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<!DOCTYPE html>
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<h2><a href="https://www.lyh.me/featran.html">Featran</a></h2>
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<i class="fa fa-calendar"></i><time datetime="2017-12-11T13:58:00-05:00"> Mon 11 December 2017</time>
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<div class="summary"><p>Featran is a Scala feature transformation library for data science and machine learning. We recently open sourced it and I gave talks at <a href="http://scale.bythebay.io/">Scale By the Bay</a> and <a href="http://www.criteo.com/events/nabdconf-palo-alto/">NABDConf</a>. Here are the <a href="/slides/featran.html">slides</a>.</p>
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<h2><a href="https://www.lyh.me/automatic-type-class-derivation-with-shapeless.html">Automatic type-class derivation with Shapeless</a></h2>
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<i class="fa fa-calendar"></i><time datetime="2017-08-21T09:30:00-04:00"> Mon 21 August 2017</time>
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<div class="summary"><p>We had a knowledge sharing session at work recently on <a href="https://github.com/milessabin/shapeless/">Shapeless</a> for automatic type class derivation. Here is a little write-up for the topic.</p>
<h2>Scala List</h2>
<p>First let’s review how <code>List</code> works in Scala. A <code>List</code> is a linked list with <code>head</code> and <code>tail</code>, plus <code>Nil</code> for empty list. It can be represented with the following abstract data type:</p>
<div class="highlight"><pre><span></span><span class="k">sealed</span> <span class="k">trait</span> <span class="nc">List</span><span class="o">[</span><span class="kt">+A</span><span class="o">]</span> <span class="o">{</span>
<span class="k">def</span> <span class="o">::[</span><span class="kt">B</span> <span class="k">>:</span> <span class="kt">A</span><span class="o">](</span><span class="n">head</span><span class="k">:</span> <span class="kt">B</span><span class="o">)</span><span class="k">:</span> <span class="kt">List</span><span class="o">[</span><span class="kt">B</span><span class="o">]</span> <span class="k">=</span> <span class="nc">Cons</span><span class="o">(</span><span class="n">head</span><span class="o">,</span> <span class="k">this</span><span class="o">)</span>
<span class="o">}</span>
<span class="k">case</span> <span class="k">object</span> <span class="nc">Nil</span> <span class="k">extends</span> <span class="nc">List</span><span class="o">[</span><span class="kt">Nothing</span><span class="o">]</span> <span class="c1">// Nothing is a sub-type of every other type</span>
<span class="k">case</span> <span class="k">class</span> <span class="nc">Cons</span><span class="o">[</span><span class="kt">+A</span><span class="o">](</span><span class="n">head</span><span class="k">:</span> <span class="kt">A</span><span class="o">,</span> <span class="n">tail</span><span class="k">:</span> <span class="kt">List</span><span class="o">[</span><span class="kt">A</span><span class="o">])</span> <span class="k">extends</span> <span class="nc">List</span><span class="o">[</span><span class="kt">A</span><span class="o">]</span>
</pre></div>
<p>Notice that <code>::</code>, the list concatenation operation, is just a method on trait <code>List[+A]</code>. Since Scala operators that end with <code>:</code> are right-associative, we can conveniently create lists by chaining multiple <code>::</code>s. Therefore the following expressions are equivalent:</p>
<div class="highlight"><pre><span></span><span class="mi">1</span> <span class="o">::</span> <span class="mi">2</span> <span class="o">::</span> <span class="nc">Nil</span>
<span class="mi">1</span> <span class="o">::</span> <span class="o">(</span><span class="mi">2</span> <span class="o">::</span> <span class="nc">Nil</span><span class="o">)</span>
<span class="nc">Nil</span><span class="o">.::(</span><span class="mi">2</span><span class="o">).::(</span><span class="mi">1</span><span class="o">)</span>
<span class="nc">Cons</span><span class="o">(</span><span class="mi">1</span><span class="o">,</span> <span class="nc">Cons</span><span class="o">(</span><span class="mi">2</span><span class="o">,</span> <span class="nc">Nil</span><span class="o">))</span>
</pre></div>
<p>It’s important to point out here that Scala <code>List</code> is homogeneous, i.e. it has a single type parameter <code>A</code> and thus can only store elements of <code>A</code> and its sub-types. On the other hand, it can have varying numbers of elements at runtime.</p>
<h2>Shapeless HList</h2>
<p>Since <code>List …</code></p>
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<h2><a href="https://www.lyh.me/lambda-serialization.html">Lambda serialization</a></h2>
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<div class="summary"><p>Lambda serialization is one of the more confusion issues in distributed data processing in Scala. No matter which framework you choose, whether it’s Scalding, Spark, Flink or Scio, sooner or later you’ll be hit by the dreaded <code>NotSerializableException</code>. In this post we’ll take a closer look at the common causes and solutions to this problem.</p>
<h2>Setup</h2>
<p>To demonstrate the problem, first we need a minimal setup that minics the behavior of a distributed data processing system. We start with a utility method that roundtrips an object throguh Java serialization. Anonymous functions, or lambdas, in such systems are serialized so that they can be distributed to workers for parallel processing.</p>
<div class="highlight"><pre><span></span><span class="k">import</span> <span class="nn">java.io.</span><span class="o">{</span><span class="nc">ByteArrayInputStream</span><span class="o">,</span> <span class="nc">ByteArrayOutputStream</span><span class="o">,</span> <span class="nc">ObjectInputStream</span><span class="o">,</span> <span class="nc">ObjectOutputStream</span><span class="o">}</span>
<span class="k">object</span> <span class="nc">SerDeUtil</span> <span class="o">{</span>
<span class="k">def</span> <span class="n">serDe</span><span class="o">[</span><span class="kt">T</span><span class="o">](</span><span class="n">obj</span><span class="k">:</span> <span class="kt">T</span><span class="o">)</span><span class="k">:</span> <span class="kt">T</span> <span class="o">=</span> <span class="o">{</span>
<span class="k">val</span> <span class="n">buffer</span> <span class="k">=</span> <span class="k">new</span> <span class="nc">ByteArrayOutputStream</span><span class="o">()</span>
<span class="k">val</span> <span class="n">out</span> <span class="k">=</span> <span class="k">new</span> <span class="nc">ObjectOutputStream</span><span class="o">(</span><span class="n">buffer</span><span class="o">)</span>
<span class="n">out</span><span class="o">.</span><span class="n">writeObject</span><span class="o">(</span><span class="n">obj</span><span class="o">)</span>
<span class="n">out</span><span class="o">.</span><span class="n">close</span><span class="o">()</span>
<span class="k">val</span> <span class="n">in</span> <span class="k">=</span> <span class="k">new</span> <span class="nc">ObjectInputStream</span><span class="o">(</span><span class="k">new</span> <span class="nc">ByteArrayInputStream</span><span class="o">(</span><span class="n">buffer</span><span class="o">.</span><span class="n">toByteArray</span><span class="o">))</span>
<span class="n">in</span><span class="o">.</span><span class="n">readObject</span><span class="o">().</span><span class="n">asInstanceOf</span><span class="o">[</span><span class="kt">T</span><span class="o">]</span>
<span class="o">}</span>
<span class="o">}</span>
</pre></div>
<p>Next we create a bare minimal <code>Collection[T]</code> type that mimics an abstract distributed data set, akin to <code>TypedPipe</code>, <code>RDD</code>, or <code>SCollection</code> in Scalding, Spark or Scio respectively. Our implementation is backed by a local in-memory <code>Seq[T]</code> but does pass the function <code>f</code> through serialization like …</p>
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<h2><a href="https://www.lyh.me/lawfulness-of-aggregatebykey.html">Lawfulness of aggregateByKey</a></h2>
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<div class="summary"><p>I spent a couple hours yesterday debugging what I thought was a Protobuf serialization issue, which turns out to be an unlawful Monoid-like use of <code>aggregateByKey</code> in <a href="https://github.com/spotify/scio">Scio</a>.</p>
<h2>The Problem</h2>
<p>Both Scio and Spark have <code>aggregate</code> and <code>aggregateByKey</code> transformations that look like this:</p>
<div class="highlight"><pre><span></span><span class="c1">// on SCollection[V]</span>
<span class="k">def</span> <span class="n">aggregate</span><span class="o">[</span><span class="kt">U</span><span class="o">](</span><span class="n">zeroValue</span><span class="k">:</span> <span class="kt">U</span><span class="o">)(</span><span class="n">seqOp</span><span class="k">:</span> <span class="o">(</span><span class="kt">U</span><span class="o">,</span> <span class="kt">V</span><span class="o">)</span> <span class="k">=></span> <span class="n">U</span><span class="o">,</span> <span class="n">combOp</span><span class="k">:</span> <span class="o">(</span><span class="kt">U</span><span class="o">,</span> <span class="kt">U</span><span class="o">)</span> <span class="k">=></span> <span class="n">U</span><span class="o">)</span><span class="k">:</span> <span class="kt">SCollection</span><span class="o">[</span><span class="kt">U</span><span class="o">]</span>
<span class="c1">// on SCollection[(K, V)]</span>
<span class="k">def</span> <span class="n">aggregateByKey</span><span class="o">[</span><span class="kt">U</span><span class="o">](</span><span class="n">zeroValue</span><span class="k">:</span> <span class="kt">U</span><span class="o">)(</span><span class="n">seqOp</span><span class="k">:</span> <span class="o">(</span><span class="kt">U</span><span class="o">,</span> <span class="kt">V</span><span class="o">)</span> <span class="k">=></span> <span class="n">U</span><span class="o">,</span> <span class="n">combOp</span><span class="k">:</span> <span class="o">(</span><span class="kt">U</span><span class="o">,</span> <span class="kt">U</span><span class="o">)</span> <span class="k">=></span> <span class="n">U</span><span class="o">)</span><span class="k">:</span> <span class="kt">SCollection</span><span class="o">[(</span><span class="kt">K</span>, <span class="kt">U</span><span class="o">)]</span>
</pre></div>
<p>And we have some business logic that looks like this:</p>
<div class="highlight"><pre><span></span><span class="k">case</span> <span class="k">class</span> <span class="nc">Count</span><span class="o">(</span><span class="n">id</span><span class="k">:</span> <span class="kt">String</span><span class="o">,</span> <span class="n">count</span><span class="k">:</span> <span class="kt">Int</span><span class="o">)</span>
<span class="k">val</span> <span class="n">z</span> <span class="k">=</span> <span class="nc">Count</span><span class="o">(</span><span class="s">""</span><span class="o">,</span> <span class="mi">0</span><span class="o">)</span> <span class="c1">// zeroValue</span>
<span class="k">def</span> <span class="n">seqOp</span><span class="o">(</span><span class="n">acc</span><span class="k">:</span> <span class="kt">Count</span><span class="o">,</span> <span class="n">v</span><span class="k">:</span> <span class="kt">Count</span><span class="o">)</span> <span class="k">=</span> <span class="nc">Count</span><span class="o">(</span><span class="n">v</span><span class="o">.</span><span class="n">id</span><span class="o">,</span> <span class="n">acc</span><span class="o">.</span><span class="n">count</span> <span class="o">+</span> <span class="n">v</span><span class="o">.</span><span class="n">count</span><span class="o">)</span>
<span class="k">def</span> <span class="n">combOp</span><span class="o">(</span><span class="n">x</span><span class="k">:</span> <span class="kt">Count</span><span class="o">,</span> <span class="n">y</span><span class="k">:</span> <span class="kt">Count</span><span class="o">)</span> <span class="k">=</span> <span class="nc">Count</span><span class="o">(</span><span class="n">x</span><span class="o">.</span><span class="n">id</span><span class="o">,</span> <span class="n">x</span><span class="o">.</span><span class="n">count</span> <span class="o">+</span> <span class="n">y</span><span class="o">.</span><span class="n">count</span><span class="o">)</span>
<span class="n">sc</span><span class="o">.</span><span class="n">parallelize</span><span class="o">(</span><span class="nc">Seq</span><span class="o">(</span><span class="nc">Count</span><span class="o">(</span><span class="s">"a"</span><span class="o">,</span> <span class="mi">10</span><span class="o">),</span> <span class="nc">Count</span><span class="o">(</span><span class="s">"a"</span><span class="o">,</span> <span class="mi">100</span><span class="o">),</span> <span class="nc">Count</span><span class="o">(</span><span class="s">"b"</span><span class="o">,</span> <span class="mi">5</span><span class="o">),</span> <span class="nc">Count</span><span class="o">(</span><span class="s">"b"</span><span class="o">,</span> <span class="mi">50</span><span class="o">)))</span>
<span class="o">.</span><span class="n">groupBy</span><span class="o">(</span><span class="k">_</span><span class="o">.</span><span class="n">id</span><span class="o">)</span>
<span class="o">.</span><span class="n">aggregateByKey</span><span class="o">(</span><span class="n">z</span><span class="o">)(</span><span class="n">seqOp</span><span class="o">,</span> <span class="n">combOp</span><span class="o">)</span>
</pre></div>
<p>This code however, only works correctly locally with <code>DirectRunner</code> and always produces results with <code>id == ""</code> when running on Dataflow service with the <code>DataflowRunner</code>. Can you spot the bug?</p>
<h2>Monoid laws</h2>
<p>You might notice that <code>zeroValue</code> and <code>combOp</code> together resemble a <a href="https://en.wikipedia.org/wiki/Monoid">Monoid</a>, which should satisfy the identity law:</p>
<div class="highlight"><pre><span></span><span class="n">combOp</span><span class="o">(</span><span class="n">zeroValue …</span></pre></div>
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<h2><a href="https://www.lyh.me/canbuildfrom.html">CanBuildFrom</a></h2>
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<div class="summary"><p>We recently had an internal knowledge sharing on higher-kinded types and <code>CanBuildFrom</code> type classes in Scala. Here’s a short summary.</p>
<h2>Basics</h2>
<p>Let’s start by implementing <code>map</code>.</p>
<div class="highlight"><pre><span></span><span class="k">def</span> <span class="n">map</span><span class="o">(</span><span class="n">xs</span><span class="k">:</span> <span class="kt">Seq</span><span class="o">[</span><span class="kt">Int</span><span class="o">],</span> <span class="n">f</span><span class="k">:</span> <span class="kt">Int</span> <span class="o">=></span> <span class="nc">Double</span><span class="o">)</span><span class="k">:</span> <span class="kt">Seq</span><span class="o">[</span><span class="kt">Double</span><span class="o">]</span> <span class="k">=</span> <span class="n">xs</span><span class="o">.</span><span class="n">map</span><span class="o">(</span><span class="n">f</span><span class="o">)</span>
<span class="n">map</span><span class="o">(</span><span class="nc">Seq</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="mi">3</span><span class="o">),</span> <span class="k">_</span> <span class="o">+</span> <span class="mf">0.1</span><span class="o">)</span>
</pre></div>
<p>This implementation is not very good since it only works with <code>Seq[Int]</code> and <code>Int => Double</code>. It’s easy to parameterize <code>Int</code> and <code>Double</code>.</p>
<div class="highlight"><pre><span></span><span class="k">def</span> <span class="n">map</span><span class="o">[</span><span class="kt">A</span>, <span class="kt">B</span><span class="o">](</span><span class="n">xs</span><span class="k">:</span> <span class="kt">Seq</span><span class="o">[</span><span class="kt">A</span><span class="o">],</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="o">)</span><span class="k">:</span> <span class="kt">Seq</span><span class="o">[</span><span class="kt">B</span><span class="o">]</span> <span class="k">=</span> <span class="n">xs</span><span class="o">.</span><span class="n">map</span><span class="o">(</span><span class="n">f</span><span class="o">)</span>
</pre></div>
<p>However <code>map(Seq(1, 2, 3), _ + 0.1)</code> now fails to compile with a message <code>missing parameter type for expanded function ((x$1) => x$1.$plus(10))</code></p>
<p>This is because inference of <code>A</code> in <code>f: A => B</code> depends on the type of <code>xs: Seq[A]</code>, and limitation of Scala type inference. A common workaround is to curry arguments.</p>
<div class="highlight"><pre><span></span><span class="k">def</span> <span class="n">map</span><span class="o">[</span><span class="kt">A</span>, <span class="kt">B</span><span class="o">](</span><span class="n">xs</span><span class="k">:</span> <span class="kt">Seq</span><span class="o">[</span><span class="kt">A</span><span class="o">])(</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="o">)</span><span class="k">:</span> <span class="kt">Seq</span><span class="o">[</span><span class="kt">B</span><span class="o">]</span> <span class="k">=</span> <span class="n">xs</span><span class="o">.</span><span class="n">map</span><span class="o">(</span><span class="n">f</span><span class="o">)</span>
<span class="n">map</span><span class="o">(</span><span class="nc">Seq</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="mi">3</span><span class="o">))(</span><span class="k">_</span> <span class="o">+</span> <span class="mf">0.1</span><span class="o">)</span>
</pre></div>
<p>Similar pattern is commonly seen in Scala, like <code>foldLeft(z: B)(op: (B, A) => B)</code>. Another benefit is we can now write <code>f</code> in a multi-line <code>{}</code> block more elegantly.</p>
<div class="highlight"><pre><span></span><span class="n">map …</span></pre></div>
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<h2><a href="https://www.lyh.me/decompiling-scala-code.html">Decompiling Scala code</a></h2>
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<div class="summary"><p>I was bored today and decided to decompile some Scala code for fun and profit. I’m using Scala 2.12.2 and Java 1.8.0_121.</p>
<h2>Scala object</h2>
<div class="highlight"><pre><span></span><span class="k">package</span> <span class="nn">javap</span>
<span class="k">object</span> <span class="nc">Test01</span> <span class="o">{</span>
<span class="k">def</span> <span class="n">main</span><span class="o">(</span><span class="n">args</span><span class="k">:</span> <span class="kt">Array</span><span class="o">[</span><span class="kt">String</span><span class="o">])</span><span class="k">:</span> <span class="kt">Unit</span> <span class="o">=</span> <span class="nc">Unit</span>
<span class="o">}</span>
</pre></div>
<div class="highlight"><pre><span></span><span class="kd">public</span> <span class="kd">final</span> <span class="kd">class</span> <span class="nc">javap</span><span class="o">.</span><span class="na">Test01$</span> <span class="o">{</span>
<span class="kd">public</span> <span class="kd">static</span> <span class="n">javap</span><span class="o">.</span><span class="na">Test01$</span> <span class="n">MODULE$</span><span class="o">;</span>
<span class="kd">public</span> <span class="kd">static</span> <span class="o">{};</span>
<span class="kd">public</span> <span class="kt">void</span> <span class="nf">main</span><span class="o">(</span><span class="n">java</span><span class="o">.</span><span class="na">lang</span><span class="o">.</span><span class="na">String</span><span class="o">[]);</span>
<span class="kd">private</span> <span class="n">javap</span><span class="o">.</span><span class="na">Test01$</span><span class="o">();</span>
<span class="o">}</span>
<span class="kd">public</span> <span class="kd">final</span> <span class="kd">class</span> <span class="nc">javap</span><span class="o">.</span><span class="na">Test01</span> <span class="o">{</span>
<span class="kd">public</span> <span class="kd">static</span> <span class="kt">void</span> <span class="nf">main</span><span class="o">(</span><span class="n">java</span><span class="o">.</span><span class="na">lang</span><span class="o">.</span><span class="na">String</span><span class="o">[]);</span>
<span class="o">}</span>
</pre></div>
<p>As we can see a Scala object is compiled to 2 Java classes, <code>Test01</code> with static methods for Java compatibility and a <code>Test01$</code> with a static instance of itself as <code>MODULE$</code>, so that <code>Test01</code> can be used as an instance value in Scala.</p>
<h2>Class constructors</h2>
<div class="highlight"><pre><span></span><span class="k">package</span> <span class="nn">javap</span>
<span class="k">class</span> <span class="nc">Test02</span><span class="o">(</span><span class="k">val</span> <span class="n">x</span><span class="k">:</span> <span class="kt">Int</span><span class="o">,</span> <span class="k">val</span> <span class="n">y</span><span class="k">:</span> <span class="kt">Int</span><span class="o">,</span> <span class="n">z</span><span class="k">:</span> <span class="kt">Int</span><span class="o">)</span> <span class="o">{</span>
<span class="k">def</span> <span class="k">this</span><span class="o">(</span><span class="n">x</span><span class="k">:</span> <span class="kt">Int</span><span class="o">,</span> <span class="n">y</span><span class="k">:</span> <span class="kt">Int</span><span class="o">)</span> <span class="k">=</span> <span class="k">this</span><span class="o">(</span><span class="n">x</span><span class="o">,</span> <span class="n">y</span><span class="o">,</span> <span class="mi">0</span><span class="o">)</span>
<span class="o">}</span>
</pre></div>
<div class="highlight"><pre><span></span><span class="kd">public</span> <span class="kd">class</span> <span class="nc">javap</span><span class="o">.</span><span class="na">Test02</span> <span class="o">{</span>
<span class="kd">private</span> <span class="kd">final</span> <span class="kt">int</span> <span class="n">x</span><span class="o">;</span>
<span class="kd">private</span> <span class="kd">final</span> <span class="kt">int</span> <span class="n">y</span><span class="o">;</span>
<span class="kd">public</span> <span class="kt">int</span> <span class="nf">x</span><span class="o">();</span>
<span class="kd">public</span> <span class="kt">int</span> <span class="nf">y</span><span class="o">();</span>
<span class="kd">public</span> <span class="n">javap</span><span class="o">.</span><span class="na">Test02</span><span class="o">(</span><span class="kt">int</span><span class="o">,</span> <span class="kt">int</span><span class="o">,</span> <span class="kt">int</span><span class="o">);</span>
<span class="kd">public</span> <span class="n">javap</span><span class="o">.</span><span class="na">Test02</span><span class="o">(</span><span class="kt">int</span><span class="o">,</span> <span class="kt">int</span><span class="o">);</span>
<span class="o">}</span>
</pre></div>
<p>Looks like the default constructor <code>(val x: Int, val y: Int, z: Int)</code> and the overloaded one <code>(x: Int, y: Int)</code> each generated a Java constructor. However only <code>x</code> and <code>y …</code></p>
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<h2><a href="https://www.lyh.me/implicits.html">Implicits</a></h2>
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<div class="summary"><p>In this post we’re going to take a closer look at Scala implicits and various use cases.</p>
<h2>Basics</h2>
<p>Let’s first look at the basics. There’re 3 main basic uses of implicits, as an argument, as a conversion method, and enhancing an existing class, a.k.a. the “Pimp My Library” pattern.</p>
<h3>Implicit arguments</h3>
<p>Suppose we have a basic function like this.</p>
<div class="highlight"><pre><span></span><span class="k">def</span> <span class="n">plus</span><span class="o">(</span><span class="n">x</span><span class="k">:</span> <span class="kt">Int</span><span class="o">)</span> <span class="k">=</span> <span class="n">x</span> <span class="o">+</span> <span class="mi">1</span>
<span class="n">plus</span><span class="o">(</span><span class="mi">10</span><span class="o">)</span> <span class="c1">// => 11</span>
</pre></div>
<p>We can add a second argument and make it a curried function.</p>
<div class="highlight"><pre><span></span><span class="k">def</span> <span class="n">plus</span><span class="o">(</span><span class="n">x</span><span class="k">:</span> <span class="kt">Int</span><span class="o">)(</span><span class="n">y</span><span class="k">:</span> <span class="kt">Int</span><span class="o">)</span> <span class="k">=</span> <span class="n">x</span> <span class="o">+</span> <span class="n">y</span>
<span class="n">plus</span><span class="o">(</span><span class="mi">10</span><span class="o">)(</span><span class="mi">1</span><span class="o">)</span> <span class="c1">// => 11</span>
</pre></div>
<p>We can then make the second argument implicit and supply it via an <code>implicit val</code>.</p>
<div class="highlight"><pre><span></span><span class="k">def</span> <span class="n">plus</span><span class="o">(</span><span class="n">x</span><span class="k">:</span> <span class="kt">Int</span><span class="o">)(</span><span class="k">implicit</span> <span class="n">y</span><span class="k">:</span> <span class="kt">Int</span><span class="o">)</span> <span class="k">=</span> <span class="n">x</span> <span class="o">+</span> <span class="n">y</span>
<span class="k">implicit</span> <span class="k">val</span> <span class="n">one</span> <span class="k">=</span> <span class="mi">1</span>
<span class="n">plus</span><span class="o">(</span><span class="mi">10</span><span class="o">)</span> <span class="c1">// => 11</span>
</pre></div>
<p>Since <code>plus</code> needs an implicit argument of type <code>Int</code> and there happens to be one in the scope, <code>one</code> is applied automatically. However it won’t work if there are multiple <code>implicit val</code>s.</p>
<div class="highlight"><pre><span></span><span class="k">implicit</span> <span class="k">val</span> <span class="n">one</span> <span class="k">=</span> <span class="mi">1</span>
<span class="k">implicit</span> <span class="k">val</span> <span class="n">two</span> <span class="k">=</span> <span class="mi">2</span>
<span class="n">plus</span><span class="o">(</span><span class="mi">10</span><span class="o">)</span> <span class="c1">// => ambiguous implicit values</span>
</pre></div>
<p>This example isn’t very interesting and one can usually use argument with a default value instead. However implicit arguments are handy for decoupling behavior …</p>
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<h2><a href="https://www.lyh.me/scio-at-philly-ete.html">Scio at Philly <span class="caps">ETE</span></a></h2>
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<div class="summary"><p>It’s been another 6 months since my talk about <a href="https://github.com/spotify/scio">Scio</a> at <a href="http://scala.bythebay.io">Scala by the Bay</a>. We’ve seen huge adoption and improvements since then. The number of production Scio pipelines has grown from ~70 to 400+ within <a href="https://www.spotify.com/">Spotify</a>. A lot of other companies are using and contributing to it as well. In the most recent edition of the Spotify data university, an internal week long big data training camp for non-data engineers, we revamped the curriculum to cover Scio, BigQuery and other <a href="https://cloud.google.com/solutions/big-data/">Google Cloud Big Data</a> products instead of Hadoop, Scalding and Hive.</p>
<blockquote class="twitter-tweet" data-lang="en"><p lang="en" dir="ltr">Spotify data university round 3 <span class="amp">&</span> 1st time covering Scio, <a href="https://twitter.com/ApacheBeam">@ApacheBeam</a> <span class="amp">&</span> <a href="https://twitter.com/GCPBigData">@GCPBigData</a> 👋 Hadoop, <span class="caps">HDFS</span>, M/R, <span class="caps">YARN</span> 🍾 batch + streaming <a href="https://t.co/1gWIEbN0mW">pic.twitter.com/1gWIEbN0mW</a></p>— Neville Li (@sinisa_lyh) <a href="https://twitter.com/sinisa_lyh/status/846549633367265281">March 28, 2017</a></blockquote>
<script async src="//platform.twitter.com/widgets.js" charset="utf-8"></script>
<p>And here’s a list of some notable improvements in Scio.</p>
<ul>
<li>Master branch is now based on <a href="https://beam.apache.org/">Apache Beam</a></li>
<li>Graduate type safe BigQuery <span class="caps">API</span> form experimental to stable</li>
<li><a href="https://github.com/spotify/sparkey-java">Sparkey</a> side input support</li>
<li><a href="https://www.tensorflow.org/">TensorFlow</a> TFRecord file <span class="caps">IO</span></li>
<li><a href="https://cloud.google.com/pubsub/">Cloud Pub/Sub</a> attributes support</li>
<li>Named transformations for streaming update</li>
<li>Safe-guard against malformed tests and better error messages</li>
<li>Flexible custom <span class="caps">IO</span> wiring</li>
<li>KryoRegistrar for custom Kryo serialization</li>
<li>Table description for type-safe BigQuery</li>
<li>Lots of performance improvements and bug fixes</li>
</ul>
<p>I talked about …</p>
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<h2><a href="https://www.lyh.me/joins.html">Joins</a></h2>
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<div class="summary"><p>We recently started teaching <a href="https://github.com/spotify/scio">Scio</a> at Spotify’s internal data university and I made these <a href="/slides/joins.html">slides</a> to explain how joins work and some lower level details.</p>
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<h2><a href="https://www.lyh.me/for-comprehensions.html">For comprehensions</a></h2>
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<i class="fa fa-calendar"></i><time datetime="2017-03-28T12:23:00-04:00"> Tue 28 March 2017</time>
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<div class="summary"><p>I made these <a href="/slides/for-yield.html">slides</a> on for comprehension a while ago but forgot to post them but here you go.</p>
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