Binary Methods Programming: the Clos Perspective

Implementing binary methods in traditional object-oriented languages is dicult: numerous problems arise regarding the relationship between types and classes in the context of inheritance, or the need for privileged access to the internal repre- sentation of objects. Most of these problems occur in the context of statically typed languages that lack multi-methods (polymorphism on multiple arguments). The pur- pose of this paper is twofold: rst, we show why some of these problems are either non-issues, or easily solved in Common Lisp. Then, we demonstrate how the Com- mon Lisp Object System (Clos) allows us not only to implement binary methods in a straightforward way, but also to support the concept directly, and even enforce it at dierent levels (usage and implementation).