;;; http://www.gigamonkeys.com/book/object-reorientation-generic-functions.html

(defpackage #:ultra-game
  (:use :common-lisp)
  (:export :say-my-name :ship))

(in-package :ultra-game)

(defclass point ()
  ((x
    :initform 0
    :accessor point-x)
   (y
    :initform 0
    :accessor point-y)))

(defclass entity (point)
  ((name
    :initarg :name
    :initform (error "Must supply a name")
    :accessor entity-name)))

(defgeneric say-my-name (entity)
  (:documentation "Print entity name to the *standard-output*"))

(defmethod say-my-name ((entity entity))
  (format *standard-output* "My name is ~a~%" (entity-name entity)))


(defclass engine ()
  ((angle
    :initform 0
    :accessor engine-angle)
   (acceleration
    :initform 9000
    :accessor engine-acceleration)))

(defclass ship (entity engine)
  ((cost
   :initform 0)))

(defmethod say-my-name ((ship ship))
  (with-slots (name acceleration) ship
    (format *standard-output* "Colonial spaceship ~a; Acceleration: ~a~%" name acceleration)))

;;; usage example

(in-package :cl-user)

(let ((ship (make-instance 'ultra-game:ship :name "Foo")))
  (ultra-game:say-my-name ship))

;; output:
;; Colonial spaceship Foo; Acceleration: 9000
;; My name is Alarm!


;; note '::'
(let ((entity (make-instance 'ultra-game::entity :name "Alarm!")))
  (ultra-game:say-my-name entity))

;; output:
;; My name is Alarm!