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    <p>First, let's address the problem of finding the exact instance you want:</p> <pre><code>scala&gt; trait AbstractOp defined trait AbstractOp scala&gt; case class Pop() extends AbstractOp { | override def equals(other: Any) = other match { | case that: Pop =&gt; this eq that | case _ =&gt; false | } | } defined class Pop scala&gt; case class PushInt(val i: Int) extends AbstractOp { | override def equals(other: Any) = other match { | case that: PushInt =&gt; this eq that | case _ =&gt; false | } | } defined class PushInt scala&gt; val l = List(PushInt(1), PushInt(1), Pop(), Pop()) l: List[Product with AbstractOp] = List(PushInt(1), PushInt(1), Pop(), Pop()) scala&gt; val op = l(1) op: Product with AbstractOp = PushInt(1) scala&gt; println( l.indexOf( op ) ) 1 </code></pre> <p>That, of course, mean <code>PushInt(1) != PushInt(1)</code>, unless it is the exact same instance of <code>PushInt(1)</code>. It doesn't break <code>equals</code>/<code>hashCode</code> contract because <code>a.equals(b) =&gt; a.hashCode == b.hashCode</code>, but <code>a.hashCode == b.hashCode</code> doesn't imply anything. But if your only use is <em>finding</em> that instance, try this instead:</p> <pre><code>scala&gt; case class Pop() extends AbstractOp defined class Pop scala&gt; case class PushInt(val i: Int) extends AbstractOp defined class PushInt scala&gt; val l = List(PushInt(1), PushInt(1), Pop(), Pop()) l: List[Product with AbstractOp] = List(PushInt(1), PushInt(1), Pop(), Pop()) scala&gt; val op = l(1) op: Product with AbstractOp = PushInt(1) scala&gt; println( l.findIndexOf( op eq _ ) ) 1 </code></pre> <p>Either way, if you reinsert that instance in the list you'll have trouble. You have to make sure that each instance you insert is unique. You might even write your own collection, either throwing an exception if a repeated instance is inserted, or make a copy of any instance passed to it (easy enough with case classes and <code>copy</code> method on Scala 2.8).</p>
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    1. USDaniel C. Sobral
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    1. CODaniel, what about the mapping problem if I would use your second suggestion? I think it does not work, so I should override equals probably, right?
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      1. USJoa Ebert
    2. COWhat mapping problem? _(reads Patrick)_ Oh, right. Yes, there is that. In that case, you may override `equals` and create an alternate comparison operator for those times you want `Push(1) == Push(1)`. Something like `=~=`, for instance.
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    3. COFor the first solution, please note that you need not check the type of that given that you'll check for identity afterwards. Therefore it is enough to have: override def equals(other: Any) = this eq other
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