Scala特征案例类和继承

Question

when defining a type hierarchy of case classes in scala:

sealed trait FooBar {
  def A:Int
  def B:Int
  def C:Int
}

// works
final case class Bar(A:Int, B:Int, C:Int)extends FooBar

// fails
final case class Bar extends FooBar(A:Int, B:Int, C:Int)

// fails
final case class Foo extends FooBar

how can I avoid to specify the already inherited parameters when defining an inherited type? Is this possible without any macros: Scala case classes and constructors

Would an abstract class be better suited for this purpose?

Answer 1

Well, what you've declared is a trait with three abstract methods.

Your first implementation declares a case class with 3 values that match the abstract methods declared in FooBar . Since methods without parenthesis and values are basically the same in Scala, this works.

Your second implementation calls FooBar 's constructor with 3 values that don't exist. Where does it get the A from?

Your third implementation is declaring a concrete class that does not implement abstract methods, and cannot compile.

I do not know of a (sane) solution for what you're asking. You're declaring abstract methods and want to not implement them. It's probably feasible through macros, but it seems like a lot of work for not much benefit.