;;; Model
(defclass model ()
  ((data :accessor model-data :initarg :data :initform nil)))


;;; View
(defclass view ()
  ((model :accessor view-model :initarg :model :initform nil)))

(defgeneric show (obj))

(defmethod show ((v view))
  (print (model-data (view-model v))))


;;; Controller
(defclass controller ()
  ((model :accessor controller-model :initarg :model :initform nil)
   (view  :accessor controller-view  :initarg :view  :initform nil)))

(defmethod show ((c controller))
  (show (controller-view c)))

(defgeneric init (obj x))

(defmethod init ((c controller) x)
  (setf (model-data (controller-model c)) x)
  (show c))

(defgeneric inc (obj &optional x))

(defmethod inc ((c controller) &optional (x 1))
  (incf (model-data (controller-model c)) x)
  (show c))


;;; Demo
(let* ((model-1 (make-instance 'model :data 0))
       (view-1  (make-instance 'view :model model-1))
       (ctl-1   (make-instance 'controller :model model-1 :view view-1)))
  (show ctl-1)     ; => prints 0
  (inc  ctl-1)     ; => prints 1
  (inc  ctl-1)     ; => prints 2
  (init ctl-1 0)   ; => prints 0
  (inc  ctl-1 0)   ; => prints 0
  (inc  ctl-1 3))  ; => prints 3