A naive covariant set

gistfile1.scala

trait Set[+A] extends (Any => Boolean) { self =>

  def contains(x: Any): Boolean

  def apply(x: Any): Boolean = contains(x)

  def + [B >: A](x: B): Set[B] = new Set[B] {
    def contains(y: Any) = (y == x) || self.contains(y)
  }

  def union[B >: A](that: Set[B]): Set[B] = new Set[B] {
    def contains(x: Any) = self.contains(x) || that.contains(x)
  }

  def intersect[B >: A](that: Set[B]): Set[B] = new Set[B] {
    def contains(x: Any) = self.contains(x) && that.contains(x)
  }

}

object EmptySet extends Set[Nothing] {
  def contains(x: Any) = false
}