with-accessors (ANSI and GNU Common Lisp Document)

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#### 7.7.23 with-accessors [Macro]

`with-accessors` ({slot-entry}*) instance-form {declaration}* {form}*
⇒ {result}*

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

#### Arguments and Values::

variable-name—a variable name; not evaluated.

accessor-name—a function name; not evaluated.

instance-form—a form; evaluated.

declaration—a declare expression; not evaluated.

forms—an implicit progn.

results—the values returned by the forms.

#### Description::

Creates a lexical environment in which the slots specified by slot-entry are lexically available through their accessors as if they were variables. The macro with-accessors invokes the appropriate accessors to access the slots specified by slot-entry. Both setf and setq can be used to set the value of the slot.

#### 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 thing1 (make-instance 'thing :x 1 :y 2)) ⇒  #<THING 43135676>
     (setq thing2 (make-instance 'thing :x 7 :y 8)) ⇒  #<THING 43147374>
     (with-accessors ((x1 thing-x) (y1 thing-y))
                     thing1
       (with-accessors ((x2 thing-x) (y2 thing-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 1 to 9 in #<THING 43135676>.
     |>  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 accessor-name is not the name of an accessor for the instance.

#### See Also::

with-slots , symbol-macrolet

#### Notes::

A with-accessors expression of the form:

    (with-accessors (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

    (variable-name_i ()
    (accessor-name_i in))

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