for-comprehension-translation-rules.scala

object forTranslationRules extends App {
  val l1 = 1 to 4
  val l2 = 3 to 7
  val l3 = 2 to 8

  // Simple generator
  val a1 = for (a <- l1) yield a * 2
  val a2 = l1 map (_ * 2)
  assert(a1 == a2)

  // Sequence of generators
  val b1 = for (a <- l1; b <- l2) yield (a, b)
  val b2 = l1.flatMap(x => l2.map(y => (x, y)))
  assert(b1 == b2)

  // generators with guards
  val c1 = for (a <- l1; if a % 2 == 0) yield a * 2
  val c2 = l1.withFilter(_ % 2 == 0).map(_ * 2)
  assert(c1 == c2)


  def f(a: Int) = List(a * 2)

  def g(a: Int) = List(a * 3)

  val d1 = for (x <- l1;
                y <- f(x);
                z <- g(y)) yield z

  val d2 = for (y <- for (x <- l1; y <- f(x)) yield y;
                z <- g(y)) yield z

  val d3 = l1.flatMap(x => f(x).flatMap(y => g(y)))

  assert(d1 == d2)
  assert(d2 == d3)

}