AOP-style Mixin Composition in Scala

Question

I'm trying to use mixins to get AOP/interceptor-like style of programming.

Here's my code:

trait X {
  def run
}

trait Y extends X {
  abstract override def run = {
    println("hi")
    super.run
  }
}

object Z extends X with Y {
  def run = println("bye")
}

I get the following error when trying to compile:

method run needs `override' modifier

What am I doing wrong here? I tried to add the 'override' modifier, but than it gives me:

method run needs `abstract override' modifiers

Answer 1

One way you can do it, is to define base functionality in a class A extending the base X, and to provide an object (or a class) that mixes A and Y (and all the other traits you may define):

trait X {
  def run
}

trait Y extends X {
  abstract override def run = {
    println("hi")
    super.run
  }
}

class A extends X {
  def run = println("bye")
}

object Z extends A with Y

Z.run will print

hi
bye