动态定义 setf 扩展器

Question

我正在尝试定义一个宏，它将采用结构的名称、键和结构中的哈希表的名称，并定义函数来访问和修改哈希中键下的值。

(defmacro make-hash-accessor (struct-name key hash)
  (let ((key-accessor  (gensym))
        (hash-accessor (gensym)))
    `(let ((,key-accessor  (accessor-name ,struct-name ,key))
           (,hash-accessor (accessor-name ,struct-name ,hash)))
       (setf (fdefinition ,key-accessor) ; reads
             (lambda (instance)
               (gethash ',key
                (funcall ,hash-accessor instance))))
       (setf (fdefinition '(setf ,key-accessor)) ; modifies
             (lambda (instance to-value)
               (setf (gethash ',key
                      (funcall ,hash-accessor instance))
                 to-value))))))

;; Returns the symbol that would be the name of an accessor for a struct's slot
(defmacro accessor-name (struct-name slot)
  `(intern
    (concatenate 'string (symbol-name ',struct-name) "-" (symbol-name ',slot))))

为了测试这个我有：

(defstruct tester
  (hash (make-hash-table)))

(defvar too (make-tester))
(setf (gethash 'x (tester-hash too)) 3)

当我跑步时

(make-hash-accessor tester x hash)

然后

(tester-x too)

它应该返回3 T，但是

(setf (tester-x too) 5)

给出错误：

The function (COMMON-LISP:SETF COMMON-LISP-USER::TESTER-X) is undefined.
   [Condition of type UNDEFINED-FUNCTION]

(macroexpand-1 '(make-hash-accessor tester x hash)) 扩展为

(LET ((#:G690 (ACCESSOR-NAME TESTER X)) (#:G691 (ACCESSOR-NAME TESTER HASH)))
  (SETF (FDEFINITION #:G690)
        (LAMBDA (INSTANCE) (GETHASH 'X (FUNCALL #:G691 INSTANCE))))
  (SETF (FDEFINITION '(SETF #:G690))
        (LAMBDA (INSTANCE TO-VALUE)
          (SETF (GETHASH 'X (FUNCALL #:G691 INSTANCE)) TO-VALUE))))
T

我正在使用 SBCL。我做错了什么？

Answer 1

您应该尽可能使用defun。 具体来说，这里用 defmacro 代替 accessor-name 代替 (setf fdefinition) 代替您的访问器：

(defmacro define-hash-accessor (struct-name key hash)
  (flet ((concat-symbols (s1 s2)
           (intern (concatenate 'string (symbol-name s1) "-" (symbol-name s2)))))
    (let ((hash-key (concat-symbols struct-name key))
          (get-hash (concat-symbols struct-name hash)))
      `(progn
         (defun ,hash-key (instance)
           (gethash ',key (,get-hash instance)))
         (defun (setf ,hash-key) (to-value instance)
           (setf (gethash ',key (,get-hash instance)) to-value))
         ',hash-key))))
(defstruct tester
  (hash (make-hash-table)))
(defvar too (make-tester))
(setf (gethash 'x (tester-hash too)) 3)
too
==> #S(TESTER :HASH #S(HASH-TABLE :TEST FASTHASH-EQL (X . 3)))
(define-hash-accessor tester x hash)
==> tester-x
(tester-x too)
==> 7; T
(setf (tester-x too) 5)
too
==> #S(TESTER :HASH #S(HASH-TABLE :TEST FASTHASH-EQL (X . 5)))

请注意，我为宏使用了一个更传统的名称：因为它定义访问器，所以通常将其命名为define-...（参见define-condition、defpackage）。 make-... 通常用于函数返回对象（参见make-package）。

另见Is defun or setf preferred for creating function definitions in common lisp and why? 请记住，样式很重要，无论是缩进还是命名变量、函数和宏。