lisp:如何在作用域内创建临时方法专门化

Question

在Common lisp中:重新定义范围内的现有函数？OP要求类似的东西.但是我想创建一个方法专用器,而不是一个函数.基本上假设一个方法定义如下:

defmethod my-meth ((objA classA) (objB classB)) (...)

我想做的是(伪代码):

(labels ((my-meth ((objA classA) (objB (eql some-object)))))
   do stuff calling my-meth with the object...)

真正的用途是我想创建一个临时环境,setf slot-value-using-class专门用于eql创建特定对象的插槽写入的按需拦截.(目的是记录旧的和新的槽值,然后调用next方法.)我不想创建元类,因为我可能想拦截已经实例化的标准对象.

当然我尝试了它并且它没有用(因为你是怎么做DEFMETHOD的LABELS？)但我想要一些更有经验的人来验证它是不可行的和/或提出合适的方式.

评论？

编辑:

Daniel和Terje提供了很好的链接,可以扩展我对可能性的认识,但是我想在去那里之前更多地寻求更加普遍的方法.我一直在研究在进入环境时做一个add-method,它将专注于eql,并在退出时执行remove-method.我还没完呢.如果有人玩过那些,评论会很好.将使线程保持最新.

编辑2:我接近使用add-method场景,但是有一个问题.这是我尝试过的:

    (defun inject-slot-write-interceptor (object fun)
    (let* ((gf (fdefinition '(setf sb-mop:slot-value-using-class)))
            (mc (sb-mop:generic-function-method-class gf))
            (mc-instance (make-instance (class-name mc) 
                            :qualifiers '(:after)
                            :specializers (list (find-class 't)
                                                (find-class 'SB-PCL::STD-CLASS)
                                                (sb-mop::intern-eql-specializer object) 
                                                (find-class 'SB-MOP:STANDARD-EFFECTIVE-SLOT-DEFINITION))
                           :lambda-list '(new-value class object slot)
                           :function (compile nil (lambda (new-value class object slot) (funcall fun new-value class object slot))))))
         (add-method gf mc-instance)
         (defun remove-slot-write-interceptor ()
            (remove-method gf mc-instance))
       ))

(defun my-test (object slot-name data)
     (let ((test-data "No results yet") 
           (gf (fdefinition '(setf sb-mop::slot-value-using-class))))
       (labels ((show-applicable-methods () (format t "~%Applicable methods: ~a" (length (sb-mop:compute-applicable-methods gf (list data (class-of object) object (slot-def-from-name (class-of object) slot-name)))))))
            (format t "~%Starting test: ~a" test-data)
            (show-applicable-methods)
            (format t "~%Injecting interceptor.")
            (inject-slot-write-interceptor object (compile nil (lambda (a b c d) (setf test-data "SUCCESS !!!!!!!"))))
            (show-applicable-methods)
            (format t "~%About to write slot.")
            (setf (slot-value object slot-name) data)
            (format t "~%Wrote slot: ~a" test-data)
            (remove-slot-write-interceptor)
            (format t "~%Removed interceptor.")
            (show-applicable-methods)
       )))      

使用一些对象槽和数据作为args调用(my-test)会导致:

Starting test: No results yet 
Applicable methods: 1 
Injecting interceptor. 
Applicable methods: 2 
About to write slot.     
Wrote slot: No results yet  <----- Expecting SUCCESS here....
Removed interceptor. 
Applicable methods: 1

所以我被困在这里.专业化工作,因为适用的方法现在包括eql-specialized:after方法,但不幸的是它似乎没有被调用.任何人都可以帮忙,所以我可以完成它并重构为一个甜蜜的小实用宏？

Answer 1

不,您无法在Common Lisp中定义动态范围或词法范围的专用方法.

面向方面编程可以用作解决潜在问题的方法.另请参见面向上下文的编程.

ContextL是一个为Common Lisp/CLOS提供面向方面/上下文的扩展的库.

轻量级替代方法是使用特殊/动态变量来指示方法何时应该进行日志记录:

(defparameter *logging* NIL "Bind to a true value to activate logging")

(defmethod my-meth :around ((objA classA) (objB (eql some-object)))
  (prog2 
   (when *logging*
     (logging "Enter my-meth"))
   (call-next-method)
   (when *logging*
     (logging "Exit my-meth"))))

(let ((*logging* T))
   (do stuff calling my-meth with the object...))

请注意,禁用日志记录时也会调用:around方法.