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Common Lisp Object System

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For other uses, see Clos (disambiguation).

The Common Lisp Object System (CLOS) is the facility for object-oriented programming which is part of ANSI Common Lisp. CLOS is a powerful dynamic object system which differs radically from the OOP facilities found in more static languages such as C++ or Java. CLOS was inspired by earlier Lisp object systems such as MIT Flavors and CommonLOOPS, although it is more general than either. Originally proposed as an add-on, CLOS was adopted as part of the ANSI standard for Common Lisp and has been adapted into other Lisp dialects like EuLisp or Emacs Lisp.[1]

ULOL

Metaobject Protocol

Outside of the ANSI Common Lisp standard, there is a widely implemented extension to CLOS called the Metaobject Protocol (MOP). The MOP defines a standard interface to the underpinnings of the CLOS implementation, treating classes, slot-descriptions, generic-functions and methods themselves as instances of metaclasses, and allows the definition of new metaclasses and the modification of all CLOS behavior. The flexibility of the CLOS MOP prefigures aspect-oriented programming, which was later developed by some of the same engineers, such as Gregor Kiczales. The MOP defines the behavior of the whole object system by a set of protocols. These are defined in terms of CLOS. Thus it is possible to create new object-systems by extending or changing the provided CLOS functionality. The book The Art of the Metaobject Protocol describes the use and implementation of the CLOS MOP.

The various Common Lisp implementations have slightly different support for the Meta-Object Protocol. The Closer[2] project aims to provide the missing features.

Influences from older Lisp-based object systems

Flavors (and its successor New Flavors) was the object system on the MIT Lisp Machine. Large parts of the Lisp Machine operating systems and many applications for it use Flavors or New Flavors. Flavors introduced multiple inheritance and mixins, amongst other features. Flavors is mostly obsolete, though implementations for Common Lisp do exist. Flavors was using the message passing paradigm. New Flavors introduced generic functions.

CommonLoops was the successor of LOOPS (from Xerox Interlisp-D). CommonLoops was implemented for Common Lisp. A portable implementation called Portable CommonLoops (PCL) was the first implementation of CLOS. PCL is widely ported and still provides the base for the CLOS implementation of several Common Lisp implementations. PCL is implemented mostly in portable Common Lisp with only a few system dependent parts.

CLOS in other programming languages

Because of the power and expressivity of CLOS, as well as the historical availability of TinyCLOS (a simplified portable CLOS implementation written by Gregor Kiczales for use with Scheme), CLOS-like MOP-based object systems have become the de facto norm in most Lisp dialect implementations, as well as finding their way into some other languages' OOP facilities:

[5]

References

  1. ^ "CLOS is a standard. Multiple vendors supply CLOS. CLOS (or parts of it) is being used to add object-orientation to other Lisp dialects such as EuLisp or Emacs Lisp." pg 110 of Veitch 1998
  2. ^ Closer Project: Closer to MOP
  3. ^ COS, the C Object System
  4. ^ VCLOS, CLOS for Skill
  5. ^ Tiny CLOS, developed by Gregor Kiczales
  • "CommonLoops: merging Lisp and object-oriented programming", by Daniel G. Bobrow, Kenneth Kahn, Gregor Kiczales, Larry Masinter, Mark Stefik, Frank Zdybel. 1986, Portland, Oregon, United States. Pages 17 - 29 of the Conference on Object-oriented Programming Systems Languages and Applications, ISSN 0362-1340.
  • "A History and Description of CLOS", by Jim Veitch. Pages 107-158 of Handbook of Programming Languages, Volume IV: Functional and Logic Programming Languages, ed. Peter H. Salus. 1998 (1st edition), Macmillan Technical Publishing; ISBN 1-57870-011-6

Literature

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