Scala: How can I make my immutable classes easier to subclass?

Question

I\'ve recently created an immutable class supporting operations like +, -, etc. that returns a new instance of that class when it is changed.

I wanted to make a subclass

Answer 1

You could use an implementation trait, like the collection classes do, which is parametrized by the concrete type. E.g., something like:

trait FooLike[+A] {
  protected def bar: Int

  protected def copy(newBar: Int): A
  def +(other: Foo): A = copy(bar + other.bar)
}

class Foo(val bar: Int) extends FooLike[Foo] {
  protected def copy(newBar: Int): Foo = new Foo(newBar)
}

class Subclass(barbar: Int) extends Foo(barbar) with FooLike[Subclass] {
  protected def copy(newBar: Int): Subclass = new Subclass(newBar)
}