How to create object/singleton of generic type in Scala?

Question

In the code shown below, how can I convert EmptyTree to object (Singleton) ?

trait Tree[T] {
    def contains(num: T): Boolean
    def inc( num: T )         


        

Answer 1

If keeping generics, also there is an option to add empty factory - like it's done for Map and Vector. Off course, with such an implementation it will not be a unique instance object for every creation, but when using inc method, it will not produce new objects, it will just reference itself.

object DataTree {
  def empty[T <% Ordered[T]] = new Tree[T] {
      def contains(num: T):Boolean = false
      def inc(num: T): Tree[T] = {
        new DataTree(num, this, this)
      }
      override def toString = "."
  }
}

So you can instantiate it as following:

val t = new DataTree(20, DataTree.empty[Int], DataTree.empty[Int])