Force Scala trait to implement a certain method
mixins, scala, traits
Solution
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`).
Problem
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?