Re: Empty Iterators

Burak Emir wrote,
> Iterator[+a] is covariant in a. That means that X<:Y implies
> Iterator[X]<:Iterator[Y].
> And since Nothing <: T for any T, you have Iterator[Nothing] <:
> Iterator[T] for any T you can think of, even if it is a type
> parameter A.
>
> In short, having an empty iterator of type Iterator[Nothing] is just
> as good as having one of type Iterator[A].

I don't think it is in the example I gave,

  class DisjointUnion[A](family : List[Set[A]])
    extends Set[A]
  {
    def elements =
      family.foldLeft(Iterator.empty)((i, s) => i ++ s.elements)
  }

gives me the compiler error,

  type mismatch;
     found   : scala.Iterator[A]
     required: scala.Iterator[scala.Nothing]    

I could do this,

    def elements =
      family.foldLeft(Iterator.empty.asInstanceOf[Iterator[A]])(
        (i, s) => i ++ s.elements)

but that's pretty ugly. Or I can add the following,

  object Collections
  {
    def emptyIterator[A] : Iterator[A] = Iterator.empty
  }

and do this,

  def elements =
    family.foldLeft(Collections.emptyIterator[A])(
      (i, s) => i ++ s.elements)

but it'd nicer if emptyIterator (or something equivalent) lived on 
scala.Iterator.

Or am I missing something?

Cheers,


Miles