Abstract
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: first, we show why some of these problems are either non-issues, or easily solved in Common Lisp. Then, we demonstrate how the Common 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 different levels (usage and implementation).