Editing Dynamic programming language (section)

===Object runtime alteration===
This example shows how an existing instance can be changed to include a new slot when its class changes and that an existing method can be replaced with a new version.

<syntaxhighlight lang="lisp">
; a person class. The person has a name.
CL-USER > (defclass person () ((name :initarg :name)))
#<STANDARD-CLASS PERSON 4020081FB3>

; a custom printing method for the objects of class person
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) 4020066E5B>

; one example person instance
CL-USER > (setf *person-1* (make-instance 'person :name "Eva Luator"))
#<PERSON Eva Luator>

; the class person gets a second slot. It then has the slots name and age.
CL-USER > (defclass person () ((name :initarg :name) (age :initarg :age :initform :unknown)))
#<STANDARD-CLASS PERSON 4220333E23>

; updating the method to print the object
CL-USER > (defmethod print-object ((p person) stream)
            (print-unreadable-object (p stream :type t)
              (format stream "~a age: ~" (slot-value p 'name) (slot-value p 'age))))
#<STANDARD-METHOD PRINT-OBJECT NIL (PERSON T) 402022ADE3>

; the existing object has now changed, it has an additional slot and a new print method
CL-USER > *person-1*
#<PERSON Eva Luator age: UNKNOWN>

; we can set the new age slot of instance
CL-USER > (setf (slot-value *person-1* 'age) 25)
25

; the object has been updated
CL-USER > *person-1*
#<PERSON Eva Luator age: 25>
</syntaxhighlight>
let foo = 42; // foo is now a number
foo = "bar"; // foo is now a string
foo = true; // foo is now a boolean