defgeneric

Defgeneric is a macro in Common Lisp's Object System (CLOS) that defines a generic function. A generic function is a callable entity whose behavior is chosen at runtime by selecting one of its defined methods based on the runtime classes of its arguments. The defgeneric form creates the generic function’s name, its lambda list, and the associated dispatch mechanism, but does not supply any method bodies by itself.

Methods are added with defmethod, each specifying one or more specializers for the corresponding arguments, typically classes. Example: defgeneric area (shape) and then defmethod area ((shape circle)) ... and defmethod area ((shape rectangle)) .... When area is invoked with a circle instance, the (shape circle) method runs; when called with a rectangle instance, the (shape rectangle) method runs. The same generic function can thus have several methods covering different classes, enabling polymorphic behavior across multiple argument types.

Dispatch uses the most specific applicable method; if multiple apply, the method combination rules determine the order and composition of methods (for example, standard, and with possible around, before, and after methods). The generic function may also carry documentation and can be extended with new methods without modifying existing code.

Notes and relationships: defgeneric itself does not implement method bodies—it defines the interface. defmethod provides concrete method implementations. Together, they implement multiple dispatch (multimethod) behavior across the arguments of the generic function. While most CL implementations follow the CLOS model, some Lisp environments offer similar facilities under different names or with additional customization options.