def execute[A](ui: UserInteraction[A]): A = ui match {
  case Tell(statement) =>
    println(statement)
    statement
  case Ask(question) =>
    println(question)
    val answer = scala.io.StdIn.readLine()
    answer.asInstanceOf[A] // to make compiler happy
}

def run[A](program: List[UserInteraction[A]]): Unit = program.foreach(execute)