method-combination

method-combination System Class

Class Precedence List:

method-combination, t

Description:

Every method combination object is an indirect instance of the class method-combination. A method combination object represents the information about the method combination being used by a generic function. A method combination object contains information about both the type of method combination and the arguments being used with that type.

Expanded Reference: method-combination

The method-combination System Class

method-combination objects determine how the effective method is computed from the applicable methods of a generic function. The standard method combination uses primary, :before, :after, and :around methods.

(defgeneric process (obj)
  (:method-combination standard))

(defmethod process :before ((obj string))
  (format t "Before: ~A~%" obj))

(defmethod process ((obj string))
  (format t "Primary: ~A~%" obj))

(defmethod process :after ((obj string))
  (format t "After: ~A~%" obj))

(process "test")
;; prints:
;; Before: test
;; Primary: test
;; After: test
=> NIL

Built-in Method Combinations

Common Lisp provides several built-in method combinations including +, list, progn, and, or, max, min, and append.

(defgeneric total-weight (obj)
  (:method-combination +))

(defclass container () ())
(defclass heavy-container (container) ())

(defmethod total-weight + ((obj container)) 10)
(defmethod total-weight + ((obj heavy-container)) 50)

(total-weight (make-instance 'heavy-container))
=> 60