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 an object of the same type as its implementing class. So Derived1.f() returns Derived1 and Derived2.f() returns Derived2. How do I write this in Scala?

Here is what I have come up with so far.

package com.github.wpm.cancan

abstract class Base {
  def f[C <: Base]: C
}

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

case class Derived2(x: Int) extends Base {
  def f[Derived2] = Derived2(x + 2)
}

This gives the following compiler errors:

type mismatch;
[error]  found   : com.github.wpm.cancan.Derived1
[error]  required: Derived1
[error]   def f[Derived1] = Derived1(x + 1)

type mismatch;
[error]  found   : com.github.wpm.cancan.Derived2
[error]  required: Derived2
[error]   def f[Derived2] = Derived2(x + 2)

This error message is confusing to me because I think com.github.wpm.cancan.Derived1 should be the same as Derived1 in this context.

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.

Answer 2

Just to add a small precision about (perfectly good) Travis Brown answer: It's not that C in trait Base[C <: Base[C]] let's you refer to the implementing class; it's just sticking to the convention of writing subclass extends Base[subclass] that let's you do so. There's no way that I know of to refer to this type. To clarify what I mean, this compiles

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

case class Derived1(x: Int) extends Base[Derived1] {
  def f: Derived1 = Derived1(x + 1)
}
// a Derived2 where f returns Derived1!!
case class Derived2(x: Int) extends Base[Derived1] {
  def f = Derived1(x + 2)
}

Now, if all you're going to have as implementations of Base are case classes, you can get this right through a self-type bound:

trait Base[C <: Base[C]] { self: C => 
  def f: C 
}

case class Derived1(x: Int) extends Base[Derived1] {
  def f: Derived1 = Derived1(x + 1)
}
// a Derived2 where f returns Derived1!!
// this won't compile now
case class Derived2(x: Int) extends Base[Derived1] {
  def f = Derived1(x + 2)
}