method-qualifiers

method-qualifiers Standard Generic Function

Syntax:

method-qualifiers method → qualifiers

Method Signatures:

method-qualifiers (method standard-method)

Arguments and Values:

method—a method.

qualifiers—a proper list.

Description:

Returns a list of the qualifiers of the method.

Examples:

(defmethod some-gf :before ((a integer)) a) 
→ #<STANDARD-METHOD SOME-GF (:BEFORE) (INTEGER) 42736540> 
(method-qualifiers \*) → (:BEFORE) 

See Also:

define-method-combination

Expanded Reference: method-qualifiers

Inspecting Method Qualifiers

method-qualifiers returns the list of qualifiers of a method object. Unqualified (primary) methods return an empty list. Qualified methods return a list containing their qualifier keyword(s).

(defgeneric process (x))
(defmethod process ((x integer)) x)
(defmethod process :before ((x integer)) (print "before"))
(defmethod process :after ((x integer)) (print "after"))
(defmethod process :around ((x integer)) (call-next-method))

;; Find all methods and check their qualifiers
(let ((methods (compute-applicable-methods #'process '(42))))
  (mapcar #'method-qualifiers methods))
;; The result will contain lists like: (:AROUND) (:BEFORE) NIL (:AFTER)

Primary Methods Have Empty Qualifier Lists

An unqualified (primary) method has () as its qualifier list.

(defgeneric foo (x))
(defmethod foo ((x t)) x)

(let ((method (first (compute-applicable-methods #'foo '(42)))))
  (method-qualifiers method))
=> NIL
;; (i.e., the empty list ())

Distinguishing Method Types

You can use method-qualifiers to filter methods by their role.

(defgeneric act (obj))
(defmethod act ((obj string)) obj)
(defmethod act :before ((obj string)) nil)
(defmethod act :after ((obj string)) nil)

(let* ((methods (compute-applicable-methods #'act '("test")))
       (primary (remove-if-not (lambda (m) (null (method-qualifiers m))) methods))
       (before  (remove-if-not (lambda (m) (equal (method-qualifiers m) '(:before))) methods)))
  (list (length primary) (length before)))
=> (1 1)