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?
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).
