Listing a Directory（列目录）

You can implement the function for listing a single directory, list-directory, as a thin wrapper around the standard function DIRECTORY. DIRECTORY takes a special kind of pathname, called a wild pathname, that has one or more components containing the special value :wild and returns a list of pathnames representing files in the file system that match the wild pathname. The matching algorithm--like most things having to do with the interaction between Lisp and a particular file system--isn't defined by the language standard, but most implementations on Unix and Windows follow the same basic scheme.

你可以将用于列举单独目录的函数 list-directory，实现成标准函数 DIRECTORY 外围的一个包装层。DIRECTORY 接受一种特殊类型的路径名，称为通配路径名，其带有一个或多个含有特殊值 :wild 的组件，然后返回一个路径名的列表，用来表示文件系统中匹配该通配路径名的文件。 匹配算法和多数 Lisp 与一个特定文件系统之间的交互一致，它没有被语言标准定义，但 Unix 与 Windows 上的多数实现遵循了相同的基本模式。

The DIRECTORY function has two problems that you need to address with list-directory. The main one is that certain aspects of its behavior differ fairly significantly between different Common Lisp implementations, even on the same operating system. The other is that while DIRECTORY provides a powerful interface for listing files, to use it properly requires understanding some rather subtle points about the pathname abstraction. Between these subtleties and the idiosyncrasies of different implementations, actually writing portable code that uses DIRECTORY to do something as simple as listing all the files and subdirectories in a single directory can be a frustrating experience. You can deal with those subtleties and idiosyncrasies once and for all, by writing list-directory, and forget them thereafter.

DIRECTORY 函数有两个需要在 list-directory 中解决的问题。其中主要的一个是，即便在相同的操作系统上，其行为的特定方面在不同的 Common Lisp 的实现间也具有相当大的区别。另一个问题在于，尽管 DIRECTORY 提供了一个强大的用于列举文件的接口，但要想正确地使用它需要对路径名抽象有相当细致的理解。在这些不同细微之处和不同实现的特征影响下，实际编写可移植代码来使用 DIRECTORY 一些像列出单个目录中所有文件和子目录这样简单的事情，都可能令人沮丧。你可以通过编写 list-directory 来一次性地处理所有这些细节和特征，并从此忘记它们。

One subtlety I discussed in Chapter 14 is the two ways to represent the name of a directory as a pathname: directory form and file form.

我在第 14 章里讨论过一个细节是，将一个目录的名字表示成路径名有两种方式，即目录形式和文件形式。

To get DIRECTORY to return a list of files in /home/peter/, you need to pass it a wild pathname whose directory component is the directory you want to list and whose name and type components are :wild. Thus, to get a listing of the files in /home/peter/, it might seem you could write this:

为了让 DIRECTORY 返回 /home/peter/ 中的文件列表，需要传给它一个通配路径名，其目录组件是你想要列出的目录，其名称和类型组件是 :wild。因此，为了列出 /home/peter/ 中的文件，需要这样来写：

(directory (make-pathname :name :wild :type :wild :defaults home-dir))

where home-dir is a pathname representing /home/peter/. This would work if home-dir were in directory form. But if it were in file form--for example, if it had been created by parsing the namestring "/home/peter"--then that same expression would list all the files in /home since the name component "peter" would be replaced with :wild.

其中的 home-dir 是代表 /home/peter/ 的路径名。如果 home-dir 是以目录形式表示的，那么上述写法将是有效的。但如果它以文件形式表示，例如，它通过解析名字字符串 "/home/peter" 来创建，那么同样的表达式将列出 /home 中的所有文件，因为名字组件 "peter" 将被替换成 :wild。

To avoid having to worry about explicitly converting between representations, you can define list-directory to accept a nonwild pathname in either form, which it will then convert to the appropriate wild pathname.

为了避免两种形式间的显式转换，可以定义 list-directory 来接受任何形式的非通配路径名，随后它将被转化成适当的通配路径名。

To help with this, you should define a few helper functions. One, component-present-p, will test whether a given component of a pathname is "present," meaning neither NIL nor the special value :unspecific. Another, directory-pathname-p, tests whether a pathname is already in directory form, and the third, pathname-as-directory, converts any pathname to a directory form pathname.

为此，应当定义一些助手函数。其中一个是 component-present-p，它将测试一个路径名的给定组件是否 “存在”，也就是说该组件既不是 NIL 也不是特殊值 :unspecific。 另一个函数是 directory-pathname-p，用来测试一个路径名是否已经是目录形式，而第三个函数 pathname-as-directory 可以将任何路径名转换成目录形式的路径名。

(defun component-present-p (value)
  (and value (not (eql value :unspecific))))

(defun directory-pathname-p  (p)
  (and
   (not (component-present-p (pathname-name p)))
   (not (component-present-p (pathname-type p)))
   p))

(defun pathname-as-directory (name)
  (let ((pathname (pathname name)))
    (when (wild-pathname-p pathname)
      (error "Can't reliably convert wild pathnames."))
    (if (not (directory-pathname-p name))
      (make-pathname
       :directory (append (or (pathname-directory pathname) (list :relative))
                          (list (file-namestring pathname)))
       :name      nil
       :type      nil
       :defaults pathname)
      pathname)))

Now it seems you could generate a wild pathname to pass to DIRECTORY by calling MAKE-PATHNAME with a directory form name returned by pathname-as-directory. Unfortunately, it's not quite that simple, thanks to a quirk in CLISP's implementation of DIRECTORY. In CLISP, DIRECTORY won't return files with no extension unless the type component of the wildcard is NIL rather than :wild. So you can define a function, directory-wildcard, that takes a pathname in either directory or file form and returns a proper wildcard for the given implementation using read-time conditionalization to make a pathname with a :wild type component in all implementations except for CLISP and NIL in CLISP.

现在看起来似乎你可以通过在由 pathname-as-directory 返回的目录形式名字上调用 MAKE-PATHNAME，来生成一个通配路径名并传给 DIRECTORY。不幸的是，事情没有那么简单，多亏了 CLISP 的 DIRECTORY 实现中的一个怪癖。在 CLISP 中，除非通配符中的类型组件是 NIL 而非 :wild，DIRECTORY 将不会返回那些没有扩展名的文件。因此，你可以定义一个函数 directory-wildcard，其接受一个目录形式或文件形式的路径名，并返回一个给定实现下的适当的通配符，它通过使用读取期条件化在除 CLISP 之外的所有实现里生成一个带有 :wild 类型组件的路径名，而在 CLISP 中该类型组件为 NIL。

(defun directory-wildcard (dirname)
  (make-pathname
   :name :wild
   :type #-clisp :wild #+clisp nil
   :defaults (pathname-as-directory dirname)))

Note how each read-time conditional operates at the level of a single expression After #-clisp, the expression :wild is either read or skipped; likewise, after #+clisp, the NIL is read or skipped.

注意每一个读取期条件是怎样在单个表达式层面上操作的。在 #-clisp 之后，表达式 :wild 要么被读取要么被跳过；同样，在 #+clisp 之后，NIL 要么被读取要么被跳过。

Now you can take a first crack at the list-directory function.

现在你可以首次看到 list-directory 函数了：

(defun list-directory (dirname)
  (when (wild-pathname-p dirname)
    (error "Can only list concrete directory names."))
  (directory (directory-wildcard dirname)))

As it stands, this function would work in SBCL, CMUCL, and LispWorks. Unfortunately, a couple more implementation differences remain to be smoothed over. One is that not all implementations will return subdirectories of the given directory. Allegro, SBCL, CMUCL, and LispWorks do. OpenMCL doesn't by default but will if you pass DIRECTORY a true value via the implementation-specific keyword argument :directories. CLISP's DIRECTORY returns subdirectories only when it's passed a wildcard pathname with :wild as the last element of the directory component and NIL name and type components. In this case, it returns only subdirectories, so you'll need to call DIRECTORY twice with different wildcards and combine the results.

使用上述定义，该函数在 SBCL、CMUCL 和 LispWorks 中正常工作。不幸的是，你还需要谨慎处理另外一些实现间的区别。其中一点是，并非所有实现都返回给定目录中的子目录。Allegro、SBCL、CMUCL 和 LispWorks 可以返回子目录。OpenMCL 默认不会这样做，但如果为 DIRECTORY 传递一个实现相关的、值为真的关键字参数 :directories，那么它将返回子目录。对于 CLISP 的 DIRECTORY 只有当传递给它一个以 :wild 作为目录组件的最后一个元素且名字和类型组件为 NIL 的通配路径名时，才可以返回子目录。而且，在这种情况下，它只返回子目录，因此需要使用不同的通配符，调用 DIRECTORY 两次并组合结果。

Once you get all the implementations returning directories, you'll discover they can also differ in whether they return the names of directories in directory or file form. You want list-directory to always return directory names in directory form so you can differentiate subdirectories from regular files based on just the name. Except for Allegro, all the implementations this library will support do that. Allegro, on the other hand, requires you to pass DIRECTORY the implementation-specific keyword argument :directories-are-files NIL to get it to return directories in file form.

一旦所有实现都返回目录了，你会发现它们返回的目录名有些是目录形式的，有 些是文件形式的。你想要 list-directory 总是返回目录形式的目录名，以便可以只通过名字来区分子目录和正规文件。除 Allegro 之外，所有实现都支持做到这点。Allegro 要求为 DIRECTORY 传递一个实现相关的、值为 NIL 的关键字参数 :directories-are-files，从而使其以目录形式返回目录。

Once you know how to make each implementation do what you want, actually writing list-directory is simply a matter of combining the different versions using read-time conditionals.

一旦你知道如何使每一个实现做到你想要的事，那么实际编写 list-directory 就只是简单地将不同版本用读取期条件组合起来了。

(defun list-directory (dirname)
  (when (wild-pathname-p dirname)
    (error "Can only list concrete directory names."))
  (let ((wildcard (directory-wildcard dirname)))

    #+(or sbcl cmu lispworks)
    (directory wildcard)

    #+openmcl
    (directory wildcard :directories t)

    #+allegro
    (directory wildcard :directories-are-files nil)

    #+clisp
    (nconc
     (directory wildcard)
     (directory (clisp-subdirectories-wildcard wildcard)))

    #-(or sbcl cmu lispworks openmcl allegro clisp)
    (error "list-directory not implemented")))

The function clisp-subdirectories-wildcard isn't actually specific to CLISP, but since it isn't needed by any other implementation, you can guard its definition with a read-time conditional. In this case, since the expression following the #+ is the whole DEFUN, the whole function definition will be included or not, depending on whether clisp is present in *FEATURES*.

函数 clisp-subdirectories-wildcard 事实上并是非 CLISP 相关的，由于任何其他实现不需要它，因而可以将其定义放在一个读取期条件之后。在这种情况下，由于跟在 #+ 后面的表达式是整个 DEFUN，因此整个函数定义是否被包含将取决于 clisp 是否存在于 FEATURES 中。

#+clisp
(defun clisp-subdirectories-wildcard (wildcard)
  (make-pathname
   :directory (append (pathname-directory wildcard) (list :wild))
   :name nil
   :type nil
   :defaults wildcard))