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POInheritance and type parameters of Traversable
 

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    <p>I'm studying the source code of the Scala 2.8 collection classes. I have questions about the hierarchy of <code>scala.collection.Traversable</code>. Look at the following declarations:</p> <pre><code>package scala.collection trait Traversable[+A] extends TraversableLike[A, Traversable[A]] with GenericTraversableTemplate[A, Traversable] trait TraversableLike[+A, +Repr] extends HasNewBuilder[A, Repr] with TraversableOnce[A] package scala.collection.generic trait HasNewBuilder[+A, +Repr] trait GenericTraversableTemplate[+A, +CC[X] &lt;: Traversable[X]] extends HasNewBuilder[A, CC[A] @uncheckedVariance] </code></pre> <p>Question: Why does <code>Traversable</code> extend <code>GenericTraversableTemplate</code> with type parameters <code>[A, Traversable]</code> - why not <code>[A, Traversable[A]]</code>? I tried some experimenting with a small program with the same structure and got a strange error message when I tried to change it to <code>Traversable[A]</code>:</p> <pre><code>error: Traversable[A] takes no type parameters, expected: one </code></pre> <p>I guess that the use of the <code>@uncheckedVariance</code> annotation in <code>GenericTraversableTemplate</code> also has to do with this? (That seems like a kind of potentially unsafe hack to force things to work...).</p> <p><em>edit</em> - found some useful answers about the annotation in <a href="https://stackoverflow.com/questions/2454281/when-is-uncheckedvariance-needed-in-scala-and-why-is-it-used-in-generictraversa">this question</a> (it is because <code>GenericTraversableTemplate</code> is used for both mutable and immutable collections which have different variance).</p> <p>Question: When you look at the hierarchy, you see that <code>Traversable</code> inherits <code>HasNewBuilder</code> twice (once via <code>TraversableLike</code> and once via <code>GenericTraversableTemplate</code>), but with slightly different type parameters. How does this work exactly? Why don't the different type parameters cause an error?</p>
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