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    <p>The singleton types and ETSR's do not solve the problem. I myself was looking for just the same feature in Scala, but apparently it lacks the so-called self-type annotations.</p> <p>There are circumstances where such self-type annotations could be very useful. Consider an example (adapted from <a href="https://stackoverflow.com/questions/2059601/circular-type-parameters-definition-in-scala" title="Circular type parameters question example">Circular type parameters question example</a>):</p> <pre><code>// we want a container that can store elements trait Container[E &lt;: Element[E]] { def elements: Seq[E] def add(elem: E): Unit } // we want elements be aware of their enclosing container trait Element[E &lt;: Element[E]] { def container: Container[E] } </code></pre> <p>Let's say you put that into a library. A library consumer should do the following:</p> <pre><code>object PersonContainer extends Container[Person] { // actual implementation is not important def elements = Nil def add(p: Person) = {} } class Person extends Element[Person] { // {1} def container = PersonContainer } </code></pre> <p>It is allright and everything works quite as expected. The only thing that concerns is that a library consumer is <em>supposed</em> to use the self-bound type parameter (#1 in the code). But that's not all. Now suppose you have some sort of an ActiveRecord pattern in mind, and you want to add the method <code>save</code> to <code>Element</code>, which just delegates to it's container's <code>add</code> method. Surprisingly, it is not that easy:</p> <pre><code>trait Element[E &lt;: Element[E]] { def container: Container[E] def save() = container.add(this) // won't compile } found : Element[E] required: E </code></pre> <p>Intuitively, we have a few options here:</p> <ul> <li>make <code>add</code> method accept <code>Element[E]</code> instead of <code>E</code>;</li> <li>cast <code>this</code> to <code>Element[E]</code>.</li> </ul> <p>None of these options are satisfactory, just because of the fact that <code>E</code> is not the same as <code>Element[E]</code> (implementations are not forced to use self-bound type parameters). The only way I see of solving this problem is to have that self-type concept in Scala (let's suppose we have it in our favorite language):</p> <pre><code>trait Container[E &lt;: Element] { def elements: Seq[E] def add(elem: E): Unit } trait Element { // the type parameter would be redundant ... def save() = container.add(this) // ... and this would be possible, too, ... def container: Container[this] // ... if only we could do this } </code></pre> <p>If the compiler could treat <code>this</code> (or maybe another keyword), when it is used inside square brackets, as the type of the actual implementation (i.e. the same type as the result of <code>obj.getClass</code>), then the problems would disappear. </p> <p>P.S. May someone consider including this stuff into Scala wishlist? Unfortunately, I don't know, how hard it is to implement such logic since there could be the problems with the notorious JVM's erasure.</p> <p>P.P.S. Or maybe there is some another Scala-way I'm unaware of?</p>
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    1. POIs there a "SELF" type in scala that represents the current type?
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    1. COYou need a self-type annotation in trait Element: `trait Element[E <: Element[E]] { this: E => ... }` Once you have that, you can leave out the type bound.
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