ExiSet.scala

package com.gabrieljones

/**
 * Represents a set of elements that may or may not exist
 * @tparam T the type of the elements in the set
 */
sealed trait ExiSet[T] {
  def exists(p: T => Boolean): Boolean
}

object ExiSet {
  /**
   * Represents a set with no members
   * @tparam T the type of the elements in the set
   * @return an empty ExiSet
   */
  def none[T]: ExiSet[T] = ExiSet.None.asInstanceOf[ExiSet[T]]

  /**
   * Represents a set with all potential members of type T
   * @tparam T the type of the elements in the set
   * @return an ExiSet with all potential members
   */
  def all[T]: ExiSet[T] = ExiSet.All.asInstanceOf[ExiSet[T]]

  /**
   * Represents a set with no members
   */
  private object None extends ExiSet[Any] {
    override def exists(p: Any => Boolean): Boolean = false
  }

  /**
   * Represents a set with some members of type T
   * @param elem the set of elements
   * @tparam T the type of the elements in the set
   */
  case class Some[T](elem: Set[T]) extends ExiSet[T] {
    override def exists(p: T => Boolean): Boolean = elem.exists(p)
  }

  /**
   * Represents a set with all possible members
   */
  private object All extends ExiSet[Any] {
    override def exists(p: Any => Boolean): Boolean = true
  }
}