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Design pattern: Decorator

Description: "Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality."

This pattern can be used with both classes and prototypes.

The sample code uses a parametric object to represent the decorator object with a parameter being used to pass the decorated object. This is a clean, declarative solution given that object parameters are logical variables. It also supports defining decorations dynamically by simply constructing identifier terms for the parametric object. In those cases where we need to persist across backtracking the association between a decorator object and its decorated object, we can use in alternative a dynamic predicate in the decorator object.

As a decorator object should accept all messages that the decorated object accepts, we use the forward/1 handler for unknown messages to forward to the decorated objects all messages that are not defined in the decorator itself. By using this handler, it is simple to define pipelines of decorators where a decorator acts as the decorated object for another decorator.

Another possible implementation of this pattern is to use a complementing category (i.e. hot patching) to decorate an object. This can be a good alternative solution when we need only to decorated a few specific objects (although a complementing category can complement multiple objects). The advantage of using a decorator object is that it can be used to decorate any object at compilation time or at runtime while with a complementing category we need to know in advance which objects we will be decorating.