Scala Language Self types Simple self type example


Self types can be used in traits and classes to define constraints on the concrete classes it is mixed to. It is also possible to use a different identifier for the this using this syntax (useful when outer object has to be referenced from an inner object).

Assume you want to store some objects. For that, you create interfaces for the storage and to add values to a container:

 trait Container[+T] {
   def add(o: T): Unit

 trait PermanentStorage[T] {
   /* Constraint on self type: it should be Container
    * we can refer to that type as `identifier`, usually `this` or `self`
    * or the type's name is used. */
   identifier: Container[T] =>

   def save(o: T): Unit = {
     //Do something to persist too.

This way those are not in the same object hierarchy, but PermanentStorage cannot be implemented without also implementing Container.