How do you define an enum that can only be created by its companion object? (think smart constructor-CodePudding

I've tried making the constructor private, like this:

enum X {
  case Stuff private (value: Int)
}

object X {
  def stuff(s: String) = 
    X.Stuff(performSomeValidationAndCalculation(s))
}

but it complains:

method apply cannot be accessed as a member of Playground.X.Stuff.type from module class X$

I would like to force callers to use the smart constructor to prevent an invalid enum from being instantiated and to limit the number of introduction forms.

CodePudding user response：

Just add the class name to private to limit the scope:

enum X {
  case Stuff private[X] (value: Int)
}

object X {
  def stuff(s: String) = 
    X.Stuff(s.toInt)
}

Sample working code: https://scastie.scala-lang.org/hUPECAJFSzqAus6c5slBHQ