builders.scala

object builders {

  trait Builder[From, To, Elem1] {
    def += (elem: Elem1)
    def result: To
  }
  
  implicit def listBuilder[A, B] = new Builder[List[A], List[B], B] {
    private val buf = new scala.collection.mutable.ListBuffer[B]
    def += (elem: B) { buf += elem }
    def result: List[B] = buf.toList
  }

  implicit def arrayBuilder[A, B] = new Builder[Array[A], Array[B], B] {
    private val buf = new scala.collection.mutable.ListBuffer[B]
    def += (elem: B) { buf += elem }
    def result: Array[B] = buf.toArray
  }	

  class Iter[A, C](elems: List[A]) {
    def ++ [B  >: A, D](xs: Iterable[B])(implicit b: Builder[C, D, B]): D = {
      for (x <- elems) b += x
      for (x <- xs) b += x
      b.result
    }
    def map[B, D](f: A => B)(implicit b: Builder[C, D, B]): D = {
      for (x <- elems) b += f(x)
      b.result
    }
  }
  
  def main(args : Array[String]) : Unit = {
    val x = new Iter[Int, List[Int]](List(1, 2, 3))
    val y: List[Int] = x.map (_ + 1)
    println(y)
  }
}