Editing Dynamic programming language (section)

===Assembling of code at runtime based on the class of instances===
In the next example, the class '''person''' gets a new superclass. The '''print''' method gets redefined such that it assembles several methods into the effective method. The effective method gets assembled based on the class of the argument and the at runtime available and applicable methods.

<syntaxhighlight lang="lisp">
; the class person
CL-USER > (defclass person () ((name :initarg :name)))
#<STANDARD-CLASS PERSON 4220333E23>

; a person just prints its name
CL-USER > (defmethod print-object ((p person) stream)
            (print-unreadable-object (p stream :type t)
              (format stream "~a" (slot-value p 'name))))
#<STANDARD-METHOD PRINT-OBJECT NIL (PERSON T) 40200605AB>

; a person instance
CL-USER > (defparameter *person-1* (make-instance 'person :name "Eva Luator"))
*PERSON-1*

; displaying a person instance
CL-USER > *person-1*
#<PERSON Eva Luator>

; now redefining the print method to be extensible
; the around method creates the context for the print method and it calls the next method
CL-USER > (defmethod print-object :around ((p person) stream)
            (print-unreadable-object (p stream :type t)
              (call-next-method)))
#<STANDARD-METHOD PRINT-OBJECT (:AROUND) (PERSON T) 4020263743>

; the primary method prints the name
CL-USER > (defmethod print-object ((p person) stream)
            (format stream "~a" (slot-value p 'name)))
#<STANDARD-METHOD PRINT-OBJECT NIL (PERSON T) 40202646BB>

; a new class id-mixin provides an id
CL-USER > (defclass id-mixin () ((id :initarg :id)))
#<STANDARD-CLASS ID-MIXIN 422034A7AB>

; the print method just prints the value of the id slot
CL-USER > (defmethod print-object :after ((object id-mixin) stream)
          (format stream " ID: ~a" (slot-value object 'id)))
#<STANDARD-METHOD PRINT-OBJECT (:AFTER) (ID-MIXIN T) 4020278E33>

; now we redefine the class person to include the mixin id-mixin
CL-USER 241 > (defclass person (id-mixin) ((name :initarg :name)))
#<STANDARD-CLASS PERSON 4220333E23>

; the existing instance *person-1* now has a new slot and we set it to 42
CL-USER 242 > (setf (slot-value *person-1* 'id) 42)
42

; displaying the object again. The print-object function now has an effective method, which calls three methods: an around method, the primary method and the after method.
CL-USER 243 > *person-1*
#<PERSON Eva Luator ID: 42>
</syntaxhighlight>