scala - How does the NotNull trait work in 2.8 and does anyone actually use it?

Question

trait NotNull {}

I've been trying to see how this trait can guarantee that something is not null and I can't figure it out:

def main(args: Array[String]) {
  val i = List(1, 2) 
  foo(i) //(*)
}

def foo(a: Any) = println(a.hashCode)

def foo(@NotNull a: Any) = println(a.hashCode) //compile error: trait NotNull is abstract

def foo(a: Any with NotNull) = println(a.hashCode) //compile error: type mismatch at (*)

And:

val i = new Object with NotNull //compile-error illegal inheritance

There is obviously some special compiler treatment going on because this compiles:

trait MyTrait {}

def main(args: Array[String]) {
  val i: MyTrait = null
  println(i)
}

Whereas this does not:

def main(args: Array[String]) {
  val i: NotNull = null //compile error: found Null(null) required NotNull
  println(i)
} 

EDIT: there's nothing about this I can find in programming in Scala

Answer 1

NotNull is not yet finished. The intention is to evolve this into a usable way to check for non-nullness but it's not yet there. For the moment I would not use it. I have no concrete predictions when it will be done, only that it won't arrive for 2.8.0.