Generic Function

Summary

Returns the effective method function.

Package

clos

Signature

compute-effective-method-function-from-classes gf classes => em-function

Arguments

gf

A generic function.

classes

A list of class metaobjects.

Values

em-function

A function or nil.

Description

The generic function compute-effective-method-function-from-classes is called by LispWorks to compute the effective method function when gf is called with required argument types specified by classes. If em-function is nil, then no-applicable-method is called. Otherwise, em-function may be cached by the generic function and is called with the arguments supplied to the generic function.

The default method for compute-effective-method-function-from-classes implements the standard generic function behavior of finding the applicable methods and using the method combination to construct a function that calls them.

In order for compute-effective-method-function-from-classes to be called and the result cached, there must be methods specializing on the "interesting" arguments. For the standard behavior, this is trivially true, but if you want to implement other behavior then you need to define dummy methods even if they are never called.

Example

A "computed" generic function that returns a value based on a form chosen from the classes of the arguments rather than the methods. Note the dummy method which is specialized on null.

(defclass computed-generic-function (standard-generic-function)

  ((computer :initarg :computer

             :accessor computed-generic-function-computer))

  (:metaclass funcallable-standard-class))

(defmethod clos:compute-effective-method-function-from-classes

           ((gf computed-generic-function)

            classes)

  (apply (computed-generic-function-computer gf) gf classes))

(defmacro define-computed-generic-function (name lambda-list

                                                 specializers

                                                 &body body)

  `(dspec:def (define-computed-generic-function ,name)

     (defgeneric ,name ,lambda-list

       (:generic-function-class computed-generic-function)

       (:method ,(loop for arg in lambda-list

                       collect

                       (if (member arg specializers)

                           `(,arg null)

                         arg))))

     (setf (computed-generic-function-computer #',name)

           #'(lambda (,name ,@(loop for arg in lambda-list

                                    collect

                                    (if (member arg specializers)

                                        arg

                                      (gensym))))

               ,@body))

     ',name))

(define-computed-generic-function aaaa (x y) (x)

  (let ((something (compute-something aaaa x)))

    #'(lambda (x y)

        (declare (ignore y))

        (format nil "Something for ~a is ~a" x something))))

(defun compute-something (gf class)

   (format nil "~a-~a"

           (generic-function-name gf)

           (class-name class)))

See also