Binary Methods Programming: the Clos Perspective

Implementing binary methods in traditional object-oriented languages is difficult. Numerous problems arise regarding the relationship between types and classes in the context of inheritance, or the need for privileged access to the internal representation of objects. Most of these problems occur in the context of statically typed languages that lack multi-methods (polymorphism on multiple arguments). The purpose of this paper is twofold: to show why some of these problems are either non-issues, or easily solved in Common Lisp, and to demonstrate how the Common Lisp Object System (Clos) allows us to simply define, implement and enforce type-safe binary methods. These last considerations involve re-programming a binary method-specific object system through the Clos Meta-Object Protocol (Mop)