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POInheritance and (automatic?) type conversion
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  1. POInheritance and (automatic?) type conversion
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    copied!<p>Please have a look at the followin code, where <code>Extractor[A,B]</code> is part of a generic framework and everything else should be regarded as "client code" (I boiled it a down quite a bit and renamed everything. So don't mind that <code>Extractor</code> doesn't seem to be too usefull).</p> <pre><code>scala&gt; abstract class Extractor[A,B] { | def extract(d:A):B | def stringRepr(d:A):String | } defined class Extractor scala&gt; sealed abstract class Value defined class Value scala&gt; case class IntValue(i:Int) extends Value defined class IntValue scala&gt; case class StringValue(s:String) extends Value defined class StringValue scala&gt; case class Data(i:Int, s:String) defined class Data scala&gt; sealed abstract class MyExtractor[Value] extends Extractor[Data, Value] { | def stringRepr(d:Data) = extract(d) match { | case IntValue(i) =&gt; i.toString | case StringValue(s) =&gt; s | } | } defined class MyExtractor scala&gt; class IntExtractor(name:String) extends MyExtractor[IntValue] { | def extract(d:Data) = IntValue(d.i) | } defined class IntExtractor scala&gt; class StringExtractor(name:String) extends MyExtractor[StringValue] { | def extract(d:Data) = StringValue(d.s) | } defined class StringExtractor </code></pre> <p>so in short words <code>Extractor[A,B]</code> is used to extract some value <code>B</code> from <code>A</code> and do some other things that are not represented in this show code. The abstract classes <code>Value</code> and <code>MyExtractor</code> are used for reasons of type savety in the "client code". When I try to create a <code>List</code> of <code>MyExtractor</code>s, the following happens:</p> <pre><code>scala&gt; val l = List.empty[MyExtractor[Value]] l: List[MyExtractor[Value]] = List() scala&gt; new IntExtractor("test1") :: l res5: List[MyExtractor[_ &gt;: IntValue &lt;: Value]] = List(IntExtractor@1fd96c5) </code></pre> <p>trying to convert an <code>IntExractor</code> to a superclass</p> <pre><code>scala&gt; new IntExtractor("test"):MyExtractor[Value] &lt;console&gt;:24: error: type mismatch; found : IntExtractor required: MyExtractor[Value] new IntExtractor("test"):MyExtractor[Value] ^ scala&gt; new IntExtractor("test"):Extractor[Data,Value] &lt;console&gt;:24: error: type mismatch; found : IntExtractor required: Extractor[Data,Value] new IntExtractor("test"):Extractor[Data,Value] </code></pre> <p>I am aware that everything is fine, when I define <code>IntExtractor</code> like this</p> <pre><code>scala&gt; class IntExtractor(name:String) extends MyExtractor[Value] { | def extract(d:Data) = IntValue(d.i) | } defined class IntExtractor scala&gt; new IntExtractor("test"):Extractor[Data,Value] res17: Extractor[Data,Value] = IntExtractor@1653d7a </code></pre> <p>But I don't understand, why it doesn't work the way I tried it above. I would be thankfull for any help or hints.</p>
 

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