| Preserving entity identity over time while changing behavior is a problem often encountered in software systems.;The need for changes in behavior has multiple causes: it can stem from changes to the situation of a real-world entity for which a software entity is a proxy, from upgrades to software capabilities, or a need to utilize new interfaces when communicating with changing or new external entities.;Strategies to permit change in behavior have ranged from self-modifying programs to duck typing. Existing methods have drawbacks that limit their utility.;To understand behavior, one needs to understand types. Investigation reveals a few basic types: structure, interface, behavior, and categorization.;To implement, we use well-defined interfaces, which we call contexts, as the basis for defining entity behavior. An entity is instantiated as a structure, which may contain other structures as well as non-structure data. A structure's core behavior is defined in a template. Other behavior is implemented by contained structures, whose interfaces may be exposed as if they were the containing structure's own.;The implementation examples display the flexibility of this approach, showing definition of explicit behavior, dynamic behavior acquisition, and dynamic behavior change.;The ability to dynamically add or change behavior without the necessity of a prior road map, but with well-defined interfaces, opens up interesting possibilities for new programming paradigms. |
