如何在Emacs中提供原语 [英] How to advise primitives in Emacs

问题描述

我正在尝试回答另一个SO问题，当我打了一些非常奇怪的行为。这是我的小测试用例：

I was trying to answer another SO question when I hit upon some very odd behavior. Here's my little test case:

(make-variable-buffer-local
 (defvar my-override-mode-on-save nil
   "Can be set to automatically ignore read-only mode of a file when saving."))

(defadvice file-writable-p (around my-overide-file-writeable-p act)
  "override file-writable-p if `my-override-mode-on-save' is set."
  (or
   my-override-mode-on-save
   ad-do-it))

(defun my-override-toggle-read-only ()
  "Toggle buffer's read-only status, keeping `my-override-mode-on-save' in sync."
  (interactive)
  (setq my-override-mode-on-save (not my-override-mode-on-save))
  (toggle-read-only))

(defun tester-fn ()
  (interactive)
  (let ((xxx (file-writable-p "/tmp/foofoo"))
        (yyy (file-writable-p "/tmp/fooxxfoo")))
    (message (concat "XXX: " (if xxx "yes" "no") "   -   YYY: " (if yyy "yes" "no")))))

其中：

• / tmp / foofoo 是一个只读文件，我访问过并运行 my-override-toggle-read-只有 in。


• / tmp / fooxxfoo 不存在。


• / tmp 可由用户登录。

• /tmp/foofoo is a read-only file that I've visited and run my-override-toggle-read-only in.
• /tmp/fooxxfoo does not exist.
• /tmp is writable by the user I'm logged in as.

如果我在缓冲区中运行 tester-fn ，其中设置了 my-override-mode-on-save 到 t 然后我得到一个意想不到的结果： XXX：否 - YYY：否。如果我在某些其他缓冲区（例如 scratch ）中运行 tester-fn ，我会在minibuffer中获得预期的响应： XXX：不 - YYY：是。通过调试器跟踪建议表明它正在做我正在做的事情，执行我期望的部分，跳过我期望的部分，返回我期望的值。然而，通过调试器跟踪 tester-fn 显示返回的值非常不同（ nil & t 如果变量评估为nil， nil & nil 为非零）。 nil & nil 返回真的是我发现奇怪的。

If I run tester-fn in a buffer where my-override-mode-on-save is set to t then I get an unexpected result: XXX: no - YYY: no. If I run tester-fn while in some other buffer (e.g. scratch) I get the expected response in the minibuffer: XXX: no - YYY: yes. Tracing the advice through the debugger shows it to be doing exactly what I think it should be doing, executing the parts I expect it to, skipping the parts I expect it to, returning the value I expect it to. However, tracing tester-fn through the debugger shows very different values being returned (nil & t if the variable evaluates as nil, nil & nil if the variable evaluates as non-nil). The nil & nil return is really what I find bizarre.

我不知道这里发生了什么。任何人知道为什么我没有得到我期望的结果？

I have no clue what's happening here. Anyone know why I'm not getting the results I expect?

推荐答案

你的代码看起来不错，除了一个缺少的键。您需要适当地设置返回值：

Your code looks good, except the one missing key. You need to set the return value appropriately:

(defadvice file-writable-p (around my-overide-file-writeable-p act)
  "override file-writable-p if `my-override-mode-on-save' is set."
  (setq ad-return-value
        (or
         my-override-mode-on-save
         ad-do-it)))

这在建议手册中有记录。

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