Scala do-notation overloading

for.scala

case class Foo(a: Int) {
  def map(f : Int => Int) = Foo(f(a))
  def flatMap(f : Int => Bar): Bar = f(a)
}

case class Bar(a: Int) {
  def map(f : Int => Int) = Bar(f(a))
  def flatMap(f : Int => Bar): Bar = f(a)
}


object Main {
  val foo: Bar = for {
    x <- Foo(1)
    y <- Bar(x)
  } yield y

  def main(args: Array[String]): Unit = println(foo)
}