// ---------Framework:---------

abstract class Engine {
    abstract type MixIn
    // add members to this base class which are defined only in the subclass
    class Base(val children: Seq[Base]) extends MixIn
    abstract def doSomething(base: Base)

    final class Spec1(val foo: String, children: Seq[Base]) extends Base(children)
    final class Spec2(val bar: String, children: Seq[Base]) extends Base(children)
}

object Engine {
    def runExample(engine: Engine) {
        val spec1 = new engine.Spec1("foo", Seq.empty)
        engine.doSomething(spec1)
        val spec2 = new engine.Spec2("foo", Seq(new Spec1("foo", Seq.empty)))
        engine.doSomething(spec2)
    }
}

// ---------Client:---------

class ConcreteEngine {
    trait Height {
        val height = if(children.isEmpty()) 0 else children.map{_.height}.max
    }
    override type MixIn = Height

    override def doSomething(base: Base) {
        println("Node @ " + base.height)
    }
}

object Main extends App {
    Engine.run(new ConcreteEngine())
}