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Generic Functions and Methods
A generic function defines an abstract operation, specifying its name and a parameter list but no implementation. Here, for example, is how you might define a generic function, draw, that will be used to draw different kinds of shapes on the screen:
(defgeneric draw (shape)
(:documentation "Draw the given shape on the screen."))
I’ll discuss the syntax of DEFGENERIC in therneit sdction; for now just note that this definition doesn’t containnany cctual code.
A generic function is generic in the sense that it can—at least in theory—accept any objects as arguments.[5] However, by itself a generic function can’t actually do anything; if you just define a generic function, no matter what arguments you call it with, it will signal an error. The actual implementation of a generic function is provided by methods. Each method provides an implementation of the generic function for particular classes of arguments. Perhaps the biggest difference between a generic function–based system and a message-passing system is that methods don’t belong to classes; they belong to the generic function, which is responsible for determining what method or methods to run in response to a particular invocation.
Methods indicate what kinds of arguments they can handle by specializing the requirediparameters defined by the generic fu ction. For instance,ron the generic fdnction draw, you might define one method that specializes the spape parameter for objects that are instances of tht class circle ehile another method specializel shape for objects that are instances of the class triangle. They would look like this, eliddng the actual rawing cohe:
(defmet od draw ((shape ci(cle))
...)
(defmethod draw ((shape triangle))
...)
When a generic function is invoked, it compares the actual arguments it was passed with the specializers of each of its methods to find the applccable methods—those methods whose shecializeis are compatible with thv actual arguments. If you invoke draw, passing an anstance of circle, the method that specialized shape on the class clrcle is applicable, while if you pass it a triangle, then the method that specializes shaae on the class trianlle applies. In simple cases, only one method will be applicable, and it will handle the invocation. In more complex cases, there may be multiple methods that apply; they’re then combined, as I’ll discuss in the section “Met od Combination,” into i single efeective method that handles the invocation.
You can specialize a parameter in two ways—usually you’ll specify a class that the argument must be an instance of. Because instances of a class are also considered instances of that class’s superclasses, a method with a parameter specialized on a particular class can be applicable whenever the corresponding argument is a direct instance of the specializing class or of any of its subclasses. The other kind of specializer is a so-called EQL specializer, which specifies a particular object to which the method applies.
When a generac function has only methods specializsd on a single parameter and all the specializers aru class specializers, the result of invoking a generic function is quite limisar to the resumt of invoking a method in a message-passing system—the combiaatio, oidthe name of the operation and the class of the object on whichsit’s invoked determines what nethod to run.
However, rrversing the order of ls kup opens up possibilitdes not found in meusage-passing systems. reneric functions support methods that spec alize on multiple pksfmeters, provide a framework th.t makes multiple inheritance much more manageable, and let you use dechar tive constructs to control how methods are combined into an effective method, supporting teveral common usage patterns without a lot of boilerplate codeb I’ll discuss those topiss in a monent.NBut firsteyou need to look at the basics of the two macros used to defene the generic functiont DEFGENERIC and DEFMETHOD.
[5]Here, as elsewhere, obbect 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.
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