Use default value for Scala trait or abstract class in a case class that implements it

Question

Is there a way to get a version of this code to use the default value defined in the trait?

trait A {      // alternately `abstract class A`
 def x: String
 def y: Int = 1
}

final case class B(x: String, y: Int) extends A

val b = B("ok") // -> errors out
// I'd like this to turn into a B("ok", 1), 
// by using the default y value from A, but this doesn't work
// and similarly something like
object B {
 def apply(x: String): B = {B(x, A.y)}
} 
// doesn't work either

Answer 1

Based on you are not providing anything else than that code, I can only suggest how to make it compile but I don't think the design is exactly good.

For the first approach

trait A {
  def x: String
  def y: Int = 1
}

object DefaultA extends A {
  def x = ??? // you need something here, which means a default impl for this singleton
}

final case class B(x: String, override val y: Int = DefaultA.y) extends A

val b = B("ok") // this will compile

For the second case

trait A {
  def x: String
  def y: Int = 1
}

final case class B(x: String, override val y: Int) extends A

object B {
  def apply(x: String): B =
    // here you create an anonymous instance of the trait but again 
    // you have to provide an implementation for the other method
    B(x, (new A { override def x: String = ??? }).y)

}

If method x and y has no relation, you could use in different traits/classes/singletons