macros - 嵌套在宏内的 let 中的 if 子句未按预期工作

Question

好吧，首先这是我的第一个问题，所以我对任何不好的做法表示歉意，并且如果您告诉我我做错了什么，我会很感激。

我正在尝试编写一个宏来减少重复代码，即在 Common Lisp 中创建一个包、系统或代码文件，并在其名称中包含章节号。下面的代码是我所拥有的，当 :chapter-number 作为字符串传递时它工作得很好，但当它作为数字传递时会出错:

(defmacro with-open-chapter-file
  ((streamvar (component &key
                         (type "lisp")
                         (directory (sb-posix:getcwd))
                         chapter-number))
   (&body body))
  `(let ((chapter-number ,(if (numberp chapter-number) ; the problem is at this if clause.
                              (write-to-string chapter-number) ; My intention was to convert it to a string if it was a number or leave it as is otherwise.
                            chapter-number)))
     (with-open-file (,streamvar (make-pathname
                                  :name ,(if chapter-number ; the variable manipulated in the if clause is used in this expression
                                             (concatenate 'string "chapter-" chapter-number "-" (string component)) 
                                           component)
                                  :type ,type
                                  :defaults ,directory)
                                 :direction :output)
       ,body)))

当我运行以下测试时:

(macroexpand-1 '(with-open-chapter-file (out ("pack" :chapter-number 10))
                   (format t "Hey!")))

我收到错误:

The value
  10
is not of type
  SEQUENCE
   [Condition of type TYPE-ERROR]

以及回溯:

  0: (LENGTH 10)
  1: (SB-KERNEL:%CONCATENATE-TO-STRING "chapter-" 10 "-" "pack")
  2: ((MACRO-FUNCTION WITH-OPEN-CHAPTER-FILE) (WITH-OPEN-CHAPTER-FILE (OUT ("pack" :CHAPTER-NUMBER 10)) (FORMAT T "Hey!")) #<unused argument>)
  3: ((FLET SB-IMPL::PERFORM-EXPANSION :IN MACROEXPAND-1) #<FUNCTION (MACRO-FUNCTION WITH-OPEN-CHAPTER-FILE) {2278173B}> NIL)
  4: (SB-INT:SIMPLE-EVAL-IN-LEXENV (MACROEXPAND-1 (QUOTE (WITH-OPEN-CHAPTER-FILE # #))) #<NULL-LEXENV>)
  5: (EVAL (MACROEXPAND-1 (QUOTE (WITH-OPEN-CHAPTER-FILE # #))))

如果你们能帮助我，我将非常感激。

Answer 1

在代码中:

  :name ,(if chapter-number ; the variable manipulated in the if clause is used in this expression
             (concatenate 'string "chapter-" chapter-number "-" (string component)) 
           component)

您正在使用宏的 chapter-number 参数，而不是在扩展中使用 let 绑定(bind)的变量，因为此代码位于逗号之后。

您不应该在扩展中绑定(bind)该变量，您应该只更新宏本身中的变量。

(defmacro with-open-chapter-file
  ((streamvar (component &key
                         (type "lisp") (directory (sb-posix:getcwd)) chapter-number))
   (&body body))
  (when (numberp chapter-number)
    (setq chapter-number (write-to-string chapter-number)))
  `(with-open-file (,streamvar (make-pathname
                                :name ,(if chapter-number
                                           (concatenate 'string "chapter-" chapter-number "-" (string component)) 
                                         component)
                                :type ,type
                                :defaults ,directory)
                                :direction :output)
     ,@body))

另一个不需要测试 chapter-number 类型的解决方案是将使用 concatenate 的代码更改为使用 format:

(if chapter-number
    (format nil "chapter-%A-%A" chapter-number component)
    component)

一个不相关的错误是您应该使用 ,@body 来替换正文，因为它是一个必须拼接到表达式中的列表。