case class Twice(i: Int) {                              

  def apply(x: Int, y: Int): Int = x * y

  //def unapply(z: Int): Option[Int] = if (z % 2 == 0) Some(z/2) else None

}

Twice(21) match {

  case Twice(n) => println(n)

}

Twice(21, 3) match {

  case Twice(n) => println(n)

}

package Main

object ComprehensionsTest extends App {

  def even(from: Int, to: Int): List[Int] =

    for (i <- List.range(from, to) if i % 2 == 0) yield i

  Console.println(even(0, 20))

}

object ComprehensionsTest2 extends App {

  def foo(n: Int, v: Int) =

    for (

      i <- 0 until n;

      j <- i + 1 until n if i + j == v

    ) yield Pair(i, j);

  foo(20, 32) foreach {

    case (i, j) =>

      println("(" + i + "," + j + ")")

  }

}

object ComprehensionTest3 extends App {

  for (

    i <- Iterator.range(0, 20);

    j <- Iterator.range(i + 1, 20) if i + j == 32

  ) println("(" + i + ", " + j + ")")

}