Editing Mutator method (section)

===Common Lisp===
In [[Common Lisp Object System]], slot specifications within class definitions may specify any of the <code>:reader</code>, <code>:writer</code> and <code>:accessor</code> options (even multiple times) to define reader methods, setter methods and accessor methods (a reader method and the respective <code>setf</code> method).<ref name="defclass">{{cite web |url=http://www.lispworks.com/documentation/HyperSpec/Body/m_defcla.htm |title=CLHS: Macro DEFCLASS |accessdate=2011-03-29}}</ref> Slots are always directly accessible through their names with the use of <code>with-slots</code> and <code>slot-value</code>, and the slot accessor options define specialized methods that use <code>slot-value</code>.<ref name="defclass-accessors">{{cite web |url=http://www.lispworks.com/documentation/HyperSpec/Body/07_eb.htm |title=CLHS: 7.5.2 Accessing Slots |accessdate=2011-03-29}}</ref>

CLOS itself has no notion of properties, although the [[Common Lisp Object System#Metaobject Protocol|MetaObject Protocol]] extension specifies means to access a slot's reader and writer function names, including the ones generated with the <code>:accessor</code> option.<ref name="mop-slot-definitions">{{cite web |url=http://www.lisp.org/mop/concepts.html#slot-definitions |title=MOP: Slot Definitions |accessdate=2011-03-29}}</ref>

The following example shows a definition of a student class using these slot options and direct slot access:
<syntaxhighlight lang="lisp">
(defclass student ()
  ((name      :initarg :name      :initform "" :accessor student-name) ; student-name is setf'able
   (birthdate :initarg :birthdate :initform 0  :reader student-birthdate)
   (number    :initarg :number    :initform 0  :reader student-number :writer set-student-number)))

;; Example of a calculated property getter (this is simply a method)
(defmethod student-age ((self student))
  (- (get-universal-time) (student-birthdate self)))

;; Example of direct slot access within a calculated property setter
(defmethod (setf student-age) (new-age (self student))
  (with-slots (birthdate) self
    (setf birthdate (- (get-universal-time) new-age))
    new-age))

;; The slot accessing options generate methods, thus allowing further method definitions
(defmethod set-student-number :before (new-number (self student))
  ;; You could also check if a student with the new-number already exists.
  (check-type new-number (integer 1 *)))
</syntaxhighlight>