在不不断复制构造函数 vals 的情况下扩展 Scala 案例类？

Question

Is there a way to extend a case class without constantly picking up new vals along the way?

For example this doesn't work:

case class Edge(a: Strl, b: Strl)
case class EdgeQA(a: Strl, b: Strl, right: Int, asked: Int) extends Edge(a, b)

"a" conflicts with "a" , so I'm forced to rename to a1 . But I don't want all kinds of extra public copies of "a" so I made it private.

case class Edge(a: Strl, b: Strl)
case class EdgeQA(private val a1: Strl, private val b1: Strl, right: Int, asked: Int) extends Edge(a, b)

This just doesn't seem clean to me... Am I missing something?

Answer 1

As the previous commenter mentioned: case class extension should be avoided but you could convert your Edge class into a trait.

If you want to avoid the private statements you can also mark the variables as override

trait Edge{
  def a:Strl
  def b:Strl
}

case class EdgeQA(override val a:Strl, override val b:Strl, right:Int, asked:Int ) extends Edge

Don't forget to prefer def over val in traits

Answer 2

This solution offers some advantages over the previous solutions:

trait BaseEdge {
  def a: Strl
  def b: Strl
}
case class Edge(a:Strl, b:Strl) extends BaseEdge
case class EdgeQA(a:Strl, b:Strl, right:Int, asked:Int ) extends BaseEdge

In this way:

• you don't have redundant val s, and
• you have 2 case classes.

Answer 3

Case classes can't be extended via subclassing. Or rather, the sub-class of a case class cannot be a case class itself.

Answer 4

Starting in Scala 3 , traits can have parameters :

trait Edge(a: Strl, b: Strl)
case class EdgeQA(a: Strl, b: Strl, c: Int, d: Int) extends Edge(a, b)