Case class for value object visibility?

Question

Is it worth, to control the visibility of case class that represent value object ? If so, is a visibility modifier on the case class enough, or using an explicit companion object, and a private constructor, would be better ?

Version 1:

case class MyClass private/protected/private[] {}

Version 2:

case class MyClass private

object MyClass {

  def apply = {

    new MyClass

  }
}

In sum, the question could be summarize as how to deal with value object in scala. I personally want to enforce the no new, so that is i want to change something in the creation of the object, I can do it at any time. That is, by simply adding a companion object when necessary.

Rex Kerr Rex Kerr · Accepted Answer · 2014-08-23T19:54:30

~~A visibility modifier on the case class constructor does not propagate to the companion object, so creating one yourself by hand with the exact same apply method is unnecessary.~~

You can verify this yourself trivially:

$ scala
Welcome to Scala version 2.11.2 (Java HotSpot(TM) 64-Bit Server VM, Java 1.6.0_37).
Type in expressions to have them evaluated.
Type :help for more information.

scala> case class X private () {}
defined class X

scala> new X
<console>:10: error: constructor X in class X cannot be accessed in object $iw
              new X
              ^

scala> X()
res1: X = X()

Note that just plain X is the name of the companion object. You can't get a new X by naming the companion object; that won't call its apply method even if a no-argument apply exists. So you can't omit the empty parameter list ().

The above is all true in the REPL, but compiled code fails.

Case class for value object visibility?

1 Answers