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

You have to fix the generic argument because that's the only time you can provide it:

scala> trait A[T]
defined trait A

scala> object B extends A[Int]
defined object B

Obviously you want to reuse EmptyTree for all types of T, so instead of defining A[SOMETYPE] for each type just use bottom type Nothing:

scala> object B extends A[Nothing]
defined object B

This object can be used with any tree.

That's exactly how Option[T] is implemented in Scala. Here is how None is defined:

case object None extends Option[Nothing]