Object Reorientation: Generic Functions

Abstract

Because the invention of Lisp predated the rise of object-oriented programming by a couple decades,¹ new Lispers are sometimes surprised to discover what a thoroughly object-oriented language Common Lisp is. Common Lisp’s immediate predecessors were developed at a time when object orientation was an exciting new idea and there were many experiments with ways to incorporate the ideas of object orientation, especially as manifested in Smalltalk, into Lisp. As part of the Common Lisp standardization, a synthesis of several of these experiments emerged under the name Common Lisp Object System, or CLOS. The ANSI standard incorporated CLOS into the language, so it no longer really makes sense to speak of CLOS as a separate entity.

The language now generally considered the first object-oriented language, Simula, was invented in the early 1960s, only a few years after McCarthy’s first Lisp. However, object orientation didn’t really take off until the 1980s when the first widely available version of Smalltalk was released, followed by the release of C++ a few years later. Smalltalk took quite a bit of inspiration from Lisp and combined it with ideas from Simula to produce a dynamic object-oriented language, while C++ combined Simula with C, another fairly static language, to yield a static object-oriented language. This early split has led to much confusion in the definition of object orientation. Folks who come from the C++ tradition tend to consider certain aspects of C++, such as strict data encapsulation, to be key characteristics of object orientation. Folks from the Smalltalk tradition, however, consider many features of C++ to be just that, features of C++, and not core to object orientation. Indeed, Alan Kay, the inventor of Smalltalk, is reported to have said, “I invented the term object oriented, and I can tell you that C++ wasn’t what I had in mind.”