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

Let me detalize Alexey's answer. Here is full implementation with some code style improvements:

First define your trait with aknowledgment of its covariance:

 trait Tree[+T] {
    def contains[U >: T : Ordering](num: U): Boolean

    def inc[U >: T : Ordering](num: U): Tree[U]
  }

Next define your subtype-of-all-trees object

  case object EmptyTree extends Tree[Nothing] {
    def contains[U >: Nothing : Ordering](num: U): Boolean = false
    def inc[U >: Nothing : Ordering](num: U): Tree[U] =
      DataTree(num, EmptyTree, EmptyTree)
    override def toString = "."
  }

Now change your general case implementation:

  case class DataTree[T: Ordering](x: T, left: Tree[T], right: Tree[T]) extends Tree[T] {
    import Ordering.Implicits._
    def contains[U >: T : Ordering](num: U): Boolean = 
      if (num < x) left.contains(x)
      else if (num > x) right.contains(x)
      else true

    def inc[U >: T : Ordering](num: U): Tree[U] = 
      if (num < x) DataTree(x, left.inc(num), right)
      else if (num > x) DataTree(x, left, right.inc(num))
      else this

    override def toString = "{" + left + x + right + "}"
  }

You could be a little bit frustrated since I replaced your Ordered with Ordering, but you should know that view bounds are deprecated