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<!DOCTYPE html>
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Copyright 2010 Google Inc.
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you may not use this file except in compliance with the License.
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limitations under the License.
Original slides: Marcin Wichary (mwichary@google.com)
Modifications: Ernest Delgado (ernestd@google.com)
Alex Russell (slightlyoff@chromium.org)
landslide modifications: Adam Zapletal (adamzap@gmail.com)
Nicolas Perriault (nperriault@gmail.com)
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<section><div style="border-radius: 10px; background: #EEEEEE; padding: 20px; text-align: center; font-size: 1.5em">
<big><b>scalaz "For the Rest of Us"</b></big> </br>
</br>
Adam Rosien <br/>
<code>arosien@box.com && adam@rosien.net</code> <br/>
<br/>
<code>@arosien #scalasv #scalaz</code>
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<section><p><img alt="" src="img/ma.jpg" /></p>
<p><code>scalaz</code> has a (undeserved?) reputation as being, well, kind of crazy.</p>
<p>So this talk is specifically <em>not</em> about:</p>
<ul>
<li>Functors, Monads, or Applicative Functors</li>
<li>Category theory</li>
<li>Other really cool stuff you should learn about (eventually)</li>
</ul></section>
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<section><p>This talk <em>is</em> about <strong>every-day situations</strong> where <code>scalaz</code> can:</p>
<ul>
<li><em>Reduce</em> syntactical noise</li>
<li>Provide useful types that solve <em>many classes</em> of problems</li>
<li><em>Add</em> type-safety with minimal "extra work"</li>
</ul>
<p class="notes">We'll talk about <code>scalaz</code> for (1) memoization, (2) domain model validation, (3) dependency injection and (4) better style.</p></section>
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<header><h1>Getting Started</h1></header>
<section><p>In <code>build.sbt</code>:</p>
<table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre>1
2</pre></div></td><td class="code"><div class="highlight"><pre><span class="n">libraryDependencies</span> <span class="o">+=</span>
<span class="s">"org.scalaz"</span> <span class="o">%%</span> <span class="s">"scalaz-core"</span> <span class="o">%</span> <span class="s">"6.0.4"</span>
</pre></div>
</td></tr></table>
<p>Then:</p>
<table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre>1
2
3
4</pre></div></td><td class="code"><div class="highlight"><pre><span class="k">import</span> <span class="nn">scalaz._</span>
<span class="k">import</span> <span class="nn">Scalaz._</span>
<span class="c1">// profit</span>
</pre></div>
</td></tr></table>
<p class="notes">This is <em>scalaz 6.</em> Also, assume this is imported in all code snippets.</p></section>
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<header><h1>Presenter Notes</h1></header>
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<header><h1>Memoization</h1></header>
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<header><h1>Presenter Notes</h1></header>
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<header><h1>Memoization</h1></header>
<section><p>The goal: cache the result of an expensive computation.</p>
<table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre>1
2
3
4
5
6
7</pre></div></td><td class="code"><div class="highlight"><pre><span class="k">def</span> <span class="n">expensive</span><span class="o">(</span><span class="n">foo</span><span class="k">:</span> <span class="kt">Foo</span><span class="o">)</span><span class="k">:</span> <span class="kt">Bar</span> <span class="o">=</span> <span class="o">...</span>
<span class="k">val</span> <span class="n">f</span><span class="k">:</span> <span class="kt">Foo</span>
<span class="n">expensive</span><span class="o">(</span><span class="n">f</span><span class="o">)</span> <span class="c1">// $$$</span>
<span class="n">expensive</span><span class="o">(</span><span class="n">f</span><span class="o">)</span> <span class="c1">// $$$</span>
<span class="o">...</span>
</pre></div>
</td></tr></table>
<p class="notes">Assumption: <code>expensive</code> produces the same output for every input, i.e., is referentially-transparent.</p></section>
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<header><h1>Memoization</h1></header>
<section><p>Typically you might use a <code>mutable.Map</code> to cache results:</p>
<table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre>1
2
3
4</pre></div></td><td class="code"><div class="highlight"><pre><span class="k">val</span> <span class="n">cache</span> <span class="k">=</span> <span class="n">collection</span><span class="o">.</span><span class="n">mutable</span><span class="o">.</span><span class="nc">Map</span><span class="o">[</span><span class="kt">Foo</span>, <span class="kt">Bar</span><span class="o">]()</span>
<span class="n">cache</span><span class="o">.</span><span class="n">getOrElseUpdate</span><span class="o">(</span><span class="n">f</span><span class="o">,</span> <span class="n">expensive</span><span class="o">(</span><span class="n">f</span><span class="o">))</span> <span class="c1">// $$$</span>
<span class="n">cache</span><span class="o">.</span><span class="n">getOrElseUpdate</span><span class="o">(</span><span class="n">f</span><span class="o">,</span> <span class="n">expensive</span><span class="o">(</span><span class="n">f</span><span class="o">))</span> <span class="c1">// 1¢</span>
</pre></div>
</td></tr></table>
<p class="notes">Downsides: the cache is not the same type as the function: <code>Foo => Bar</code> vs. <code>Map[Foo, Bar]</code>. It's also not DRY.</p></section>
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<header><h1>Memoization</h1></header>
<section><p>You can try to make it look like a regular function, avoiding the <code>getOrElseUpdate()</code> call:</p>
<table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre>1
2
3
4
5
6</pre></div></td><td class="code"><div class="highlight"><pre><span class="k">val</span> <span class="n">cache</span><span class="k">:</span> <span class="kt">Foo</span> <span class="o">=></span> <span class="nc">Bar</span> <span class="k">=</span>
<span class="n">collection</span><span class="o">.</span><span class="n">mutable</span><span class="o">.</span><span class="nc">Map</span><span class="o">[</span><span class="kt">Foo</span>, <span class="kt">Bar</span><span class="o">]()</span>
<span class="o">.</span><span class="n">withDefault</span><span class="o">(</span><span class="n">expensive</span> <span class="k">_</span><span class="o">)</span>
<span class="n">cache</span><span class="o">(</span><span class="n">f</span><span class="o">)</span> <span class="c1">// $$$ (miss & NO fill)</span>
<span class="n">cache</span><span class="o">(</span><span class="n">f</span><span class="o">)</span> <span class="c1">// $$$ (miss & NO fill)</span>
</pre></div>
</td></tr></table>
<p>But it doesn't actually cache.</p></section>
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<header><h1>Memoization</h1></header>
<section><p>In <code>scalaz</code>:</p>
<table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre> 1
2
3
4
5
6
7
8
9
10
11</pre></div></td><td class="code"><div class="highlight"><pre><span class="k">def</span> <span class="n">expensive</span><span class="o">(</span><span class="n">foo</span><span class="k">:</span> <span class="kt">Foo</span><span class="o">)</span><span class="k">:</span> <span class="kt">Bar</span> <span class="o">=</span> <span class="o">...</span>
<span class="c1">// Memo[Foo, Bar]</span>
<span class="k">val</span> <span class="n">memo</span> <span class="k">=</span> <span class="n">immutableHashMapMemo</span> <span class="o">{</span>
<span class="n">foo</span><span class="k">:</span> <span class="kt">Foo</span> <span class="o">=></span> <span class="n">expensive</span><span class="o">(</span><span class="n">foo</span><span class="o">)</span>
<span class="o">}</span>
<span class="k">val</span> <span class="n">f</span><span class="k">:</span> <span class="kt">Foo</span>
<span class="n">memo</span><span class="o">(</span><span class="n">f</span><span class="o">)</span> <span class="c1">// $$$ (cache miss & fill)</span>
<span class="n">memo</span><span class="o">(</span><span class="n">f</span><span class="o">)</span> <span class="c1">// 1¢ (cache hit)</span>
</pre></div>
</td></tr></table></section>
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<header><h1>Memoization</h1></header>
<section><p>Many memoization strategies:</p>
<table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre>1
2
3
4
5
6
7
8
9</pre></div></td><td class="code"><div class="highlight"><pre><span class="n">immutableHashMapMemo</span><span class="o">[</span><span class="kt">K</span>, <span class="kt">V</span><span class="o">]</span>
<span class="n">mutableHashMapMemo</span><span class="o">[</span><span class="kt">K</span>, <span class="kt">V</span><span class="o">]</span>
<span class="c1">// remove + gc unreferenced entries</span>
<span class="n">weakHashMapMemo</span><span class="o">[</span><span class="kt">K</span>, <span class="kt">V</span><span class="o">]</span>
<span class="c1">// fixed size, K = Int</span>
<span class="n">arrayMemo</span><span class="o">[</span><span class="kt">V</span><span class="o">](</span><span class="n">size</span><span class="k">:</span> <span class="kt">Int</span><span class="o">)</span>
</pre></div>
</td></tr></table>
<p class="notes">Super-nerdy: the memoizing strategies are just functions of <code>K => V</code>, which means the generic <code>memo()</code> constructor has the same signature as the Y-combinator!</p></section>
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<section><p>Remove the need for temporary variables:</p>
<table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre> 1
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13</pre></div></td><td class="code"><div class="highlight"><pre><span class="k">val</span> <span class="n">f</span><span class="k">:</span> <span class="kt">A</span> <span class="o">=></span> <span class="n">B</span>
<span class="k">val</span> <span class="n">g</span><span class="k">:</span> <span class="kt">B</span> <span class="o">=></span> <span class="n">C</span>
<span class="c1">// using temps:</span>
<span class="k">val</span> <span class="n">a</span><span class="k">:</span> <span class="kt">A</span> <span class="o">=</span> <span class="o">...</span>
<span class="k">val</span> <span class="n">b</span> <span class="k">=</span> <span class="n">f</span><span class="o">(</span><span class="n">a</span><span class="o">)</span>
<span class="k">val</span> <span class="n">c</span> <span class="k">=</span> <span class="n">g</span><span class="o">(</span><span class="n">b</span><span class="o">)</span>
<span class="c1">// or via composition, which is a bit ugly:</span>
<span class="k">val</span> <span class="n">c</span> <span class="k">=</span> <span class="n">g</span><span class="o">(</span><span class="n">f</span><span class="o">(</span><span class="n">a</span><span class="o">))</span>
<span class="c1">// "unix-pipey"!</span>
<span class="k">val</span> <span class="n">c</span> <span class="k">=</span> <span class="n">a</span> <span class="o">|></span> <span class="n">f</span> <span class="o">|></span> <span class="n">g</span>
</pre></div>
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<section><p>When you just can't stand all that (keyboard) typing: </p>
<table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre>1
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<span class="c1">// ternary-operator-ish</span>
<span class="n">p</span> <span class="o">?</span> <span class="s">"yes"</span> <span class="o">|</span> <span class="s">"no"</span> <span class="c1">// if (p) "yes" else "no"</span>
<span class="k">val</span> <span class="n">o</span><span class="k">:</span> <span class="kt">Option</span><span class="o">[</span><span class="kt">String</span><span class="o">]</span>
<span class="n">o</span> <span class="o">|</span> <span class="s">"meh"</span> <span class="c1">// o.getOrElse("meh")</span>
</pre></div>
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<section><p>More legible (and more type-safe):</p>
<table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre>1
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<span class="nc">Some</span><span class="o">(</span><span class="s">"foo"</span><span class="o">)</span> <span class="c1">// Some[String]</span>
<span class="nc">None</span> <span class="c1">// None.type</span>
<span class="c1">// scalaz</span>
<span class="s">"foo"</span><span class="o">.</span><span class="n">some</span> <span class="c1">// Option[String]</span>
<span class="n">none</span> <span class="c1">// Option[Nothing], oops!</span>
<span class="n">none</span><span class="o">[</span><span class="kt">String</span><span class="o">]</span> <span class="c1">// Option[String]</span>
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<section><p>More legible (and more type-safe):</p>
<table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre>1
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9</pre></div></td><td class="code"><div class="highlight"><pre><span class="c1">// scala</span>
<span class="nc">Right</span><span class="o">(</span><span class="mi">42</span><span class="o">)</span> <span class="c1">// Right[Nothing, Int], oops!</span>
<span class="nc">Left</span><span class="o">(</span><span class="s">"meh"</span><span class="o">)</span> <span class="c1">// Left[String, Nothing], oops!</span>
<span class="nc">Right</span><span class="o">[</span><span class="kt">String</span>, <span class="kt">Int</span><span class="o">](</span><span class="mi">42</span><span class="o">)</span> <span class="c1">// verbose</span>
<span class="nc">Left</span><span class="o">[</span><span class="kt">String</span>, <span class="kt">Int</span><span class="o">](</span><span class="s">"meh"</span><span class="o">)</span> <span class="c1">// verbose</span>
<span class="c1">// scalaz</span>
<span class="mf">42.</span><span class="n">right</span><span class="o">[</span><span class="kt">String</span><span class="o">]</span> <span class="c1">// Either[String, Int]</span>
<span class="s">"meh"</span><span class="o">.</span><span class="n">left</span><span class="o">[</span><span class="kt">Int</span><span class="o">]</span> <span class="c1">// Either[String, Int]</span>
</pre></div>
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<section><p>This isn't good:</p>
<table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre>1
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<span class="n">first3</span><span class="k">:</span> <span class="kt">Int</span><span class="o">,</span>
<span class="n">second2</span><span class="k">:</span> <span class="kt">Int</span><span class="o">,</span>
<span class="n">third4</span><span class="k">:</span> <span class="kt">Int</span><span class="o">)</span>
<span class="nc">SSN</span><span class="o">(</span><span class="mi">123</span><span class="o">,</span> <span class="mi">123</span><span class="o">,</span> <span class="mi">1234</span><span class="o">)</span>
<span class="c1">// ^^^ nooooo!</span>
</pre></div>
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<section><p>This shouldn't be possible:</p>
<table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre>1
2
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4</pre></div></td><td class="code"><div class="highlight"><pre><span class="k">case</span> <span class="k">class</span> <span class="nc">Version</span><span class="o">(</span><span class="n">major</span><span class="k">:</span> <span class="kt">Int</span><span class="o">,</span> <span class="n">minor</span><span class="k">:</span> <span class="kt">Int</span><span class="o">)</span>
<span class="nc">Version</span><span class="o">(</span><span class="mi">1</span><span class="o">,</span> <span class="o">-</span><span class="mi">1</span><span class="o">)</span>
<span class="c1">// ^^ nooooo!</span>
</pre></div>
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<section><p>Meh:</p>
<table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre>1
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7</pre></div></td><td class="code"><div class="highlight"><pre><span class="k">case</span> <span class="k">class</span> <span class="nc">Dependency</span><span class="o">(</span>
<span class="n">organization</span><span class="k">:</span> <span class="kt">String</span><span class="o">,</span>
<span class="n">artifactId</span><span class="k">:</span> <span class="kt">String</span><span class="o">,</span>
<span class="n">version</span><span class="k">:</span> <span class="kt">Version</span><span class="o">)</span>
<span class="nc">Dependency</span><span class="o">(</span><span class="s">"zerb"</span><span class="o">,</span> <span class="s">""</span><span class="o">,</span> <span class="nc">Version</span><span class="o">(</span><span class="mi">1</span><span class="o">,</span> <span class="mi">2</span><span class="o">))</span>
<span class="c1">// ^^ nooo!</span>
</pre></div>
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<section><p><img alt="noooooo" src="img/no.jpg" /></p></section>
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<section><p>The problem is that the types as-is aren't really accurate.<br />
<code>String</code>s and <code>Int</code>s are being used too broadly.
We really want "<code>Int</code>s greater than zero",
"<code>String</code>s that match a pattern", etc.</p>
<p>You can do the checks in the constructor:</p>
<table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre>1
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8</pre></div></td><td class="code"><div class="highlight"><pre><span class="k">case</span> <span class="k">class</span> <span class="nc">Version</span><span class="o">(</span><span class="n">major</span><span class="k">:</span> <span class="kt">Int</span><span class="o">,</span> <span class="n">minor</span><span class="k">:</span> <span class="kt">Int</span><span class="o">)</span> <span class="o">{</span>
<span class="n">require</span><span class="o">(</span>
<span class="n">major</span> <span class="o">>=</span> <span class="mi">0</span><span class="o">,</span>
<span class="s">"major must be >= 0: %s"</span><span class="o">.</span><span class="n">format</span><span class="o">(</span><span class="n">major</span><span class="o">))</span>
<span class="n">require</span><span class="o">(</span>
<span class="n">minor</span> <span class="o">>=</span> <span class="mi">0</span><span class="o">,</span>
<span class="s">"minor must be >= 0: %s"</span><span class="o">.</span><span class="n">format</span><span class="o">(</span><span class="n">minor</span><span class="o">))</span>
<span class="o">}</span>
</pre></div>
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