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<!DOCTYPE html>
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<header><h1>scalaz.concurrent.Task</h1></header>
<section><h2>Why? Why not?</h2>
<h3>IndyScala, November 3, 2014</h3></section>
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<header><h1>Presenter Notes</h1></header>
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<header><h1>Probably 1 of 3? Yeah, probably.</h1></header>
<section><ol>
<li><strong>scalaz.concurrent.Task</strong> (November, 2014)</li>
<li>scalaz-stream (???)</li>
<li>http4s (???)</li>
</ol></section>
</div>
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<header><h1>Presenter Notes</h1></header>
<section>
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<header><h1>Ye Olden Tymes</h1></header>
<section><p><em>Way back in Scala 2.9.2</em></p>
<ul>
<li>java.util.concurrent.Future ☠☠☠<ul>
<li>com.google.common.util.concurrent.ListenableFuture</li>
</ul>
</li>
<li>scala.actors.Future</li>
<li>akka.dispatch.Future</li>
<li>com.twitter.util.Future</li>
<li>scalaz.concurrent.Promise</li>
</ul></section>
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<header><h1>Presenter Notes</h1></header>
<section>
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<header><h1>That begat <a href="https://github.com/eed3si9n/sff4s">sff4s</a></h1></header>
<section><div class="highlight"><pre><span class="lineno">1</span> <span class="k">val</span> <span class="n">factory</span> <span class="k">=</span> <span class="n">sff4s</span><span class="o">.</span><span class="n">impl</span><span class="o">.</span><span class="nc">ActorsFuture</span>
<span class="lineno">2</span> <span class="k">val</span> <span class="n">f</span> <span class="k">=</span> <span class="n">factory</span> <span class="n">future</span> <span class="o">{</span>
<span class="lineno">3</span> <span class="nc">Thread</span><span class="o">.</span><span class="n">sleep</span><span class="o">(</span><span class="mi">1000</span><span class="o">)</span>
<span class="lineno">4</span> <span class="mi">1</span>
<span class="lineno">5</span> <span class="o">}</span>
<span class="lineno">6</span> <span class="k">val</span> <span class="n">g</span> <span class="k">=</span> <span class="n">f</span> <span class="n">map</span> <span class="o">{</span> <span class="k">_</span> <span class="o">+</span> <span class="mi">1</span> <span class="o">}</span>
<span class="lineno">7</span> <span class="n">g</span><span class="o">(</span><span class="mi">2000</span><span class="o">)</span>
</pre></div>
<p>... which was nice for unopinionated frameworks, but never really caught on.</p></section>
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<header><h1>Enter SIP-14</h1></header>
<section><ul>
<li><a href="http://docs.scala-lang.org/sips/completed/futures-promises.html">Futures and Promises</a></li>
<li>Introduced in Scala 2.10</li>
<li>Backported to Scala 2.9.3</li>
<li>You know it as <code>scala.concurrent.Future</code><ul>
<li>You know, the thing <code>?</code> returns from an actor.</li>
</ul>
</li>
</ul></section>
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<header><h1>Presenter Notes</h1></header>
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<header><h1>Standardization???</h1></header>
<section><ul>
<li>java.util.concurrent.Future ☠☠☠<ul>
<li>com.google.common.util.concurrent.ListenableFuture</li>
<li>java.util.concurrent.CompletableFuture</li>
</ul>
</li>
<li><strike>scala.actors.Future</strike></li>
<li><strike>scalaz.concurrent.Promise</strike></li>
<li><strike>akka.dispatch.Future</strike></li>
<li>com.twitter.util.Future</li>
<li><em>scala.concurrent.Future</em></li>
<li><strike>scalaz.concurrent.Promise</strike></li>
<li><em>scalaz.concurrent.Task</em></li>
</ul></section>
</div>
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<header><h1>Presenter Notes</h1></header>
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<header><h1><img alt="This is why we can't have nice things" src="file:///home/ross/src/indyscala/indyscala-tasks/images/nice-things.jpeg" /></h1></header>
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<header><h1>Wrong presentation</h1></header>
<section><ul>
<li>Tomorrow night is why we can't have nice things. </li>
<li>For concurrent programming in Scala, we <em>do</em> have two rather nice things.</li>
</ul></section>
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<header><h1>Presenter Notes</h1></header>
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<header><h1>A review of scala.concurrent.Future</h1></header>
<section><div class="highlight"><pre><span class="lineno">1</span> <span class="k">def</span> <span class="n">isOk</span><span class="o">(</span><span class="n">url</span><span class="k">:</span> <span class="kt">String</span><span class="o">)</span><span class="k">:</span> <span class="kt">Future</span><span class="o">[</span><span class="kt">Boolean</span><span class="o">]</span> <span class="k">=</span> <span class="n">get</span><span class="o">(</span><span class="n">url</span><span class="o">)</span>
<span class="lineno">2</span> <span class="o">.</span><span class="n">map</span> <span class="o">{</span> <span class="k">_</span><span class="o">.</span><span class="n">status</span> <span class="o">==</span> <span class="mi">200</span> <span class="o">}</span> <span class="c1">// If we got a response, was it a 200?</span>
<span class="lineno">3</span> <span class="o">.</span><span class="n">recover</span> <span class="o">{</span> <span class="k">case</span> <span class="n">e</span> <span class="k">=></span> <span class="n">e</span><span class="o">.</span><span class="n">printStackTrace</span><span class="o">();</span> <span class="kc">false</span><span class="o">}</span>
<span class="lineno">4</span> <span class="k">val</span> <span class="n">sites</span> <span class="k">=</span> <span class="nc">Seq</span><span class="o">(</span><span class="s">"http://twitter.com"</span><span class="o">,</span> <span class="s">"http://github.com/"</span><span class="o">)</span>
<span class="lineno">5</span> <span class="k">val</span> <span class="n">panic</span> <span class="k">=</span> <span class="nc">Future</span><span class="o">.</span><span class="n">fold</span><span class="o">(</span><span class="n">sites</span><span class="o">.</span><span class="n">map</span><span class="o">(</span><span class="n">isOk</span><span class="o">))(</span><span class="kc">false</span><span class="o">)</span> <span class="o">{</span>
<span class="lineno">6</span> <span class="o">(</span><span class="n">acc</span><span class="o">,</span> <span class="n">isSiteOk</span><span class="o">)</span> <span class="k">=></span> <span class="n">acc</span> <span class="o">||</span> <span class="o">!</span><span class="n">isSiteOk</span>
<span class="lineno">7</span> <span class="o">}</span>
<span class="lineno">8</span> <span class="n">println</span><span class="o">(</span><span class="s">"Panic: "</span><span class="o">+</span><span class="nc">Await</span><span class="o">.</span><span class="n">result</span><span class="o">(</span><span class="n">panic</span><span class="o">,</span> <span class="mf">10.</span><span class="n">seconds</span><span class="o">))</span>
</pre></div>
<ul>
<li>A Future is an asynchronous computation.</li>
<li>A Future tries (that is, <code>scala.util.Try</code>s) to complete that computation.</li>
<li>Yup. Future is a monad.</li>
</ul></section>
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<header><h1>Presenter Notes</h1></header>
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<header><h1>Enter scalaz.concurrent.Future</h1></header>
<section><ul>
<li>Like Scala Future, a Scalaz Future is an asynchronous computation.</li>
<li>Yup. This Future is a monad, too.</li>
<li>But there's no error handling in this monad.</li>
</ul></section>
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<header><h1>Presenter Notes</h1></header>
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<header><h1>Enter scalaz.concurrent.<strike>Future</strike>Task</h1></header>
<section><div class="highlight"><pre><span class="lineno">1</span> <span class="k">def</span> <span class="n">isDown</span><span class="o">(</span><span class="n">url</span><span class="k">:</span> <span class="kt">String</span><span class="o">)</span><span class="k">:</span> <span class="kt">Future</span><span class="o">[</span><span class="kt">Boolean</span><span class="o">]</span> <span class="k">=</span> <span class="n">get</span><span class="o">(</span><span class="n">url</span><span class="o">)</span>
<span class="lineno">2</span> <span class="o">.</span><span class="n">map</span> <span class="o">{</span> <span class="k">_</span><span class="o">.</span><span class="n">status</span> <span class="o">!=</span> <span class="mi">200</span> <span class="o">}</span> <span class="c1">// If we got a response, was it a 200?</span>
<span class="lineno">3</span> <span class="o">.</span><span class="n">recover</span> <span class="o">{</span> <span class="k">case</span> <span class="n">e</span> <span class="k">=></span> <span class="n">e</span><span class="o">.</span><span class="n">printStackTrace</span><span class="o">();</span> <span class="kc">false</span><span class="o">}</span>
<span class="lineno">4</span> <span class="k">val</span> <span class="n">sites</span> <span class="k">=</span> <span class="nc">Seq</span><span class="o">(</span><span class="s">"http://twitter.com"</span><span class="o">,</span> <span class="s">"http://github.com/"</span><span class="o">)</span>
<span class="lineno">5</span> <span class="k">val</span> <span class="n">panic</span> <span class="k">=</span> <span class="nc">Future</span><span class="o">.</span><span class="n">fold</span><span class="o">(</span><span class="n">sites</span><span class="o">.</span><span class="n">map</span><span class="o">(</span><span class="n">isDown</span><span class="o">))(</span><span class="kc">false</span><span class="o">)(</span><span class="k">_</span> <span class="o">||</span> <span class="k">_</span><span class="o">)</span>
<span class="lineno">6</span> <span class="n">println</span><span class="o">(</span><span class="s">"Panic: "</span><span class="o">+</span><span class="nc">Await</span><span class="o">.</span><span class="n">result</span><span class="o">(</span><span class="n">panic</span><span class="o">,</span> <span class="mf">10.</span><span class="n">seconds</span><span class="o">))</span>
</pre></div>
<ul>
<li>A <code>Task[A]</code> is a thin wrapper around <code>Future[Throwable \/ A]</code>. <ul>
<li><code>scalaz.\/</code> is commonly pronounced as <em>either</em>.<ul>
<li>Because <em>either</em> it worked or it didn't.</li>
<li>Yup. <code>\/</code> is a monad. (scala.Either, incidentally, is not.)</li>
</ul>
</li>
<li>Yup. <code>Task</code> is a monad.</li>
</ul>
</li>
</ul></section>
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<div class="presenter_notes">
<header><h1>Presenter Notes</h1></header>
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<header><h1>So the difference is just in method names?</h1></header>
<section><ul>
<li>This is just more of that NIH Syndrome that Scala is famous for, isn't it?</li>
</ul>
<p><img alt="No." src="file:///home/ross/src/indyscala/indyscala-tasks/images/no.jpg" /></p></section>
</div>
<div class="presenter_notes">
<header><h1>Presenter Notes</h1></header>
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<header><h1>Future.apply immediately submits to executor</h1></header>
<section><div class="highlight"><pre><span class="lineno">1</span> <span class="nc">Future</span> <span class="o">{</span> <span class="nc">TheNukes</span><span class="o">.</span><span class="n">launch</span><span class="o">()</span> <span class="o">}</span>
</pre></div>
<p><img alt="Mushroom cloud" src="file:///home/ross/src/indyscala/indyscala-tasks/images/shroom.jpg" /></p>
<p><font size="-3">By Federal Government of the United States [Public domain], via Wikimedia Commons</font></p></section>
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<header><h1>Presenter Notes</h1></header>
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<header><h1>Tasks don't run ...</h1></header>
<section><div class="highlight"><pre><span class="lineno">1</span> <span class="nc">Task</span> <span class="o">{</span> <span class="nc">TheNukes</span><span class="o">.</span><span class="n">launch</span><span class="o">()</span> <span class="o">}</span>
</pre></div>
<p><img alt="Bucolic scene" src="file:///home/ross/src/indyscala/indyscala-tasks/images/bucolic.jpg" /></p>
<p><font size="-3">Alison Rawson <a href="http://creativecommons.org/licenses/by-sa/2.0">CC-BY-SA-2.0</a>, via Wikimedia Commons</font></p></section>
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<div class="presenter_notes">
<header><h1>Presenter Notes</h1></header>
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<header><h1>... until you ask them to.</h1></header>
<section><div class="highlight"><pre><span class="lineno">1</span> <span class="k">val</span> <span class="n">task</span> <span class="k">=</span> <span class="nc">Task</span> <span class="o">{</span> <span class="nc">TheNukes</span><span class="o">.</span><span class="n">launch</span><span class="o">()</span> <span class="o">}</span>
<span class="lineno">2</span> <span class="n">headForShelter</span><span class="o">()</span>
<span class="lineno">3</span> <span class="n">task</span><span class="o">.</span><span class="n">run</span>
</pre></div>
<p><img alt="Mushroom cloud" src="file:///home/ross/src/indyscala/indyscala-tasks/images/shroom.jpg" /></p></section>
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<div class="presenter_notes">
<header><h1>Presenter Notes</h1></header>
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<header><h1>A Future's side effects run once.</h1></header>
<section><div class="highlight"><pre><span class="lineno">1</span> <span class="k">val</span> <span class="n">f</span> <span class="k">=</span> <span class="nc">Future</span> <span class="o">{</span> <span class="nc">TheNukes</span><span class="o">.</span><span class="n">launch</span><span class="o">()</span> <span class="o">}</span>
<span class="lineno">2</span> <span class="nc">Await</span><span class="o">.</span><span class="n">result</span><span class="o">(</span><span class="n">f</span><span class="o">,</span> <span class="mf">1.</span><span class="n">second</span><span class="o">)</span>
<span class="lineno">3</span> <span class="nc">Await</span><span class="o">.</span><span class="n">result</span><span class="o">(</span><span class="n">f</span><span class="o">,</span> <span class="mf">1.</span><span class="n">second</span><span class="o">)</span>
</pre></div>
<p><img alt="Mushroom cloud" src="file:///home/ross/src/indyscala/indyscala-tasks/images/shroom.jpg" /></p></section>
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<header><h1>Presenter Notes</h1></header>
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<header><h1>A Task's side effects run each time.</h1></header>
<section><div class="highlight"><pre><span class="lineno">1</span> <span class="k">val</span> <span class="n">t</span> <span class="k">=</span> <span class="nc">Task</span> <span class="o">{</span> <span class="nc">TheNukes</span><span class="o">.</span><span class="n">launch</span><span class="o">()</span> <span class="o">}</span>
<span class="lineno">2</span> <span class="n">t</span><span class="o">.</span><span class="n">run</span>
<span class="lineno">3</span> <span class="n">t</span><span class="o">.</span><span class="n">run</span>
</pre></div>
<p><img alt="Mushroom cloud" src="file:///home/ross/src/indyscala/indyscala-tasks/images/shroom.jpg" />
<img alt="Mushroom cloud" src="file:///home/ross/src/indyscala/indyscala-tasks/images/shroom.jpg" /> </p></section>
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<header><h1>Presenter Notes</h1></header>
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<header><h1>I like how Future does it.</h1></header>
<section><p>Fine.</p>
<div class="highlight"><pre><span class="lineno">1</span> <span class="k">val</span> <span class="n">t</span> <span class="k">=</span> <span class="nc">Task</span><span class="o">.</span><span class="n">unsafeStart</span> <span class="o">{</span> <span class="nc">TheNukes</span><span class="o">.</span><span class="n">launch</span><span class="o">()</span> <span class="o">}</span>
<span class="lineno">2</span> <span class="n">t</span><span class="o">.</span><span class="n">run</span>
<span class="lineno">3</span> <span class="n">t</span><span class="o">.</span><span class="n">run</span>
</pre></div>
<p><img alt="Mushroom cloud" src="file:///home/ross/src/indyscala/indyscala-tasks/images/shroom.jpg" /> </p></section>
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<header><h1>Presenter Notes</h1></header>
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<header><h1>Site monitor reprise</h1></header>
<section><div class="highlight"><pre><span class="lineno">1</span> <span class="k">val</span> <span class="n">f</span> <span class="k">=</span> <span class="nc">Future</span><span class="o">.</span><span class="n">fold</span><span class="o">(</span><span class="n">sites</span><span class="o">.</span><span class="n">map</span><span class="o">(</span><span class="n">isDown</span><span class="o">))(</span><span class="kc">false</span><span class="o">)(</span><span class="k">_</span> <span class="o">||</span> <span class="k">_</span><span class="o">)</span>
<span class="lineno">2</span> <span class="k">val</span> <span class="n">t</span> <span class="k">=</span> <span class="nc">Task</span><span class="o">.</span><span class="n">reduceUnordered</span><span class="o">[</span><span class="kt">Boolean</span>, <span class="kt">Boolean</span><span class="o">](</span><span class="n">sites</span><span class="o">.</span><span class="n">map</span><span class="o">(</span><span class="n">isDown</span><span class="o">))</span>
</pre></div>
<ul>
<li><code>f</code> will always contain the same value when completed.</li>
<li><code>t</code> will rerun the monitoring and produce a current value on each run.</li>
</ul></section>
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<header><h1>Presenter Notes</h1></header>
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<header><h1>Future methods that submit to executor:</h1></header>
<section><ul>
<li><code>andThen</code></li>
<li><code>collect</code></li>
<li><code>filter</code></li>
<li><code>flatMap</code></li>
<li><code>foreach</code></li>
<li><code>onFailure</code></li>
<li><code>onSuccess</code></li>
<li><code>recover</code></li>
<li><code>transform</code></li>
<li><code>withFilter</code></li>
</ul></section>
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<header><h1>Task methods that submit to executor:</h1></header>
<section><p>[This slide intentionally left blank]</p></section>
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<header><h1>Execution model</h1></header>
<section><ul>
<li>
<p>Future makes you opt out of thread hopping by explicitly setting an
executor that stays on the same thread.</p>
<ul>
<li>It's easy to saturate your thread pool. </li>
<li>It's also easy to waste time submitting trivial work to it.</li>
<li>For optimal performance, swap custom execution contexts into the hotspots.</li>
</ul>
</li>
<li>
<p>Task makes you opt into thread hopping.</p>
<ul>
<li>It's easy to accidentally only use one core in your complicated</li>
<li>But it'll work really efficiently on that one core.</li>
<li>For optimal performance, salt to taste with Task.apply and Task.fork</li>
</ul>
</li>
<li>
<p>Bottom line: both ultimately offer the same control, promote a different
kind of naive mistake.</p>
</li>
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<header><h1>Hey, what about that Twitter thing?</h1></header>
<section><ul>
<li>Twitter's Future is cancellable.</li>
<li>Scala's Future is not cancellable.</li>
<li>Scalaz's Task is cancellable.</li>
</ul></section>
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<header><h1>Tasks are cancellable</h1></header>
<section><div class="highlight"><pre><span class="lineno"> 1</span> <span class="k">val</span> <span class="n">neverMind</span> <span class="k">=</span> <span class="k">new</span> <span class="nc">AtomicBoolean</span><span class="o">(</span><span class="kc">false</span><span class="o">)</span>
<span class="lineno"> 2</span> <span class="nc">Task</span> <span class="o">{</span>
<span class="lineno"> 3</span> <span class="nc">Thread</span><span class="o">.</span><span class="n">sleep</span><span class="o">(</span><span class="mi">500</span><span class="o">)</span>
<span class="lineno"> 4</span> <span class="n">neverMind</span><span class="o">.</span><span class="n">set</span><span class="o">(</span><span class="kc">true</span><span class="o">)</span>
<span class="lineno"> 5</span> <span class="o">}.</span><span class="n">runAsync</span> <span class="o">{</span>
<span class="lineno"> 6</span> <span class="k">case</span> <span class="o">-\/(</span><span class="n">t</span><span class="o">)</span> <span class="k">=></span> <span class="n">t</span><span class="o">.</span><span class="n">printStackTrace</span><span class="o">()</span>
<span class="lineno"> 7</span> <span class="k">case</span> <span class="o">\/-(())</span> <span class="k">=></span> <span class="n">println</span><span class="o">(</span><span class="s">"Cancelled"</span><span class="o">)</span>
<span class="lineno"> 8</span> <span class="o">}</span>
<span class="lineno"> 9</span> <span class="k">val</span> <span class="n">t</span> <span class="k">=</span> <span class="nc">Task</span><span class="o">.</span><span class="n">delay</span> <span class="o">{</span>
<span class="lineno">10</span> <span class="nc">Thread</span><span class="o">.</span><span class="n">sleep</span><span class="o">(</span><span class="mi">1000</span><span class="o">)</span>
<span class="lineno">11</span> <span class="nc">TheNukes</span><span class="o">.</span><span class="n">launch</span><span class="o">()</span>
<span class="lineno">12</span> <span class="o">}</span>
<span class="lineno">13</span> <span class="n">t</span><span class="o">.</span><span class="n">runAsyncInterruptibly</span><span class="o">({</span>
<span class="lineno">14</span> <span class="k">case</span> <span class="o">-\/(</span><span class="n">t</span><span class="o">)</span> <span class="k">=></span> <span class="n">t</span><span class="o">.</span><span class="n">printStackTrace</span><span class="o">()</span>
<span class="lineno">15</span> <span class="k">case</span> <span class="o">\/-(())</span> <span class="k">=></span> <span class="n">println</span><span class="o">(</span><span class="s">"Completed"</span><span class="o">)</span>
<span class="lineno">16</span> <span class="o">},</span> <span class="n">neverMind</span><span class="o">)</span>
</pre></div>
<p><img alt="bucolic cow" src="file:///home/ross/src/indyscala/indyscala-tasks/images/bucolic.jpg" /> </p></section>
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<header><h1>... but not to the point of nuclear safety</h1></header>
<section><div class="highlight"><pre><span class="lineno"> 1</span> <span class="k">val</span> <span class="n">neverMind</span> <span class="k">=</span> <span class="k">new</span> <span class="nc">AtomicBoolean</span><span class="o">(</span><span class="kc">false</span><span class="o">)</span>
<span class="lineno"> 2</span> <span class="nc">Task</span> <span class="o">{</span>
<span class="lineno"> 3</span> <span class="nc">Thread</span><span class="o">.</span><span class="n">sleep</span><span class="o">(</span><span class="mi">500</span><span class="o">)</span>
<span class="lineno"> 4</span> <span class="n">neverMind</span><span class="o">.</span><span class="n">set</span><span class="o">(</span><span class="kc">true</span><span class="o">)</span>
<span class="lineno"> 5</span> <span class="o">}.</span><span class="n">runAsync</span> <span class="o">{</span>
<span class="lineno"> 6</span> <span class="k">case</span> <span class="o">-\/(</span><span class="n">t</span><span class="o">)</span> <span class="k">=></span> <span class="n">t</span><span class="o">.</span><span class="n">printStackTrace</span><span class="o">()</span>
<span class="lineno"> 7</span> <span class="k">case</span> <span class="o">\/-(())</span> <span class="k">=></span> <span class="n">println</span><span class="o">(</span><span class="s">"Cancelled"</span><span class="o">)</span>
<span class="lineno"> 8</span> <span class="o">}</span>
<span class="lineno"> 9</span>
<span class="lineno">10</span> <span class="k">val</span> <span class="n">t</span> <span class="k">=</span> <span class="nc">Task</span><span class="o">.</span><span class="n">delay</span> <span class="o">{</span>
<span class="lineno">11</span> <span class="nc">Thread</span><span class="o">.</span><span class="n">sleep</span><span class="o">(</span><span class="mi">1000</span><span class="o">)</span>
<span class="lineno">12</span> <span class="nc">TheNukes</span><span class="o">.</span><span class="n">launch</span><span class="o">()</span>
<span class="lineno">13</span> <span class="o">}</span>
<span class="lineno">14</span> <span class="n">t</span><span class="o">.</span><span class="n">runAsyncInterruptibly</span><span class="o">({</span>
<span class="lineno">15</span> <span class="k">case</span> <span class="o">-\/(</span><span class="n">t</span><span class="o">)</span> <span class="k">=></span> <span class="n">t</span><span class="o">.</span><span class="n">printStackTrace</span><span class="o">()</span>
<span class="lineno">16</span> <span class="k">case</span> <span class="o">\/-(())</span> <span class="k">=></span> <span class="n">println</span><span class="o">(</span><span class="s">"Completed"</span><span class="o">)</span>
<span class="lineno">17</span> <span class="o">},</span> <span class="n">neverMind</span><span class="o">)</span>
</pre></div>
<p><img alt="Mushroom cloud" src="file:///home/ross/src/indyscala/indyscala-tasks/images/shroom.jpg" /> </p></section>
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Source: <a href="slides.md">slides.md</a>
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25/31
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