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  ANSI Common Lisp   7 Objects   7.7 Dictionary of Objects

7.7.24 with-slots Macro

with-slots ({slot-entry}*) instance-form {declaration}* {form}*

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.

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)))
 (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))
   (with-slots ((x2 x) (y2 y))
     (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 2 2 7 7 8 8) 
     (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:
with-accessors, slot-value, symbol-macrolet

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 Qi is

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

if slot-entryi is a symbol and is

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

if slot-entryi is of the form

(variable-namei slot-namei)

Allegro CL Implementation Details:

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