Question

I want to concatenate a traversable once to a traversable once without resolving either.
This is the solution I have come up with as an implicit, but I don’t know if I am missing a native solution…

object ImplicitTraversableOnce {
  implicit def extendTraversableOnce[T](t : TraversableOnce[T]) = new TraversableOnceExtension(t)
}

class TraversableOnceExtension[T <: Any](t : TraversableOnce[T]) {

  def ++ (t2:TraversableOnce[T]):TraversableOnce[T] = new concat(t.toIterator, t2.toIterator)

  private class concat(i1:Iterator[T], i2:Iterator[T]) extends Iterator[T] {
    private var isOnSecond = false

    def hasNext:Boolean =
      if (isOnSecond) i2.hasNext
      else if (!i1.hasNext) {
        isOnSecond = true
        hasNext
      }
    else true

    def next():T = if (isOnSecond) i2.next() else i1.next()
  }
}

Answer 1

You can join iterators with ++. So if you’re going to use iterators anyway, just

def ++(t2: TraversableOnce[T]): TraversableOnce[T] = t.toIterator ++ t2.toIterator

The reason to not do this is to supply an efficient foreach for Traversables that are not Iterable/Iterator, but then you need to fill in all the TraversableOnce abstract methods.