Scala immutable objects and traits with val fields

Question

I would like to construct my domain model using immutable objects only. But I also want to use traits with val fields and move some functionality to traits. Please look at the f

Answer 1

You cleanest solution is probably to drop some implementation logic from Versionable, and push it down the type stack to a case class (where the copy method will be available to you). Give the version property a default value to complete the design.

trait Versioned {
  def version : Int
  def nextVersion = version + 1 
}

case class Customer(name: String, version : Int = 0) extends Versioned {
  def withName(newName: String) = copy(name = newName, version = nextVersion)
}

If you want, you can also define a type alias somewhere for the version numbering:

type Version = Int
val initialVersion = 0

trait Versioned {
  def version : Version
  def nextVersion = version + 1 
}

case class Customer(name: String, version : Version = initialVersion)
extends Versioned {
  def withName(newName: String) = copy(name = newName, version = nextVersion)
}