with-slots
with-slots Macro
Syntax:
with-slots ({slot-entry}*) instance-form {declaration}* {form}*
→ {result}*
slot-entry::=slot-name | (variable-name slot-name)
Arguments and Values:
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.
Description:
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.
Examples:
(defclass thing ()
**with-slots**
((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))
Affected By:
defclassExceptional Situations:
The consequences are undefined if any slot-name is not the name of a slot in the instance.
See Also:
with-accessors, slot-value, symbol-macrolet
Notes:
A with-slots expression of the form:
(with-slots (slot-entry1. . . slot-entryn) instance-form form1. . . formk)
expands into the equivalent of
(let ((in instance-form))
(symbol-macrolet (Q1. . . Qn) form1. . . formk))
where Qiis
(slot-entryi() (slot-value in ’slot-entryi))
if slot-entryiis a symbol and is
(variable-namei () (slot-value in ’slot-namei))
if slot-entryiis of the form
(variable-namei slot-namei)
Expanded Reference: with-slots
TODO: Please contribute to this page by adding explanations and examples
(with-slots )