Another in the endless parade.

unsoundagain.scala

package p

/** Super basic map/flatMap fusion ADT.
 */
sealed trait View[A] {
  def map[B](f: A => B): View[B]           = CView(this, Mapped(f))
  def flatMap[B](f: A => View[B]): View[B] = CView(this, FlatMap(f))

  def foreach(f: A => Unit): Unit = this match {
    case IdView(xs)            => xs foreach f
    case Optimize(xs)          => xs foreach f
    case CView(xs, Mapped(g))  => xs foreach (g andThen f)
    case CView(xs, FlatMap(g)) => xs foreach (x => g(x) foreach f)
  }
}
final case class IdView[A, B](xs: Vector[A]) extends View[A]
final case class CView[A, B](prev: View[A], op: Op[A, B]) extends View[B]

sealed trait Op[A, B]
final case class FlatMap[A, B](f: A => View[B]) extends Op[A, B]
final case class Mapped[A, B](f: A => B)        extends Op[A, B]

/** Super simple peephole optimizer.
 *
 *  Can you spot the type error? Don't worry, neither can Scala.
 */
object Optimize {
  def unapply[A](v: View[A]): Option[View[A]] = v match {
    case CView(CView(xs, Mapped(f)), Mapped(g))   => Some(CView(xs, Mapped(f andThen g)))
    case CView(CView(xs, FlatMap(f)), FlatMap(g)) => Some(CView(xs, FlatMap(f andThen g)))
    case _                                        => None
  }
}

/** No warnings, just throw the ClassCastException.
 */
object Run {
  def view[A](xs: A*): View[A] = IdView(xs.toVector)

  def main(argv: Array[String]): Unit = {
    view(1, 2, 3) map (_ + 1) map (_ + 1) foreach println // ok
    view(1, 2, 3) flatMap (x => view(x)) foreach println  // ok

    // java.lang.ClassCastException: p.IdView cannot be cast to java.lang.Integer
    view(1, 2, 3) flatMap (x => view(x)) flatMap (x => view(x)) foreach println
  }
}