BadStack.scala

object BadStack {
  class Stack[+A](elements: List[A]) {
    def this() { this(Nil) }
    def isEmpty: Boolean = elements == Nil
    //push1 doesn't compile because the compiler cannot guarantee type safety.
    //def push1(e: A): Stack[A] = new Stack(e::elements)
    //push2 compiles. 
    def push2[B >: A](e: B): Stack[B] = new Stack(e::elements)
    def pop: (A, Stack[A]) = elements match {
      case x::xs => (x, new Stack(xs))
      case Nil => error("pop cannot be called for empty stack")
    }
    def peek: A = elements match {
      case x::xs => x
      case Nil => error("peek cannot be called for empty stack")
    }
  }
  def main(args: Array[String]) {
    val strings: Stack[String] = new Stack("foo"::Nil)
    val anys: Stack[Any] = strings //Because Stack is covariant, this line is OK.
    // anys.push1(1) //If push1 compiles, this line breaks type safety.
    anys.push2(1) //This line doens't break type safety.
  }
}