Polymorphic Scala return type

Question

I have an abstract Scala class Base which has subclasses Derived1 and Derived2. Base defines a function f() which returns

Answer 1

Randall Schulz pointed out one of the reasons your current code doesn't work. It is possible to get what you want, though, with F-bounded polymorphism:

trait Base[C <: Base[C]] { def f: C }

case class Derived1(x: Int) extends Base[Derived1] {
  def f: Derived1 = Derived1(x + 1)
}

case class Derived2(x: Int) extends Base[Derived2] {
  // Note that you don't have to provide the return type here.
  def f = Derived2(x + 2)
}

The type parameter on the base trait allows you to talk about the implementing class there—e.g. in the return type for f.