Scala Cookbook

8.6. Marking Traits So They Can Only Be Used by Subclasses of a Certain Type

Problem

You want to mark your trait so it can only be used by types that extend a given base type.

Solution

To make sure a trait named MyTrait can only be mixed into a class that is a subclass of a type named BaseType, begin your trait with a this: BaseType => declaration, as shown here:

trait MyTrait {
  this: BaseType =>

For instance, to make sure a StarfleetWarpCore can only be used in a Starship, mark the StarfleetWarpCore trait like this:

trait StarfleetWarpCore {
  this: Starship =>
  // more code here ...
}

Given that declaration, this code will work:

class Starship
class Enterprise extends Starship with StarfleetWarpCore

But other attempts like this will fail:

class RomulanShip
// this won't compile
class Warbird extends RomulanShip with StarfleetWarpCore

This second example fails with an error message similar to this:

error: illegal inheritance;
self-type Warbird does not conform to StarfleetWarpCore's selftype 
StarfleetWarpCore with Starship
class Warbird extends RomulanShip with StarfleetWarpCore
                                       ^

Discussion

As shown in the error message, this approach is referred to as a self type. The Scala Glossary includes this statement as part of its description of a self type:

“Any concrete class that mixes in the trait must ensure that its type conforms to the trait’s self type.”

A trait can also require that any type that wishes to extend it must extend multiple other types. The following WarpCore definition requires that ...