一時オブジェクトの生成コスト回避による高速化 ref: http://qiita.com/piyo7/items/c8e576c6b010256c66f9

file0.scala

scala> Seq(1, 2, 3).view.map(_ + 1).map(_ * 2)
res0: scala.collection.SeqView[Int,Seq[_]] = SeqViewMM(...)

scala> res0.force
res1: Seq[Int] = List(4, 6, 8)

file1.scala

  override def map[B, That](f: A => B)(implicit bf: CanBuildFrom[This, B, That]): That = {
    newMapped(f).asInstanceOf[That]
  }

file2.scala

  protected def newMapped[B](f: A => B): Transformed[B] = new { val mapping = f } with AbstractTransformed[B] with Mapped[B]

file3.scala

  trait Mapped[B] extends Transformed[B] {
    protected[this] val mapping: A => B
    def foreach[U](f: B => U) {
      for (x <- self)
        f(mapping(x))
    }
    final override protected[this] def viewIdentifier = "M"
  }