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            <div class="post-content">

              <section id="section-1">
                  <h2>Create new, complex Promises by composing Promises</h2>

<p>Mojolicious 7.49 added an its own implementation of the <a href="https://promisesaplus.com">Promises/A+ specification</a>. mohawk wrote about these in <a href="https://mojolicious.io/blog/2017/12/14/day-14-you-promised-to-call/">Day 14: You Promis...

              </section>
              <section id="section-2">
                  <p>A Promise is a structure designed to eliminate nested callbacks (also known as <a href="http://callbackhell.com">&quot;callback hell&quot;</a>). A properly written chain of Promises has a flat structure that easy to follow linear...

<p>A higher-order Promise is one that comprises other Promises and bases its status on them. The <a href="https://metacpan.org/pod/Mojo::Promise::Role::HigherOrder">Mojo::Promise::Role::HigherOrder</a> distribution provides three roles that you can m...

<h3>All</h3>

<p>An <code>all</code> promise resolves only when all of its Promises also resolve. If one of them is rejected, the <code>all</code> Promise is rejected. This means that the overall Promise knows what to do if one is rejected and it doesn&#39;t need ...

<pre><code>use Mojo::Promise;
use Mojo::UserAgent;
my $ua = Mojo::UserAgent-&gt;new;

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>

<ul>

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

<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>

<p>So now the formula is simple.</p>

<ul>
<li>Use libraries that return promises rather than take callbacks.</li>
<li>Use the <code>async</code> keyword when declaring functions that need to <code>await</code> promises.</li>
<li>Organize your promises using <a href="https://mojolicious.org/perldoc/Mojo/Promise#all">all</a>, <a href="https://mojolicious.org/perldoc/Mojo/Promise#race">race</a> (only wait for the first resolved promise) or some <a href="https://mojolicious....
</ul>

<p>Hopefully with Mojo::AsyncAwait, writing asynchronous code is finally going to be accessible to those users that haven&#39;t yet had Matt&#39;s &quot;aha&quot; moment.
And for those of us who have, don&#39;t worry, you&#39;ll love it too.</p>

<p><em>Another excellent resource is <a href="https://www.youtube.com/watch?v=gB-OmN1egV8">The Evolution of Async JavaScript: From Callbacks, to Promises, to Async/Await</a> by Tyler McGinnis.
It is for JavaScript, but nearly everything applies except they syntax.
Highly recommended.</em></p>