Question

I want to create a covariant class which is mutable, so I need to add a lower type bound to the setter method. But I also want the setter method to set a field, so I guess the field needs to have the same type bound?

class Thing[+F](initialValue: F) {

    private[this] var secondValue: Option[G >: F] = None

    def setSecondValue[G >: F](v: G) = {
        this.secondValue = Some(v)
     }
}

The method compiles fine. But the field called secondValue doesn’t compile at all, with the error message:

    Multiple markers at this line
        - ']' expected but '>:' found.
        - not found: type G

What do I need to do?

Answer 1

You need the forSome construct, which introduces G as existential type:

class Thing[+F](initialValue: F) {
  private[this] var secondValue: Option[G] forSome { type G >: F} = None

  def setSecondValue[G >: F](v: G) = {
    this.secondValue = Some(v)
  }
}

In your original code for secondValue, G has been pulled out of thin air, i.e., it hasn’t been introduced properly. In case of setSecondValue the user (or the compiler) binds G at call site, but for a field that’s not an option (especially, since yours is private). Read more about forSome and existential types in Scala here, here or here.