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I have the following declarations of classes and traits

class C
class D extends C
trait T extends C
trait U extends D with T

and the following assignments:

val x1:C with T with U = new D with U
val x2:D with T = new C with U
val x3:D with T = new U
val x4:U = new D with T

The first assignment works, the others don't. I want to understand why for both cases.

As far as I understand it, the linearizations are as follows (-> means extends):

  • C with T with U: U -> D -> T -> C
  • D with U: U -> T -> D -> C
  • D with T: T -> D -> C
  • C with U: U -> T -> D -> C

I drew the type hierarchy to get more insight:

type hierarchy

My understanding is the following:

  • x1: works because the linearized types are the same (the order of D and T is not important for the type)
  • x2: does not work because ...?
  • x3: Does not work because traits cannot be instantiated
  • x4: Does not work because D with T is a supertype of U

Number 2 is where I'm stuck. It should work IMO because the linearization of D with T is D -> T -> C, which is a supertype of the linearization of C with U (which is U -> T -> D -> C).

Either my assumption that the order of mixed traits/classes is not important (but then why does the first assignment compile?) or my linearization is wrong or ???

Can you help me on this?

tiefenauer
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  • The type of `x2` is not relevant, it will fail even if you don't specify a type because `new C with U` is invalid. – Tim Sep 10 '18 at 11:07
  • Although this is not the problem, your linearization of `D with T` is not correct. It should be `T->D->C` because a trait is a subclass of the class that it is applied to (so that it can see methods of that class). – Tim Sep 11 '18 at 06:28
  • Thanks for pointing that out! I edited the text accordingly. I am also going to mark your answer as the correct one. Thanks for that too! – tiefenauer Sep 11 '18 at 12:09

1 Answers1

3

The problem is that new C with U is not valid; the type of val x2 is not relevant.

The trait U extends D which means it can only be applied to a subclasses of D. Since C is not a subclass of D the trait cannot be applied to it.

A trait that extends a class is similar to a self type, but with some subtle differences. See This answer for more information.

Tim
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