Why can we use a new class as type of parent class in Scala?

Question

In the simplified implementation of Actor in the RedBook, they use node-based MPSC node based queue for Actor. They define the node by this line of code:

private class Node[A](var a: A = null.asInstanceOf[A]) extends AtomicReference[Node[A]]

But how can we use Node[A] as the type parameter of AtomicReference because we do not have class Node[A] yet? Is it a way of declaring recursive type in Scala?

Answer 1

You are allowed to use recursion in class/trait definition:

abstract class Example[A] extends (A => Example[A])

def example(prefix: String): Example[String] = new Example[String] {
  def apply(s: String): Example[String] = {
    val t = prefix + s
    println(t)
    example(t)
  }
}

example("1")("2")("3") 
//12
//123

If you have X extends F[X] then you ended up with something known to C++ developers as curiously recurring template pattern and in type theory in general as F-bounded types.

You can find it even in Java because each enum X is underneath abstract class X extends Enum[X].