Acme-CPANModulesBundle-Import-MojoliciousAdvent-2018

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devdata/https_mojolicious.io_blog_2018_12_24_async-await-the-mojo-way_  view on Meta::CPAN

$promise->then(sub ($title) {
  say $title;
})->wait;
</code></pre>

<p>At first glance this isn&#39;t too much better than the callback.
However, a few nice features emerge.
The most important of which is that the promise object can be returned to the caller and the caller can choose what to do with it.</p>

<p>In useful code you would also want to attach error handling, though I&#39;ve omitted it here for bevity.
Mojolicious&#39; promise implementation also gives us the <code>wait</code> method to start the ioloop if necessary.</p>

<p>Although it is interesting to see how a user can create a promise object to convert a callback api to promises, many libraries, including Mojolicious, now have promise variants built-in.
Rather than depending on the user to create a promise to resolve in the callback, these libraries will just return a promise of their own.
In the Mojolicious project, by convention methods that return promises end in <code>_p</code>.</p>

<p>With that we can write similar code to the one above</p>

<pre><code>my $promise = $ua-&gt;get_p($url);

devdata/https_mojolicious.io_blog_2018_12_24_async-await-the-mojo-way_  view on Meta::CPAN

  return $promise;
}

get_title_p($url)->then(sub ($title) {
  say $title;
})->wait;
</code></pre>

<p>All told, this is a step in the right direction, but it still involves a mental shift in style.
Even if this is easier than using pure callbacks, you still have to keep track of promises, consider the implications of chaining.
You still have to attach callbacks using <code>then</code>.
And don&#39;t forget error handling callbacks too!</p>

<p><em>Editor&#39;s note: to this point in the article, it is similar to the Perl Advent Calendar entry <a href="http://www.perladvent.org/2018/2018-12-19.html">posted just a few days before this one on 2018-12-19</a>, humorously presented by Mark Fo...
If you&#39;d like to see another take on promises and Mojo::Promise specifically, give it a read.
Everything in it is applicable even as this article takes it one step further below ...</em></p>

<h2>Async/Await</h2>

<p>What we really wish we could tell the Perl interpreter to do is</p>

devdata/https_mojolicious.io_blog_2018_12_24_async-await-the-mojo-way_  view on Meta::CPAN

<p>Where the example above would be at</p>

<pre><code>localhost:3000?url=https%3A%2F%2Fmojolicious.org&amp;url=https%3A%2F%2Fmojolicious.io&amp;url=https%3A%2F%2Fmetacpan.org
</code></pre>

<p>That code is almost exactly what you&#39;d write for a blocking implementation except that it would block the server and it have to fetch the urls sequentially.
Instead, since it is written nonblocking, the requests are all made concurrently and the server is still free to respond to new clients.
And yet the code is still very easy to follow.</p>

<p>Note: the <a href="https://metacpan.org/pod/Mojolicious::Plugin::PromiseActions">PromiseActions</a> plugin automatically attaches error handlers to the controller action when it returns a promise; it is highly recommended when using async actions....

<h2>A Word on Implementation</h2>

<p>As stated before Mojo::AsyncAwait requires some mechanism to suspend the interpreter and resume it at that point later on.
Currently, the module uses the somewhat controversial module <a href="https://metacpan.org/pod/Coro">Coro</a> to do it.
As a bulwark against future implimentation changes, it comes with a pluggable backend system, not unlike how Mojo::IOLoop&#39;s pluggable reactor system works.
The default implementation may change and users may choose to use any available backend if they have a preference (once new ones come along, and others  <strong>are</strong> in the works).</p>

<h2>Conclusion</h2>