Practical Common Lisp

Here, as elsewhere, object means any Lisp datum—Common Lisp doesn't distinguish, as some languages do, between objects and 'primitive' data types; all data in Common Lisp are objects, and every object is an instance of a class.

175

Technically you could skip the DEFGENERIC altogether—if you define a method with DEFMETHOD and no such generic function has been defined, one is automatically created. But it's good form to define generic functions explicitly, if only because it gives you a good place to document the intended behavior.

176

A method can 'accept' &key and &rest arguments defined in its generic function by having a &rest parameter, by having the same &key parameters, or by specifying &allow-other- keys along with &key. A method can also specify &key parameters not found in the generic function's parameter list— when the generic function is called, any &key parameter specified by the generic function or any applicable method will be accepted.

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CALL-NEXT-METHOD is roughly analogous to invoking a method on super in Java or using an explicitly class-qualified method or function name in Python or C++.

178

While building the effective method sounds time-consuming, quite a bit of the effort in developing fast Common Lisp implementations has gone into making it efficient. One strategy is to cache the effective method so future calls with the same argument types will be able to proceed directly.

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Actually, the order in which specializers are compared is customizable via the DEFGENERIC option :argument-precedence-order, though that option is rarely used.

180

In languages without multimethods, you must write dispatching code yourself to implement behavior that depends on the class of more than one object. The purpose of the popular Visitor design pattern is to structure a series of singly dispatched method calls so as to provide multiple dispatch. However, it requires one set of classes to know about the other. The Visitor pattern also quickly bogs down in a combinatorial explosion of dispatching methods if it's used to dispatch on more than two objects.

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Defining new methods for an existing class may seem strange to folks used to statically typed languages such as C++ and Java in which all the methods of a class must be defined as part of the class definition. But programmers with experience in dynamically typed object-oriented languages such as Smalltalk and Objective C will find nothing strange about adding new behaviors to existing classes.

182

In other object-oriented languages, slots might be called fields, member variables, or attributes.

183

As when naming functions and variables, it's not quite true that you can use any symbol as a class name—you can't use names defined by the language standard. You'll see in Chapter 21 how to avoid such name conflicts.

184

The argument to MAKE-INSTANCE can actually be either the name of the class or a class object returned by the function CLASS-OF or FIND-CLASS.

185

Another way to affect the values of slots is with the :default-initargs option to DEFCLASS. This option is used to specify forms that will be evaluated to provide arguments for specific initialization parameters that aren't given a value in a particular call to MAKE-INSTANCE. You don't need to worry about :default-