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Macro WITH-SLOTS

Syntax

  • with-slots (slot-entry*) instance-form declaration* form*result*

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

Arguments and Values

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 () ((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)) (defparameter 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

(defparameter thing1 (make-instance 'thing :x 1 :y 2))

#<THING 43135676>

(defparameter 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)) (setf 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

Exceptional Situations

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

See Also

Notes

A with-slots expression of the form:

(with-slots (slot-entry1 ... slot-entryn) instance-form form1 ... formn

expands into the equivalent of:

(let ((in instance-form)) (symbol-macrolet (Q1 ... Qn) form1 ... formk))

where Qi, if slot-entryi is a symbol, is:

(slot-entryi () (slot-value in 'slot-entryi))

If slot-entryi is of the form (variable-namei slot-namei)), then Qi is:

(variable-namei () (slot-value in 'slot-namei))

\issue{DECLS-AND-DOC}