在宏定义宏中引用

Question

我正试图编写一个宏，为数组结构的数据结构定义一些助手(基于此片段(。在这个宏中，我定义了另一个宏，它有助于遍历struct中的所有slot值。问题是，我无法使双引号正常工作。这是代码：

(defmacro defcomponent (name-and-options &body slots)
(setf name-and-options (ensure-list name-and-options))
(let ((struct (first name-and-options))
(slot-names (iter (for s in slots)
(collecting
(ematch s
((or (and name (symbol)
(<> _ '*)
(<> _ nil))
(list* name _ (plist :type _ :read-only _)))
name))))))
`(progn (defstruct ,name-and-options
;; some task-specific stuff omitted here
)
(defmacro ,(symbolicate 'with- struct) (components &rest body)
`(loop
,@',(iter (for s in slot-names)
(appending `(for ,s across (,(symbolicate struct '- s) components))))
do ,@body)))))

例如(defcomponent buzz x y)宏扩展到

(PROGN
(DEFSTRUCT (BUZZ)
X Y)  ;; details omitted
(DEFMACRO WITH-BUZZ (COMPONENTS &REST BODY)
`(LOOP ,@'(FOR X ACROSS (BUZZ-X COMPONENTS) FOR Y ACROSS (BUZZ-Y COMPONENTS))
DO ,@BODY))

这有点有效，但我想访问内部with-buzz宏的components参数，即类似于的参数

(DEFMACRO WITH-BUZZ (COMPONENTS &REST BODY)
`(LOOP FOR X ACROSS (BUZZ-X ,COMPONENTS) FOR Y ACROSS (BUZZ-Y ,COMPONENTS)
DO ,@BODY))

我怎么可能做到这一点？我尝试了很多,和,@的组合，但都没有成功。

Answer 1

有时不使用后引号模式会有所帮助。然后，在编译器的帮助下，范围问题可以更容易地理解，编译器会警告常见的变量范围问题。

作为一个稍微简化的练习，我们将编写一个生成代码的函数。生成的代码是一个宏定义，它本身生成代码。

(defun makeit (name slots)
(labels ((symbolicate (pattern &rest things)
(intern (apply #'format nil pattern things)))
(compute-for-clauses (slots)
(loop for s in slots
append (list ''for (list 'quote s)
''across (list 'list
(list 'quote
(symbolicate "~a-~a" name s))
'components)))))
(list 'progn
(list 'defmacro
(symbolicate "WITH-~a" name)
'(components &rest body)
(append '(list* 'loop)
(compute-for-clauses slots)
(list ''do 'body))))))

示例

CL-USER 51 > (pprint (makeit 'buzz '(x y)))
(PROGN
(DEFMACRO WITH-BUZZ (COMPONENTS &REST BODY)
(LIST* 'LOOP
'FOR
'X
'ACROSS
(LIST 'BUZZ-X COMPONENTS)
'FOR
'Y
'ACROSS
(LIST 'BUZZ-Y COMPONENTS)
'DO
BODY)))
CL-USER 52 > (eval *)
NIL
CL-USER 53 > (macroexpand-1 '(with-buzz a (+ 12) (+ 30)))
(LOOP FOR X ACROSS (BUZZ-X A) FOR Y ACROSS (BUZZ-Y A) DO (+ 12) (+ 30))
T

Answer 2

好吧，我已经通过手动列表构建+eval而不是准引用来实现了，但上帝的母亲，它看起来很沙哑。

;; skip
(defmacro ,(symbolicate 'with- struct) (components &rest body)
(append
'(loop)
(eval
`(iter (for s in ',',slot-names)
(appending `(for ,s across (,(symbolicate ',',struct '- ,'s) ,,components)))))
'(do)
body))

我很乐意接受任何其他更习惯地解决问题的答案。