with-slots ({slot-entry}*)
instance-form
{declaration}* {form}*
⇒ {result}*
| slot-entry | ::= | slot-name | (variable-name slot-name) |
| 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))
The consequences are undefined if any slot-name is not the name of a slot in the instance.
A with-slots expression of the form:
(with-slots (slot-entry_1 ...slot-entry_n) instance-form form_1 ...form_k)
expands into the equivalent of
(let ((in instance-form))
(symbol-macrolet (Q_1... Q_n) form_1 ...form_k))
where Q_i is
(slot-entry_i ()
(slot-value in 'slot-entry_i))
if slot-entry_i is a symbol and is
(variable-name_i ()
(slot-value in 'slot-name_i))
if slot-entry_i is of the form
(variable-name_i
slot-name_i)