如何修改Common Lisp中的函数绑定？

Question

以下是您可以在Scheme中执行的操作:

> (define (sum lst acc)
    (if (null? lst)
        acc
        (sum (cdr lst) (+ acc (car lst)))))
> (define sum-original sum)
> (define (sum-debug lst acc)
    (print lst)
    (print acc)
    (sum-original lst acc))
> (sum '(1 2 3) 0)
6
> (set! sum sum-debug)
> (sum '(1 2 3) 0)
(1 2 3)
0
(2 3)
1
(3)
3
()
6
6
> (set! sum sum-original)
> (sum '(1 2 3) 0)
6

如果我在Common Lisp中执行以下操作:

> (defun sum (lst acc)
    (if lst
        (sum (cdr lst) (+ acc (car lst)))
        acc))
SUM
> (defvar sum-original #'sum)
SUM-ORIGINAL
> (defun sum-debug (lst acc)
    (print lst)
    (print acc)
    (funcall sum-original lst acc))
SUM-DEBUG
> (sum '(1 2 3) 0)
6

现在我该如何做这样的事情(setf sum #'sum-debug)会改变定义的函数的绑定defun？

Answer 1

因为Common Lisp具有不同的函数名称空间,所以您需要使用symbol-function或fdefinition.

CL-USER> (defun foo (a)
           (+ 2 a))
FOO
CL-USER> (defun debug-foo (a)
           (format t " DEBUGGING FOO: ~a" a)
           (+ 2 a))
DEBUG-FOO
CL-USER> (defun debug-foo-again (a)
           (format t " DEBUGGING ANOTHER FOO: ~a" a)
           (+ 2 a))
DEBUG-FOO-AGAIN
CL-USER> (foo 4)
6
CL-USER> (setf (symbol-function 'foo) #'debug-foo)
#<FUNCTION DEBUG-FOO>
CL-USER> (foo 4)
 DEBUGGING FOO: 4
6
CL-USER> (setf (fdefinition 'foo) #'debug-foo-again)
#<FUNCTION DEBUG-FOO-AGAIN>
CL-USER> (foo 4)
 DEBUGGING ANOTHER FOO: 4
6
CL-USER>

Answer 2

以这种方式重新定义功能的典型用例是在调试期间.你的例子表明了.Common Lisp已经为此提供了更高级别的机制:TRACE.它设置了符号的功能值,但它提供了更方便的用户界面.通常类似的东西TRACE会有很多非标准的选择.

跟踪功能

以下示例使用Clozure Common Lisp,它总是编译代码:

? (defun sum (list accumulator)
    (declare (optimize debug))   ; note the debug declaration
    (if (endp list)
        accumulator
        (sum (rest list) (+ accumulator (first list)))))
SUM
? (sum '(1 2 3 4) 0)
10
? (trace sum)
NIL
? (sum '(1 2 3 4) 0)
0> Calling (SUM (1 2 3 4) 0) 
 1> Calling (SUM (2 3 4) 1) 
  2> Calling (SUM (3 4) 3) 
   3> Calling (SUM (4) 6) 
    4> Calling (SUM NIL 10) 
    <4 SUM returned 10
   <3 SUM returned 10
  <2 SUM returned 10
 <1 SUM returned 10
<0 SUM returned 10
10

然后去解雇:

? (untrace sum)

建议功能

在您的示例中,您已经打印了有关输入函数的参数.在许多Common Lisp实现中,还有另一种机制可以通过增加功能来增强功能:建议.再次使用Clozure Common Lisp及其advise宏:

? (advise sum                                 ; the name of the function
          (format t "~%Arglist: ~a" arglist)  ; the code
          :when :before)                      ; where to add the code
#<Compiled-function (CCL::ADVISED 'SUM) (Non-Global)  #x302000D7AC6F>

? (sum '(1 2 3 4) 0)

Arglist: ((1 2 3 4) 0)
Arglist: ((2 3 4) 1)
Arglist: ((3 4) 3)
Arglist: ((4) 6)
Arglist: (NIL 10)
10