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Design pattern: Marker interface

Description: Marker interfaces, also known as tagging interfaces, allows associating meta-data with a hierarchy of objects. Typically, marker interfaces are empty with its presence implying that an object implements some functionality or have some property.

This pattern can be used with both classes and prototypes.

This pattern is not described in the GoF book but is common practice in some languages (notably, Java). See e.g. the following Wikipedia page for details and references:

https://en.wikipedia.org/wiki/Marker_interface_pattern

Given Logtalk support for protocols (interfaces), this is a trivial pattern to implement. The reflection API provides built-in predicates that allows to test if an object (or a category) implements or conforms to a given protocol. It is also possible to add a marker to an existing object at runtime using a complementing category (aka hot patching).