12.10. Structural Types

You can think of structural types as a type-safe approach to duck typing, the popular name for the way method resolution works in dynamically typed languages. In Ruby, for example, when you write starFighter.shootWeapons, the runtime looks for a shootWeapons method on the object referenced by starFighter. That method, if found, might have been defined in the class used to instantiate starFighter or one of its parents or “included” modules. The method might also have been added to the object using the metaprogramming facility of Ruby. Finally, the object might override the catch-all method_missing method and do something reasonable when the object receives the shootWeapons “message.”

Scala doesn’t support this kind of method resolution, Instead, Scala allows you to specify that an object must adhere to a certain structure: that it contains certain types, fields, or methods, without concern for the actual type of the object. We first encountered structural types near the beginning of Chapter 4. Here is the example we saw then, a variation of the Observer Pattern: