A Small Example of Phantom Types (with covariance thrown in)

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.

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. James Iry’s Phantom Types In Haskell and Scala
2. Phantom Types at the Foursquare Engineering Blog
3. The Nothing type.

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