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POScala: subclassing with factories
 

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    <p>Let's say I've got two traits, one of them being a factory for another:</p> <pre><code>trait BaseT { val name: String def introduceYourself() = println("Hi, I am " +name) // some other members ... } trait BaseTBuilder { def build: BaseT } </code></pre> <p>Now, I want to extend BaseT:</p> <pre><code>trait ExtendedT extends BaseT { val someNewCoolField: Int override def introduceYourself() = { super.introduceYourself() println(someNewCoolField) } // some other extra fields </code></pre> <p>Let's say I know how to initialize the new fields, but I'd like to use <code>BaseTBuilder</code> for initializing superclass members. Is there a possibility to create a trait that would be able to instantiate <code>ExtendedT</code> somehow? This approach obviously fails:</p> <pre><code>trait ExtendedTBuilder { self: TBuilder =&gt; def build: ExtendedT = { val base = self.build() val extended = base.asInstanceOf[ExtendedT] // this cannot work extended.someNewCoolField = 4 // this cannot work either, assignment to val extended } def buildDifferently: ExtendedT = { new ExtendedT(4) // this fails, we don't know anything about constructors of ExtendedT } def build3: ExtendedT = { self.build() with {someNewCoolField=5} //that would be cool, but it cannot work either } } </code></pre> <p>I'd like to have such a set of traits (or objects) that when someone supplies concrete implementation of <code>BaseT</code> and <code>BaseTBuilder</code> I could instantiantiate <code>ExtendedT</code> by writing:</p> <pre><code>val extendedBuilder = new ConcreteBaseTBuilder with ExtendedTBuilder val e: ExtendedT = extendedBuilder.build </code></pre> <p><code>ExtendedT</code> could contain a field of type <code>BaseT</code>, but then it would require manually proxying all the necessary methods and fields, which is in my opinion a violation of DRY principle. How to solve that? </p>
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    1. USWojciech Ptak
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