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

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(with-accessors )

with-accessors

Expanded Reference: with-accessors​

Expanded Reference: with-accessors