Aug 092012
 

Stackable traits in Scala refers to being able to mix in multiple traits that work together to apply multiple modifications to a method. This involves invoking super.theMethod and modifying its input and/or output. But what is super in the context of a trait? The class (or trait) the trait extends from? The class the trait is being mixed into? It depends! All the mixed in traits and all the superclasses are linearized. super invokes the nearest preceding definition further up the chain. The general effect is that mixins to the right (and their ancestor classes) come earlier than those to the left. However, ancestors that are shared are deduped to only show up once, and they show up as late as possible. Here’s a detailed description of the Scala object hierarchy linearization algorithm.

If a trait which extends MyInterface tries to invoke super.myMethod but MyInterface.myMethod is abstract, the compiler generates this error:

error: method myMethod in trait MyInterface is accessed from super. It may not be abstract unless it is overridden by a member declared `abstract' and `override'

What this means is: generally, invoking an abstract method of a superclass is an error. However, with traits, the meaning of super is not known at compile time. The call would be valid if the trait were mixed into a class that had an implementation of the method. But the compiler errs on the side of caution unless told otherwise. abstract override def myMethod provides signals that you expect an implementation of the method to be available at run-time and to not treat the super.myMethod invocation as an error. (Note: this applies regardless of whether the trait itself provides an implementation of the method.)

Here are some examples:

trait Munger {
  def munge(l : List[String]) : List[String]
}

trait Replace1 extends Munger {
  override def munge(l : List[String]) = l :+ "Replace1"
}

trait Replace2 extends Munger {
  override def munge(l : List[String]) = l :+ "Replace2"
}

//abstract override def munge required in the Stack* classes because they invoke
//abstract super.munge

trait Stack1 extends Munger {
  abstract override def munge(l : List[String]) = super.munge(l) :+ "Stack1"
}

trait Stack2Parent extends Munger
  abstract override def munge(l : List[String]) = super.munge(l) :+ "Stack2Parent"
}

trait Stack2 extends Stack2Parent {
  abstract override def munge(l : List[String]) = super.munge(l) :+ "Stack2"
}

class Bottom {
  this : Munger =>

  def apply() {
    println(
      munge(List("bottom"))
    )
  }
}

scala> (new Bottom with Replace1)()
List(bottom, Replace1)

scala> (new Bottom with Replace1 with Replace2)()
List(bottom, Replace2) //Replace1's munge was overridden and never ran

scala> (new Bottom with Replace1 with Stack1)()
List(bottom, Replace1, Stack1) //Stack1 called super.munge, which invoked the
//munge from the trait to the left

scala> (new Bottom with Replace1 with Stack2)()
List(bottom, Replace1, Stack2Parent, Stack2) //Stack2's super.munge called to its
//superclass, whereas Stack2Parent's super.munge called the trait to the left

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