A Small Example of Phantom Types

Here is an example of so-called phantom types and covariance in Scala to improve type safety and correctness. The example is a simplified version from a real project. I present the example as source code so you can copy into an editor and play with it. References for further reading are given at the bottom of the post.

object Phantoms {

  /* Here is a Button, some implementations and an ActionPanel
   * that uses two Buttons. */
  trait Button

  case object Accept extends Button
  case object Reject extends Button
  case object Disabled extends Button

  /* Here is a panel containing an accept and reject button.
   * The types of accept and reject are simply Button because
   * we want to construct the ActionPanel with a DisabledButton
   * too. */
  class ActionPanel(accept: Button, reject: Button)

  /* The problem is that nothing is enforcing the order of the
   * arguments, it would be easy to get them the wrong way around.
   *
   * We can fix this with so-called phantom types, types used at
   * compile-time to assist in correctness but are not required at
   * runtime.
   *
   * Some phantoms... */
  trait Acceptor
  trait Rejector

  // Now a new Button parameterised by T but makes no use of T
  trait Button2[T]

  // New buttons that use the phantom types.
  case object Accept2 extends Button2[Acceptor]
  case object Reject2 extends Button2[Rejector]

  /* and now an ActionPanel that requires Buttons
   * using types to ensure the correct ones are
   * supplied */
  class ActionPanel2(accept: Button2[Acceptor],
                     reject: Button2[Rejector])

  /* Getting the order wrong results in compilation errors.
   * This will not compile:
   *
   * class ActionPanel2(accept: Button2[Rejector],
   *                    reject: Button2[Acceptor])
   */

  /* But what about the DisabledButton? As things stand,
   * subclasses of each button type would be needed,
   * which is duplication.
   *
   * Introducing the Covariant Phantom */
  trait Button3[+T]

  // and the usual subclasses
  case object Reject3 extends Button3[Rejector]
  case object Accept3 extends Button3[Acceptor]

  // and panel
  case class ActionPanel3(accept: Button3[Acceptor],
                          reject: Button3[Rejector])

  // And now the DisabledButton
  case object Disabled3 extends Button3[Nothing]

  // and in use
  ActionPanel3(Disabled3, Reject3)

  /* What?! How did that work?
   *
   * The Nothing type extends everything, its the
   * bottom type. Since Button3 is covariant,
   * Button3[Nothing] is a subclass of all Button3's
   * and can be used in their place. */
}

In summary, using a covariant phantom type has enabled the ActionPanel to enforce the types of its arguments more strongly and supporting a Disabled (default) implementation.

This was exactly what I needed in my real project, I hope its useful to you.

Further Reading

1. Phantom Types In Haskell and Scala
2. Nothing