Introduction The Common Lisp Object System is an object-oriented extension to Common Lisp as defined in Common Lisp: The Language, by Guy L. Steele Jr. It is based on generic functions, multiple inheritance, declarative method combination, and a meta-object protocol. The first two chapters of this specification present a description of the standard Programmer Interface for the Common Lisp Object System. The first chapter contains a description of the concepts of the Common Lisp Object System, and the second contains a description of the functions and macros in the Common Lisp Object System Programmer Interface. The chapter "The Common Lisp Object System Meta-Object Protocol" describes how the Common Lisp Object System can be customized. The fundamental objects of the Common Lisp Object System are classes, instances, generic functions, and methods. A class object determines the structure and behavior of a set of other objects, which are called its instances. Every Common Lisp object is an instance of a class. The class of an object determines the set of operations that can be performed on the object. A generic function is a function whose behavior depends on the classes or identities of the arguments supplied to it. A generic function object contains a set of methods, a lambda-list, a method combination type, and other information. The methods define the class-specific behavior and operations of the generic function; a method is said to specialize a generic function. When invoked, a generic function executes a subset of its methods based on the classes of its arguments. A generic function can be used in the same ways that an ordinary function can be used in Common Lisp; in particular, a generic function can be used as an argument to funcall and apply and can be given a global or a local name. A method is an object that contains a method function, a sequence of parameter speclalizers that specify when the given method is applicable, and a sequence of qualifiers that is used by the method combination facility to distinguish among methods. Each required formal parameter of each method has an associated parameter specializer, and the method will be invoked only on arguments that satisfy its parameter specializers. The method combination facility controls the selection of methods, the order in which they are run, and the values that are returned by the generic function. The Common Lisp Object System offers a default method combination type and provides a facility for declaring new types of method combination.

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The Common Lisp Object System is an object-oriented system that is based on the concepts of generic functions, multiple inheritance, and method combination. All objects in the Object System are instances of classes that form an extension to the Common Lisp type system. The Common Lisp Object System is based on a meta-object protocol that renders it possible to alter the fundamental structure of the Object System itself. The Common Lisp Object System has been proposed as a standard for ANSI Common Lisp and has been tentatively endorsed by X3J13. 2. History of the Common Lisp Object System The Common Lisp Object System is an object-oriented programming paradigm designed for Common Lisp. The lack of a standardized object-oriented extension for Common Lisp has long been regarded as a shortcoming by the Common Lisp community. Two separate and independent groups began work on an object-oriented extension to Common Lisp several years ago. One group is Symbolics, Inc. with New Flavors, and the other is Xerox PARC with CommonLoops. During the summer of 1986, these two groups met to explore combining their designs for submission to X3J13, a technical working group charged with producing an ANSI standard for Common Lisp. At the time of the exploratory meetings between Symbolics and Xerox, the authors of this paper became involved in the technical design work. The major participants in this effort were David Moon and Sonya Keene from Symbolics, Daniel Bobrow and Gregor Kiczales from Xerox, and Richard Gabriel and Linda DeMichiel from Lucid. § 2 History of the Common Lisp Object System By March 1987 this three-way collaborative effort had produced a strong draft of a specification for the bulk of the Object System. X3J13 has voted an endorsement of that specification draft, stating that it would almost certainly be adopted as part of the standard and encouraging implementors to proceed with trial implementations. This paper is a report on the specification that was presented to X3J13. 3. The Common Lisp Object System View of Object-Oriented Programming Several aspects of the Object System stand out upon inspection: a) it is a layered system designed for flexibility; b) it is based on the concept of generic functions rather than on message-passing; c) it is a multiple inheritance system; d) it provides a powerful method combination facility; e) the primary entities of the system are all first-class objects. 3.1 The Layered Approach One of the design goals of the Object System is to provide a set of layers that separate different programming language concerns from one another. The first level of the Object System provides a programmatic interface to objectoriented programming. This level is designed to meet the needs of most serious users and to provide a syntax that is crisp and understandable. The second level provides a functional interface into the heart of the Object System. This level is intended for the programmer who is writing very complex software or a programming environment. The first level is written in terms of th...

Multi-methods allow method selection to be based on the types of any number of arguments.Languages that currently support multi-methods do not support static type checking. We show how multi-methods can be statically type checked and how information collected at the time of program compilation can be used to make the run-time dispatch of multi-methods more efficient. The results presented can provide the basis for introducing multi-methods in languages with static type checking and for designing new object-oriented paradigms based on multi-methods.

Multi-methods allow method selection to be based on the types of any number of arguments.Languages that currently support multi-methods do not support static type checking. We show how multi-methods can be statically type checked and how information collected at the time of program compilation can be used to make the run-time dispatch of multi-methods more efficient. The results presented can provide the basis for introducing multi-methods in languages with static type checking and for designing new object-oriented paradigms based on multi-methods. 'The exact syntax is different in C++;we have taken liberty with the syntax to clarify the point. OOPSLA'91, pp. 113-128 arbitrary global prececence J J J J argument subtype precedence J argument order precedence J J global type precedence J J J J inheritance order precedence J J J J arbitrary local precedence J J J ?