WellTyped Programs Don't Go Wrong

build.sbt

scalaVersion := "2.11.7"

WellTyped.scala

sealed trait Observer[A] {
  def observe(in :A): Unit =
    this match {
      case Map(f, o) =>
        // This is completely unsound. b should be of type B, which we don't
        // have available at this point. The compiler allows us to insert
        // anything here, causing a runtime crash.
        val b = ()
        o.observe(b)
      case Sink(f) =>
        f(in)
    }
}
final case class Map[A,B](f: A => B, observer: Observer[B]) extends Observer[A] 
final case class Sink[A](f: A => Unit) extends Observer[A] 

object Example {
  def go = {
    val observer = Map[Int,Int](x => x + 1, Sink(x => println(x)))
    observer.observe(1)
  }
}

Nice one. It reminds me the grievance that some often aired against Akka Actors, this was before Typed Actor was introduced.