Finally, :around methods are combined much like primary methods except they're run "around" all the other methods. That is, the code from the most specific :around method is run before anything else. Within the body of an :around method, CALL-NEXT-METHOD will lead to the code of the next most specific :around method or, in the least specific :around method, to the complex of :before, primary, and :after methods. Almost all :around methods will contain such a call to CALL-NEXT-METHOD because an :around method that doesn't will completely hijack the implementation of the generic function from all the methods except for more-specific :around methods.
Occasionally that kind of hijacking is called for, but typically :around methods are used to establish some dynamic context in which the rest of the methods will run—to bind a dynamic variable, for example, or to establish an error handler (as I'll discuss in Chapter 19). About the only time it's appropriate for an :around method to not call CALL-NEXT-METHOD is when it returns a result cached from a previous call to CALL-NEXT-METHOD. At any rate, an :around method that doesn't call CALL-NEXT-METHOD is responsible for correctly implementing the semantics of the generic function for all classes of arguments to which the method may apply, including future subclasses.
Auxiliary methods are just a convenient way to express certain common patterns more concisely and concretely. They don't actually allow you to do anything you couldn't do by combining primary methods with diligent adherence to a few coding conventions and some extra typing. Perhaps their biggest benefit is that they provide a uniform framework for extending generic functions. Often a library will define a generic function and provide a default primary method, allowing users of the library to customize its behavior by defining appropriate auxiliary methods.
Other Method Combinations
In addition to the standard method combination, the language specifies nine other built-in method combinations known as the simple built-in method combinations. You can also define custom method combinations, though that's a fairly esoteric feature and beyond the scope of this book. I'll briefly cover how to use the simple built-in combinations to give you a sense of the possibilities.
All the simple combinations follow the same pattern: instead of invoking the most specific primary method and letting it invoke less-specific primary methods via CALL-NEXT-METHOD, the simple method combinations produce an effective method that contains the code of all the primary methods, one after another, all wrapped in a call to the function, macro, or special operator that gives the method combination its name. The nine combinations are named for the operators: +, AND, OR, LIST, APPEND, NCONC, MIN, MAX, and PROGN. The simple combinations also support only two kinds of methods, primary methods, which are combined as just described, and :around methods, which work like :around methods in the standard method combination.
For example, a generic function that uses the + method combination will return the sum of all the results returned by its primary methods. Note that the AND and OR method combinations won't necessarily run all the primary methods because of those macros' short-circuiting behavior—a generic function using the AND combination will return NIL as soon as one of the methods does and will return the value of the last method otherwise. Similarly, the OR combination will return the first non-NIL value returned by any of the methods.
To define a generic function that uses a particular method combination, you include a :method-combination option in the DEFGENERIC form. The value supplied with this option is the name of the method combination you want to use. For example, to define a generic function, priority, that returns the sum of values returned by individual methods using the + method combination, you might write this:
(defgeneric priority (job)
(:documentation "Return the priority at which the job should be run.")
(:method-combination +))
By default all these method combinations combine the primary methods in most-specific-first order. However, you can reverse the order by including the keyword :most-specific-last after the name of the method combination in the DEFGENERIC form. The order probably doesn't matter if you're using the + combination unless the methods have side effects, but for demonstration purposes you can change priority to use most-specific-last order like this:
(defgeneric priority (job)
(:documentation "Return the priority at which the job should be run.")
(:method-combination + :most-specific-last))
The primary methods on a generic function that uses one of these combinations must be qualified with the name of the method combination. Thus, a primary method defined on priority might look like this:
(defmethod priority + ((job express-job)) 10)
This makes it obvious when you see a method definition that it's part of a particular kind of generic function.
All the simple built-in method combinations also support :around methods that work like :around methods in the standard method combination: the most specific :around method runs before any other methods, and CALL-NEXT-METHOD is used to pass control to less-and-less-specific :around methods until it reaches the combined primary methods. The :most-specific-last option doesn't affect the order of :around methods. And, as I mentioned before, the built-in method combinations don't support :before or :after methods.
Like the standard method combination, these method combinations don't allow you to do anything you couldn't do "by hand." Rather, they allow you to express what you want and let the language take care of wiring everything together for you, making your code both more concise and more expressive.
That said, probably 99 percent of the time, the standard method combination will be exactly what you want. Of the remaining 1 percent, probably 99 percent of them will be handled by one of the simple built-in method combinations. If you run into one of the 1 percent of 1 percent of cases where none of the built-in combinations suffices, you can look up DEFINE-METHOD-COMBINATION in your favorite Common Lisp reference.
Multimethods