Associated types

trait Iterable {
    type Iterator
    def iterator(): Iterator
}

trait Iterator {
    type Element
    def next(): Option[Element]
}

class Range(start: Int, stop: Int) extends Iterable {
    override type Iterator = RangeIterator
    override def iterator(): Iterator =
        new RangeIterator(start, stop)
}

class RangeIterator(var start: Int, stop: Int) extends Iterator {
    override type Element = Int
    override def next(): Option[Int] =
        if (start >= stop) {
            None
        } else {
            val value = start
            start += 1
            Some(value)
        }
}

object Main {
    def main(args: Array[String]): Unit = {
        val range = new Range(1, 10);
        {
            val iterator = range.iterator()
            println(iterator.next())
            println(iterator.next())
            println(iterator.next())
        }
        {
            val iterator = range.iterator()
            println(iterator.next())
            println(iterator.next())
            println(iterator.next())
        }
    }
}