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  1. PO
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    <p>First thing to note is that mutable <code>HashMap</code> is invariant: <code>class HashMap [A, B]</code>. Though the immutable version is covariant on values: <code>class HashMap [A, +B]</code>.</p> <p>Second thing to note is that your <code>repos</code> variable is meant to be a polymorphic collection which means that some compile time type information is lost when you put stuff there.</p> <p>But since you use the mutable <code>HashMap</code>, the <code>repos</code> can't actually be a correct polymorphic collection because of <code>HashMap</code> invariance. To illustrate why let's suppose Page is a class (so we can instantitate it) and we put a HashMap[String, CoolPage] in the <code>repos</code> list. Then we could do this:</p> <pre><code>val m = repos.head // HashMap[String, Page] m.put("12345678", new Page) // We just added a Page to HashMap[String, CoolPage] </code></pre> <p>So the compiler gives you an error to protect you from this.</p> <p>I guess you can fix your code by making Repository covariant:</p> <pre><code>trait Repository[+T &lt;: Page] { private[this] val pages = new HashMap[String, T] register(this) def newPage: T def apply(): T = { val page = newPage pages(page.id) = page page } def apply(id: String): T = { pages.get(id) match { case Some(page) =&gt; page.lastAccessed = new Date page case None =&gt; this() } } } </code></pre> <p>And changing <code>repos</code> to be a list of <code>Repository[Page]</code>:</p> <pre><code>private var repos: List[Repository[Page]] = Nil private def register(repo: Repository[Page]) { repos = repo :: repos } </code></pre> <p>And remember that polymorphic collections (like <code>repos</code>) make you lose compile time type information of elements: if you put a <code>Repository[CoolPage]</code> there you only get <code>Repository[Page]</code> back and have to deal with it.</p> <p><strong>update</strong>: removed <code>.asInstance[T]</code> from the <code>Repository</code> code by making <code>pages</code> <code>private[this]</code>.</p>
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    1. CONice answer. I missed the bit about mutable `HashMap` being invariant. Doh! Your description is spot on. I also like the trick with `private[this]` to let the compiler know that `pages` won't be used in anywhere that might cause variance problems. Thanks for you help.
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