Question

I’m trying define a trait C which extends some traits A,B,… All traits,C and A,B,… implement a common trait T. Trait C is supposed to implement T by calling the implementations of T in A,B,..:

trait T{
  def f()
}
trait A extends T{
  def f(){
    print("A")
  }
}
trait B extends T{
  def f(){
    print("B")
  }
}

The desired behavior of trait C is as follows:

val x=new A with B with C[A,B]{}
x.f()
// should produce output
A
B

Here my attempt to define trait C, which gave compilation errors:

trait C[A<:T,B<:T] extends T{
  self:A with B =>
  override def f(){
    // error: A does not name a parent class of trait C
    super[A].f()
    // error: B does not name a parent class of trait C
    super[B].f()
  }
}

I need to call within C methods A.f() and B.f().
Is there any solution to this?

Answer 1

If you want to provide an implementation inside of a trait but also ensure that subclasses implement the definition, it is possible to tell this the compiler with the abstract override combination:

trait T {
  def f()
}
trait A extends T {
  abstract override def f() {
    super.f()
    print("A")
  }
}
trait B extends T {
  abstract override def f() {
    super.f()
    print("B")
  }
}

trait C extends T {
  override def f() {
    // do your work here ...
  }
}

val x = new C with A with B
x.f()

To call the next implementation in the mixin-hierarchy you must add a super.f() call inside of the abstract override method call. Because such a super call requires an existing implementation the first thing you need to create is an instance of C that mixins A and B. If you mixin C in A or B the compiler will complain because the mixin-hierarchy is executed from left to right, thus the implementation of C can not be seen.