Force Scala trait to implement a certain method

Question

Is there a way to specify that a trait has to provide a concrete implementation of a method?

Given some mixin

class A extends B with C {
  foo()
}

The program will compile if either of A, B, or C implements foo(). But how can we force, for example, B to contain foo's implementation?

Answer 1

You can do the following:

class A extends B with C {
  super[B].foo()
}

This will only compile if B implements foo. Use with caution though as it (potentially) introduces some unintuitive coupling. Further, if A overrides foo, still B's foo will be called.

One IMHO valid use case is conflict resolution:

trait B { def foo() = println("B") }
trait C { def foo() = println("C") }
class A extends B with C {
  override def foo() = super[B].foo()
}

If you want to make sure B declares foo, you can use type ascription:

class A extends B with C {
  (this:B).foo()
}

This will only compile if B declares foo (but it might be implemented in C or A).