCY

Because Promise::Me forks a separate process to run the code provided in the promise, two promises can run simultaneously. Let's take the following example:

use Time::HiRes;
my $result : shared = '';
my $p1 = Promise::Me->new(sub
{
    sleep(1);
    $result .= "Peter ";
})->then(sub
{
    print( "Promise 1: result is now: '$result'\n" );
});

my $p2 = Promise::Me->new(sub
{
    sleep(0.5);
    $result .= "John ";
})->then(sub
{
    print( "Promise 2: result is now: '$result'\n" );
});
await( $p1, $p2 );
print( "Result is: '$result'\n" );

This will yield:

Promise 2: result is now: 'John '
Promise 1: result is now: 'John Peter '
Result is: 'John Peter '

AUTHOR

Jacques Deguest <jack@deguest.jp>

COPYRIGHT & LICENSE

This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

To install Promise::Me, copy and paste the appropriate command in to your terminal.

cpanm

cpanm Promise::Me

CPAN shell

perl -MCPAN -e shell
install Promise::Me

For more information on module installation, please visit the detailed CPAN module installation guide.

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NAME

SYNOPSIS

VERSION

DESCRIPTION

METHODS

new

catch

reject

rejected

resolve

resolved

result

timeout

wait

CLASS FUNCTIONS

all

race

EXPORTED FUNCTIONS

async

await

lock

share

unlock

unshare

INTERNAL METHODS

add_final_handler

add_reject_handler

add_resolve_handler

args

exec

exit_bit

exit_signal

exit_status

filter

get_finally_handler

get_next_by_type

get_next_reject_handler

get_next_resolve_handler

has_coredump

is_child

is_parent

no_more_chaining

pid

share_auto_destroy

shared_mem

shared_space_destroy

use_async

PRIVATE METHODS

_browse

_parse

_reject_resolve

_set_exit_values

_set_shared_space

SHARED VARIABLES

SHARED MEMORY

CONCURRENCY

AUTHOR

SEE ALSO

COPYRIGHT & LICENSE

Module Install Instructions