Is there any way to extend an object?

Question

In scala, we cannot extend object:

object X 
object Y extends X

gives an error error: not found: type X

I

Answer 1

As so often the correct answer depends on the actual business requirement. Extending from an object would in some sense defy the purpose of that object since it wouldn't be a singleton any longer.

What might be a solution is to extract the behavior into an abstract trait. And create objects extending that trait like so:

trait T{
    // some behavior goes here
}

object X extends T

object Y extends T {
    // additional stuff here
}

Answer 2

The only way to share code between two objects is by having one or more common superclass/trait.

Answer 3

Note that with Dotty (foundation of Scala 3), you can alternatively use composition (instead of inheritance) via export clauses which allow defining aliases for selected members of an object:

object X { def f = 5 }

object Y {
  export X._
  def g = 42
  def h = f * g
}

Y.f // 5
Y.g // 42
Y.h // 210

Note that you can also restrict which members you want to export:

object X { def f = 5; def g = 6 }
object Y { export X.f }
Y.f // 5
Y.g
^^^
value g is not a member of Y

Answer 4

You can't actually extend an object, because that would create two of it, and an object by definition exists only once (edit: well, that's not quite true, because the object definition can be in a class or method).

For your purposes, try this:

object X {
}

object Y {
    def a = 5
}

implicit def xToY(x: X.type) = Y

println(X.a)

It doesn't actually extend, but it does allow you to call new methods on it than were originally defined.

Answer 5

If you want use methods and values from another object you can use import.

object X{
  def x = 5
}

object Y{
  import X._
  val y = x
}

Answer 6

You can convert parent into class + companion object, and then have child extend class E.g.

in Parent.scala

class Parent {}

object Parent extends Parent {}

And then in Child.scala

object Child extends Parent {}

Yes, it's more a hack than a solution.