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POWhy is this PartialFunction defined but still crashes (correctly) after application in Scala?
 

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  1. POWhy is this PartialFunction defined but still crashes (correctly) after application in Scala?
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    <p>I would like to try partial functions with a deep pattern matching use case. This initially (of course) didn't work after applying Some(Some(3)), but seemed defined instead :</p> <pre><code>def deepTest : PartialFunction [Option[Option[Int]], Int] = { case Some(v) =&gt; v match { case None =&gt; 3 } case None =&gt; 1 } </code></pre> <p>and I thought that by decoupling the nested pattern matching, things would be easier:</p> <pre><code>def deepTestLvl1 : PartialFunction [Option[Option[Int]], Option[Int]] = { case Some(v) =&gt; v case None =&gt; Some(1) } def deepTestLvl2 : PartialFunction [Option[Int], Int] = { case None =&gt; 3 } </code></pre> <p>but the result was the following:</p> <pre><code>scala&gt; (deepTestLvl1 andThen deepTestLvl2) isDefinedAt(Some(Some(3))) res24: Boolean = true </code></pre> <p>and after applying:</p> <pre><code>scala&gt; (deepTestLvl1 andThen deepTestLvl2) (Some(Some(3))) scala.MatchError: Some(3) (of class scala.Some) at scala.PartialFunction$$anon$1.apply(PartialFunction.scala:248) at scala.PartialFunction$$anon$1.apply(PartialFunction.scala:246) at $anonfun$deepTestLvl2$1.applyOrElse(&lt;console&gt;:7) at $anonfun$deepTestLvl2$1.applyOrElse(&lt;console&gt;:7) .... at scala.tools.nsc.MainGenericRunner.runTarget$1(MainGenericRunner.scala:83) at scala.tools.nsc.MainGenericRunner.process(MainGenericRunner.scala:96) at scala.tools.nsc.MainGenericRunner$.main(MainGenericRunner.scala:105) at scala.tools.nsc.MainGenericRunner.main(MainGenericRunner.scala) </code></pre> <p>Am I doing something incorrectly? Shouldn't the isDefinedAt be called twice when I composed sequentially deepTestLvl{1,2} and give me the correct answer?</p>
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    1. USAggelos Biboudis
    1. USAggelos Biboudis
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    1. COUnfortunately this is not documented properly in the Scala language specification - see [my anomalous partial function](http://stackoverflow.com/questions/19973026/magic-partialfunction-in-scala), which works but is not really fully explained. Does anyone know of an actual reference which _does_ document the current behaviour of Scala partial functions correctly?
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      1. USRobin Green
    2. CO@RobinGreen Please check my answer. It's not documented, because it's not the way `andThen` is defined.
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      1. USAkos Krivachy
    3. CO@krivachy.akos I was referring to `deepTest`.
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