gistfile1.txt

(defun direct-slot-defn->initarg (slot-defn)
  (list :name (slot-definition-name slot-defn)
        :readers (slot-definition-readers slot-defn)
        :writers (slot-definition-writers slot-defn)
        :initform (slot-definition-initform slot-defn)
        :initargs (slot-definition-initargs slot-defn)
        :initfunction (slot-definition-initfunction slot-defn)))

(defun add-slot-to-class (class name &key (initform nil) accessors readers writers initargs (initfunction (constantly nil)))
  (check-type class symbol)
  (let ((new-slots (list (list :name name
                               :readers (union accessors readers)
                               :writers (union writers
                                               (mapcar #'(lambda (x)
                                                           (list 'setf x))
                                                       accessors)
                                               :test #'equal)
                               :initform initform
                               :initargs initargs
                               :initfunction initfunction))))
    (dolist (slot-defn (class-direct-slots (find-class class)))
      (push (direct-slot-defn->initarg slot-defn) new-slots))
    (ensure-class class :direct-slots new-slots)))

(defclass foo ()
  ((bar :accessor bar :initform "zzzzzz")
   (baz :accessor baz :initform "zzzzzz")))

(defclass foo ()
  ((bar :accessor bar :initform "zzzzzz")
   (baz :accessor baz :initform "zzzzzz")))

(defmethod slot-missing (class (instance foo) slot-name operation &optional (new-value "defailt value"))
  (declare (ignorable class))
  (print (list class instance slot-name operation new-value))
  ;; (err 'zz)                                                                                                                                                                           
  (add-slot-to-class (class-name class) slot-name)
  (setf (slot-value instance slot-name) new-value))

(defparameter *foo* (make-instance 'foo))

(setf (slot-value *foo* 'bar) "the-bar")

(setf (slot-value *foo* 't2) "zzz")

(defparameter *foo2* (make-instance 'foo))

(slot-value *foo2* 't5)

(slot-value *foo2* 'bar)