Self in trait invisible from outside the trait?

Question

trait A { def someMethod = 1}
trait B { self : A => }

val refOfTypeB : B = new B with A

refOfTypeB.someMethod

The last line results in a type mismatch error. My question is: why it's impossible to reach the method (of A) when it's given that B is also of type A?

Answer 1

So B is not also of type A . The self type annotation that you've used here indicates specifically that B does not extend A but that instead, wherever B is mixed in, A must be mixed in at some point as well. Since you downcast refOfTypeB to a B instead of B with A you don't get access to any of type A 's methods. Inside of the implementation of the B trait you can access A 's methods since the compiler knows that you'll at some point have access to A in any implemented class. It might be easier to think about it as B depends on A instead of B is an A .

For a more thorough explanation see this answer: What is the difference between self-types and trait subclasses?

Answer 2

The problem is when you declare refOfTypeB, you have type B specified but not type B with A.

The self => syntax allow you to access the properties of A within B and pass the compilation.

However, in the runtime, refOfTypeB is not recognize as B with A and the compiler doesn't necessarily have the function map correctly.

So, the correct syntax should be:

trait A { def someMethod = 1}
trait B { self : A => }

val refOfTypeB : B with A = new B with A

refOfTypeB.someMethod //1

Indeed, this is more expressive in terms of explaining with what refOfTypeB exactly is.

This is the somehow the boilerplate of cake patterns.