with-accessors

with-accessors Macro

Syntax:

with-accessors ({slot-entry}*) instance-form {declaration}* {form}*

→ {result}*

slot-entry::=(variable-name accessor-name)

Arguments and Values:

variable-name—a variable name; not evaluated.

accessor-name—a function name; not evaluated.

instance-form—a form; evaluated.

declaration—a declare expression; not evaluated.

forms—an implicit progn.

results—the values returned by the forms.

Description:

Creates a lexical environment in which the slots specified by slot-entry are lexically available through their accessors as if they were variables. The macro with-accessors invokes the appropriate accessors to access the slots specified by slot-entry. Both setf and setq can be used to set the value of the slot.

with-accessors

Examples:

(defclass thing () 
  ((x :initarg :x :accessor thing-x) 
   (y :initarg :y :accessor thing-y))) 
→ #<STANDARD-CLASS THING 250020173> 
(defmethod (setf thing-x) :before (new-x (thing thing)) 
  (format t "~&Changing X from ~D to ~D in ~S.~%" 
	  (thing-x thing) new-x thing)) 
(setq thing1 (make-instance ’thing :x 1 :y 2)) → #<THING 43135676> 
(setq thing2 (make-instance ’thing :x 7 :y 8)) → #<THING 43147374> 
(with-accessors ((x1 thing-x) (y1 thing-y)) 
    thing1 
  (with-accessors ((x2 thing-x) (y2 thing-y)) 
      thing2 
    (list (list x1 (thing-x thing1) y1 (thing-y thing1) 
		x2 (thing-x thing2) y2 (thing-y thing2)) 
	  (setq x1 (+ y1 x2)) 
	  (list x1 (thing-x thing1) y1 (thing-y thing1) 
		x2 (thing-x thing2) y2 (thing-y thing2)) 
	  (setf (thing-x thing2) (list x1)) 
	  (list x1 (thing-x thing1) y1 (thing-y thing1) 
		x2 (thing-x thing2) y2 (thing-y thing2))))) 
▷ Changing X from 1 to 9 in #<THING 43135676>. 
▷ Changing X from 7 to (9) in #<THING 43147374>. 
→ ((1 1 2 2 7 7 8 8) 
   9 
   (9 9 2 2 7 7 8 8) 
   (9) 
   (9 9 2 2 (9) (9) 8 8)) 

Affected By:

defclass

Exceptional Situations:

The consequences are undefined if any accessor-name is not the name of an accessor for the instance.

See Also:

with-slots, symbol-macrolet

Notes:

A with-accessors expression of the form:

(with-accessors (slot-entry₁. . . slot-entry_n) instance-form form₁. . . form_k)

expands into the equivalent of

(let ((in instance-form))

(symbol-macrolet (Q₁. . . Q_n) form₁. . . form_k))

where Q_iis

(variable-name_i () (accessor-name_i in))

Expanded Reference: with-accessors

Basic Usage

with-accessors is similar to with-slots, but references go through accessor generic functions instead of slot-value. Each entry is of the form (variable-name accessor-name).

(defclass person ()
  ((name :initarg :name :accessor person-name)
   (age  :initarg :age  :accessor person-age)))

(let ((p (make-instance 'person :name "Alice" :age 30)))
  (with-accessors ((n person-name) (a person-age)) p
    (list n a)))
=> ("Alice" 30)

Modifying Through Accessors

Writes through with-accessors call the setf accessor, which means :before and :after methods on the writer are invoked.

(defclass person ()
  ((name :initarg :name :accessor person-name)
   (age  :initarg :age  :accessor person-age)))

(let ((p (make-instance 'person :name "Bob" :age 25)))
  (with-accessors ((n person-name) (a person-age)) p
    (setf n "Robert")
    (incf a 5))
  (list (person-name p) (person-age p)))
=> ("Robert" 30)

Difference from with-slots

The key difference is that with-accessors invokes accessor methods (which may have :before/:after methods), while with-slots goes directly through slot-value.

(defclass tracked ()
  ((value :initarg :value :accessor tracked-value)))

(defmethod (setf tracked-value) :before (new-val (obj tracked))
  (format t "Setting value to ~A~%" new-val))

;; with-accessors triggers the :before method
(let ((obj (make-instance 'tracked :value 0)))
  (with-accessors ((v tracked-value)) obj
    (setf v 42)))
.. Setting value to 42
..
=> 42

;; with-slots bypasses accessor methods
(let ((obj (make-instance 'tracked :value 0)))
  (with-slots (value) obj
    (setf value 42)))
=> 42

Working with Multiple Objects

Like with-slots, with-accessors forms can be nested to work with multiple objects simultaneously.

(defclass point ()
  ((x :initarg :x :accessor point-x)
   (y :initarg :y :accessor point-y)))

(defun swap-points (p1 p2)
  "Swap the coordinates of two points."
  (with-accessors ((x1 point-x) (y1 point-y)) p1
    (with-accessors ((x2 point-x) (y2 point-y)) p2
      (psetf x1 x2 x2 x1
             y1 y2 y2 y1))))

(let ((a (make-instance 'point :x 1 :y 2))
      (b (make-instance 'point :x 3 :y 4)))
  (swap-points a b)
  (list (point-x a) (point-y a) (point-x b) (point-y b)))
=> (3 4 1 2)