Gregor Kiczales, Jim des Rivières, Daniel G. Bobrow
The CLOS Specification describes the standard Programmer Interface for the Common Lisp Object System (CLOS). This document extends that specification by defining a metaobject protocol for CLOS - that is, a description of CLOS itself as an extensible CLOS program. In this description, the fundamental elements of CLOS programs (classes, slot definitions, generic functions, methods, specializers and method combinations) are represented by first-class objects. The behavior of CLOS is provided by these objects, or, more precisely, by methods specialized to the classes of these objects.
Because these objects represent pieces of CLOS programs, and because their behavior provides the behavior of the CLOS language itself, they are considered meta-level objects or metaobjects. The protocol followed by the metaobjects to provide the behavior of CLOS is called the CLOS Metaobject Protocol (MOP).
We provide the detailed specification of a metaobject protocol for CLOS. Our work with this protocol has always been rooted in our own implementation of CLOS, PCL. This has made it possible for us to have a user community, which in turn has provided us with feedback on this protocol as it has evolved. As a result, much of the design presented here is well-tested and stable. As this is being written, those parts have been implemented not only in PCL, but in at least three other CLOS implementations we know of. Other parts of the protocol, even though they have been implemented in one form or another in PCL and other implementations, are less well worked out. Work remains to improve not only the ease of use of these protocols, but also the balance they provide between user extensibility and implementor freedom.
In preparing this specification, it is our hope that it will provide a basis for the users and implementors who wish to work with a metaobject protocol for CLOS. This document should not be construed as any sort of final word or standard, but rather only as documentation of what has been done so far. We look forward to seeing the improvements, both small and large, which we hope this publication will catalyze.
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