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| ANSI Common Lisp 7 Objects 7.7 Dictionary of Objects
7.7.24 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 ()
                 ((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: defclass 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 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 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: None.

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