Dependency in traits inheritance

Question

In Scala, how can i add a container trait (like Traversable[Content]) to another one that extends the container (and thus has limited visibility of its content ?

For example, the code below tries to define a trait WithIter for a container requiring Traversable (of course, i have in fact other things in Container).

import scala.collection._

trait Container {
  type Value
}

trait WithIter extends Container with immutable.Traversable[Container#Value]

class Instance extends WithIter {
  type Value = Int
  def foreach[U](f : (Value) => (U)) : Unit = {}
}

Compiler (scalac 2.8.0.Beta1-RC8) finds an error:

error: class Instance needs to be abstract, since method foreach in trait GenericTraversableTemplate of type [U](f: (Container#Value) => U)Unit is not defined

Is there any simple way?

Answer 1

class Instance extends WithIter {
  type Value = Int
  def foreach[U](f : (Container#Value) => (U)) : Unit = {}
}

If you do not specify OuterClass# when speaking of an inner class, then this. (ie, instance-specific) will be assumed.

Answer 2

Why do you use an abstract type? Generics are straightforward:

import scala.collection._

trait Container[T] {}

trait WithIter[T] extends Container[T] with immutable.Traversable[T]

class Instance extends WithIter[Int] {
  def foreach[U](f : (Int) => (U)) : Unit = {println(f(1))}
}


new Instance().foreach( (x : Int) => x + 1)