Scala for-comprehensions presentation

sealed abstract class Perhaps[+A] {
  def foreach(f: A => Unit): Unit
  def map[B](f: A => B): Perhaps[B]
  def flatMap[B](f: A => Perhaps[B]): Perhaps[B]
  def withFilter(f: A => Boolean): Perhaps[A]
}

case class YesIHas[A](value: A) extends Perhaps[A] {
  override def foreach(f: (A) => Unit): Unit = f(value)
  override def map[B](f: (A) => B): Perhaps[B] = YesIHas(f(value))
  override def flatMap[B](f: (A) => Perhaps[B]): Perhaps[B] = f(value)
  override def withFilter(f: (A) => Boolean): Perhaps[A] = if (f(value)) this else Nope
}

case object Nope extends Perhaps[Nothing] {
  override def foreach(f: (Nothing) => Unit): Unit = ()
  override def map[B](f: (Nothing) => B): Perhaps[B] = this
  override def flatMap[B](f: (Nothing) => Perhaps[B]): Perhaps[B] = this
  override def withFilter(f: (Nothing) => Boolean): Perhaps[Nothing] = this
}

val x1 = YesIHas(3)
val x2 = YesIHas(5)

// foreach:
x1.foreach(a => println(a))

for {
  a <- x1
} println(a)

// map:
x1.map(a => a * 5)

for {
  a <- x1
} yield a * 5

// flatMap/map:
x1.flatMap(a => x2.map(b => a * b))

for {
  a <- x1
  b <- x2
  if b > 3
} yield a * b

// withFilter:
x1.withFilter(a => a > 1).flatMap(a => x2.map(b => a * b))

for {
  a <- x1
  //if a > 1
  b <- x2
} yield a * b

// assignment:
x1.flatMap { a =>
  x2.map { b =>
    val c = a * b
    c
  }
}

for {
  a <- x1
  b <- x2
  c = a * b
} yield c