Prototype object system for Common Lisp

KR is an object system implemented in Common Lisp [Steele 84]. It includes three closely integrated components: object-oriented programming, constraint maintenance, and knowledge representation.

This is KR as it appears in Garnet 3.0, made standalone and loadable with ASDF.

• The first component of KR is an object oriented programming system based on the prototype-instance paradigm. Schemata can be used as objects, and inheritance can be used to determine their properties and behavior. Objects can be sent messages, which are implemented as procedural attachments to certain slots; messages are inherited through the same mechanism as values. Instead of the class-instance paradigm, common in object-oriented programming languages, KR implements the more flexible prototype-instance paradigm [Lieberman 86], which allows properties of instances to be determined dynamically by their prototypes. This means that the class structure of a system can be modified dynamically as needed, without any need for recompilation.
• The second component of KR implements constraint maintenance. Constraint maintenance is implemented through formulas, which may be attached to slots and determine their values based on the values of other slots in the system. Constraint maintenance is closely integrated with the other components. The user, for example, does not need to know which slots in a schema contain ordinary values and which ones are constrained by a formula, since the same access primitives may be used in both cases.
• The third component, frame-based knowledge representation, stores knowledge as a network of chunks of information. Networks in KR are built out of unstructured chunks, i.e., schemata. Each schema can store arbitrary pieces of information, and is not restricted to a particular format or data structure. Information is encoded via attribute-value pairs. Values in a schema can be interpreted as links to other schemata. This enables the system to support complex network structures, which can be freely extended and modified by application programs. KR provides simple ways to specify the structure of a network and the relationship among its components.