Defmethod

Defmethod is a construct in the Common Lisp Object System (CLOS) used to define a method for a generic function. It is part of Lisp’s multiple dispatch mechanism, where the runtime classes of the arguments determine which method to execute and in what order among possible methods.

A defmethod defines one implementation for a generic function by specifying specializers for one or more parameters, typically corresponding to classes. For example, a method for the generic function area might be defined with a specialization on a circle class. Multiple methods can be defined for the same generic function, each with different specializers (such as circle, square, or rectangle), allowing the system to select the most specific applicable method at call time.

Method combination is a central feature of CLOS. In addition to primary methods, around, before, and after methods can be defined for a generic function. Around methods wrap invocation, before methods run prior to the primary method, and after methods run after. The effective behavior is determined by combining the applicable methods according to the language’s method-combination rules.

Defmethod supports extensibility and modularity. New methods can be added for existing generic functions without changing existing code, enabling behavior to be extended for new classes or circumstances. This mechanism underpins the flexible, multi-dispatch object system implemented by CLOS.

See also: defgeneric, generic function, method combination, CLOS, Common Lisp.