Referring to trait inside trait

Asked Dec 14 '21 at 06:54

Active Dec 14 '21 at 20:37

Viewed 137 times

What do I need the type of x to be in the following code to be able to refer to x.f in g()? A.B didn't work.

trait A {
    trait B {
        def f: Unit
    }
}

object O {
    def g(x : ?): Unit {
        x.f
    }
}

edited Dec 14 '21 at 20:37

Dmytro Mitin

asked Dec 14 '21 at 06:54

Clinton

2

Possibly useful: [Implementing inner traits in Scala like we do with inner interfaces in Java](https://stackoverflow.com/questions/15614849/implementing-inner-traits-in-scala-like-we-do-with-inner-interfaces-in-java) – Chris Dec 14 '21 at 06:58
6

`def g(x: A#B): Unit = { x.f }` – jwvh Dec 14 '21 at 07:44
Why would you need that? – Matthias Berndt Dec 14 '21 at 13:57
@jwvh Or with a finer type `def g(x: a.B forSome {val a: A}): Unit = { x.f }` – Dmytro Mitin Dec 14 '21 at 20:29
@DmytroMitin existential types [are no longer](https://dotty.epfl.ch/docs/reference/dropped-features/existential-types.html) supported in scala 3 :) I think projection (A#B) it is better for now, isn't? – gianluca aguzzi Dec 14 '21 at 23:43
1

@gianlucaaguzzi It's not clear whether OP asks about Scala 2 or Scala 3. Existential types (not wildcarded) are no longer supported. General projections (with abstract `A`) are not either (for traits `A#B` is ok). Projections are not better or worse than existentials, they are just different. – Dmytro Mitin Dec 15 '21 at 15:10
@gianlucaaguzzi Scala 2.12, although I'd rather not add in features that are going to be deprecated in Scala 3 (I'd like to upgrade at some point) – Clinton Dec 16 '21 at 00:17

0 Answers0