Scala in depth - Existential types - scala

Question

I am currently reading Scala in depth and I struggle with a point about existential types.
Using those sources : https://github.com/jsuereth/scala-in-depth-source/blob/master/chapter6/existential-types/existential.scala
with openjdk 7 and scala 2.10.3
The following instructions gives me a error :
val x = new VariableStore[Int](12)
val d = new Dependencies {}
val t = x.observe(println)
d.addHandle(t)
<console>:14: error: method addHandle in trait Dependencies cannot be accessed in types.Dependencies
Access to protected method addHandle not permitted because
enclosing object $iw is not a subclass of
trait Dependencies in package types where target is defined
d.addHandle(t)
^
And I can't find out why and how I arrive to this error.
Edit 1 :
I added the following code from Kihyo's answer :
class MyDependencies extends Dependencies {
override def addHandle(handle: Ref) = super.addHandle(handle)
}
val x = new VariableStore[Int](12)
val d = new MyDependencies
val t = x.observe(println)
d.addHandle(t) //compiles
It make addHandle public instead of protected.
Now I have the following error message :
type mismatch; found : x.Handle (which expands to) x.HandleClass required: d.Ref (which
expands to) x.Handle forSome { val x: sid.types.obs.Observable }
HandleClass is a Handle and Ref is a Handle of any Observer (if I get it right) so the value t should be accepted as a correct type for the exception.

Answer 1

In the trait Dependencies, addHandle is defined like that:
protected def addHandle(handle : Ref) : Unit
protected means, only subclasses can access this method and thats why you get the error. (which basically tells you exactly that)
Your code could work when you create a subclass that makes addHandle public:
class MyDependencies extends Dependencies {
override def addHandle(handle: Ref) = super.addHandle(handle)
}
val x = new VariableStore[Int](12)
val d = new MyDependencies
val t = x.observe(println)
d.addHandle(t) //compiles
But I have no idea about that example and what you want to do with it.
#Edit1:
I get the same error as you, but I can't explain why. It worked for me when I extend App instead of having a main-method:
object TestObs extends App {
val x = new VariableStore[Int](12)
val d = new MyDependencies
val t = x.observe(println)
d.addHandle(t)
}
Maybe someone else has some insight on this.