Lisp中如何通过宏遍历类插槽？遇插槽缺失错误求解

先看正常工作的代码：定义了test-class类和test方法，用print-slot宏可以直接打印指定插槽的值：

(defclass test-class ()
  ((data :initarg :data)
   (other :initarg :other)))

(defmethod test ((tc test-class) &key)
  (macrolet ((print-slot (slot)
               `(with-slots (,slot) tc
                  (format t "slot value: ~a~%" ,slot))))
    (print-slot data)
    (print-slot other)))

调用后输出正常：

CL-USER> (test (make-instance 'test-class :data "data" :other "other"))
slot value: data
slot value: other

但改成用dolist遍历插槽列表调用宏时，会触发SB-PCL::MISSING-SLOT错误：

(defmethod test ((tc test-class) &key)
  (macrolet ((print-slot (slot)
               `(with-slots (,slot) tc
                  (format t "slot value: ~a~%" ,slot))))
    (dolist (slot '(data other))
      (print-slot slot))))

错误信息：

CL-USER> (test (make-instance 'test-class :data "data" :other "other"))
When attempting to read the slot's value (slot-value), the slot
SLOT is missing from the object #<TEST-CLASS {1001E3FFB3}>.
   [Condition of type SB-PCL::MISSING-SLOT]

错误原因 #

宏是编译时展开的，不是运行时执行的。当你在dolist里写(print-slot slot)，宏展开阶段会把slot当成一个插槽名符号，生成的代码是(with-slots (slot) tc ...)——也就是尝试访问对象里名为SLOT的插槽，而不是变量slot的值对应的插槽。你的类里根本没有叫SLOT的插槽，所以报错。

解决方法 #

方法1：用函数代替宏（推荐，适合运行时动态遍历） #

把print-slot改成函数，用slot-value在运行时动态访问插槽，这样就能接收变量形式的插槽名：

(defclass test-class ()
  ((data :initarg :data)
   (other :initarg :other)))

(defmethod test ((tc test-class) &key)
  (flet ((print-slot (slot)
           (format t "slot value: ~a~%" (slot-value tc slot))))
    (dolist (slot '(data other))
      (print-slot slot))))

调用后就能正常遍历打印所有插槽的值。

方法2：用宏生成编译时的遍历代码（适合固定插槽列表） #

如果一定要用宏实现，可以定义一个print-slots宏，让它在编译时就把插槽列表展开成多个print-slot调用，这样每个插槽名都是编译时确定的：

(defclass test-class ()
  ((data :initarg :data)
   (other :initarg :other)))

(defmethod test ((tc test-class) &key)
  (macrolet ((print-slot (slot)
               `(with-slots (,slot) tc
                  (format t "slot value: ~a~%" ,slot)))
             (print-slots (&rest slots)
               `(progn
                  ,@(loop for slot in slots
                          collect `(print-slot ,slot)))))
    (print-slots data other)))

这个宏展开后和最初正常的代码完全一样，编译时就处理了每个插槽名，运行时直接访问对应插槽，不会有动态变量的问题。

内容的提问来源于stack exchange，提问作者John Graham