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    <p>Very good question.</p> <p>Let's check the source and see what's happening under the covers:</p> <pre><code>override def andThen[C](k: B =&gt; C): PartialFunction[A, C] = new AndThen[A, B, C] (this, k) </code></pre> <p>We can observe here that <code>andThen</code> <strong>doesn't even expect a Partial Function</strong>, any Function that transforms the result will do. Your code works, because: <code>trait PartialFunction[-A, +B] extends (A =&gt; B)</code>. This can actually be found in <a href="http://www.scala-lang.org/api/current/index.html#scala.PartialFunction" rel="nofollow">the documentation</a>:</p> <blockquote> <p><code>def andThen[C](k: (B) ⇒ C): PartialFunction[A, C]</code></p> <p>Composes this partial function with a transformation function that gets applied to results of this partial function.</p> <p><strong>C</strong> the result type of the transformation function.</p> <p><strong>k</strong> the transformation function</p> <p><strong>returns</strong> a partial function with the same domain as this partial function, which maps arguments <code>x</code> to <code>k(this(x))</code>.</p> </blockquote> <p>So there's currently no way to chain <code>PartialFunction</code>s in the way you would like, because as Robin said, it would require applying the function. Next to being computationally expensive <strong>it could also have side effects</strong>, which is a bigger problem.</p> <h2>Update</h2> <p>Whipped together an implementation you're looking for. <strong>Use it cautiously!</strong> As I already mentioned, if your code has side effects it will cause problems:</p> <pre><code>implicit class PartialFunctionExtension[-A, B](pf: PartialFunction[A, B]) { def andThenPf[C](pf2: PartialFunction[B, C]) = new PfAndThen(pf, pf2) class PfAndThen[+C](pf: PartialFunction[A, B], nextPf: PartialFunction[B, C]) extends PartialFunction[A, C] { def isDefinedAt(x: A) = pf.isDefinedAt(x) &amp;&amp; nextPf.isDefinedAt(pf.apply(x)) def apply(x: A): C = nextPf(pf(x)) } } </code></pre> <p>Trying it out:</p> <pre><code>deepTestLvl1.andThenPf(deepTestLvl2).isDefinedAt(Some(Some(3))) // false deepTestLvl1.andThenPf(deepTestLvl2).isDefinedAt(Some(None)) // true deepTestLvl1.andThenPf(deepTestLvl2).apply(Some(None)) // 3 </code></pre>
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    1. POWhy is this PartialFunction defined but still crashes (correctly) after application in Scala?
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