with-slots

slot-name — a slot name; not evaluated.

variable-name — a variable name; not evaluated.

instance-form — a form; evaluted to produce instance.

instance — an object.

declaration — a declare expression; not evaluated.

forms — an implicit progn.

results — the values returned by the forms.

The macro with-slots establishes a lexical environment for referring to the slots in the instance named by the given slot-names as though they were variables. Within such a context the value of the slot can be specified by using its slot name, as if it were a lexically bound variable. Both setf and setq can be used to set the value of the slot.

The macro with-slots translates an appearance of the slot name as a variable into a call to slot-value.

 (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 thing (make-instance 'thing :x 0 :y 1)) → #<THING 62310540> 
 (with-slots (x y) thing (incf x) (incf y)) → 2 
 (values (thing-x thing) (thing-y thing)) → 1, 2 
 (setq thing1 (make-instance 'thing :x 1 :y 2)) → #<THING 43135676> 
 (setq thing2 (make-instance 'thing :x 7 :y 8)) → #<THING 43147374> 
 (with-slots ((x1 x) (y1 y)) 
             thing1 
   (with-slots ((x2 x) (y2 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 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))

defclass

The consequences are undefined if any slot-name is not the name of a slot in the instance.

with-accessors, slot-value, symbol-macrolet

A with-slots expression of the form:

expands into the equivalent of

where Q_i is

if slot-entry_i is a symbol and is

if slot-entry_i is of the form