common-lisp - 从泛型方法返回实例列表

Question

我需要在通用方法中返回矩形的坐标列表。坐标是类“购物车”实例。

我尝试用 make-instance 返回它

(defclass line ()
  ((start :initarg :start :accessor line-start)
   (end   :initarg :end   :accessor line-end)))

(defmethod print-object ((lin line) stream)
  (format stream "[LINE ~s ~s]"
          (line-start lin) (line-end lin)))

(defclass cart ()
  ((x :initarg :x :reader cart-x)
   (y :initarg :y :reader cart-y)))

(defmethod print-object ((c cart) stream)
  (format stream "[CART x ~d y ~d]"
          (cart-x c) (cart-y c)))

(setq lin (make-instance 'line
             :start (make-instance 'cart :x 4 :y 3)
             :end (make-instance 'cart :x 7 :y 5)))

(defgeneric containing-rect (shape))

(defmethod containing-rect ((l line))
  (let ((x1 (cart-x (line-start l)))
        (y1 (cart-y (line-start l)))  
        (x2 (cart-x (line-end l)))
        (y2 (cart-y (line-end l))))
    (cond ((= x1 x2) 
           '((make-instance 'cart :x (1- x1) :y y1)
             (make-instance 'cart :x (1+ x1) :y y1)
             (make-instance 'cart :x (1- x2) :y y2)
             (make-instance 'cart :x (1+ x2) :y y2)))
          ((= y1 y2)
           '((make-instance 'cart :x x1 :y (1- y1))
             (make-instance 'cart :x x1 :y (1+ y1))
             (make-instance 'cart :x x2 :y (1- y2))
             (make-instance 'cart :x x2 :y (1+ y2))))
          (t 
           (rect '((make-instance 'cart :x x1 :y y1)
                   (make-instance 'cart :x x1 :y y2)
                   (make-instance 'cart :x x2 :y y2)
                   (make-instance 'cart :x x2 :y y1)))))))

(print (containing-rect lin))

我想 make-instance 应该为某物分配一个实例

所以我得到了错误的结果

((MAKE-INSTANCE 'CART :X X1 :Y Y1) (MAKE-INSTANCE 'CART :X X1 :Y Y2)
 (MAKE-INSTANCE 'CART :X X2 :Y Y2) (MAKE-INSTANCE 'CART :X X2 :Y Y1)) 

但我需要这样的输出

([CART x 4 y 3] [CART x 4 y 3] [CART x 4 y 3] [CART x 4 y 3])

Answer 1

如果您引用 某些东西，它不会被评估。

这个:

'((make-instance 'cart :x (1- x1) :y y1)
  (make-instance 'cart :x (1+ x1) :y y1)
  (make-instance 'cart :x (1- x2) :y y2)
  (make-instance 'cart :x (1+ x2) :y y2))

是一个包含四个文字列表的文字列表，每个列表都以符号MAKE-INSTANCE开头，然后有一个列表(QUOTE CART) 等。这正是您看到的结果。

您似乎想实际评估一下。最简单的事情就是这样做并列一个 list :

(list (make-instance 'cart :x (1- x1) :y y1)
      (make-instance 'cart :x (1+ x1) :y y1)
      (make-instance 'cart :x (1- x2) :y y2)
      (make-instance 'cart :x (1+ x2) :y y2))

这与引用某物有根本不同。