Why is this function not working properly, when added to a hook?

Question

Situation

I would like to have a function, that when called, creates a new file/buffer in a specific directory, indepent of the current directory. But with the same file name as the current file that the buffer is visiting.

So I created the following:

(defun change-to-foobar-directory ()
    (interactive)
    (setq buffer-temp-name (buffer-name))
    (kill-this-buffer)
    (find-file (concat "C:\\foobar\\" buffer-temp-name)))

This works great.

When I have the buffer foobar.el in directory C:\another-dir, and I call change-to-foobar-directory, then I get the buffer C:\foobar\foobar.el. The difference is that I'm editing a buffer in another directory (C:\foobar).

Problem

This solution works great so far. When I add that function to a hook, for example:

(add-hook 'emacs-lisp-mode-hook 'change-to-foobar-directory)

And when I'm in the file another-file.c, I create a new buffer foobar.el with the Evil command :e foobar.el (which stands for edit the file foobar.el in the same directory of buffer, even when the file doesn't exist yet, then a new buffer will be created).

Because the emacs.lisp-mode gets activated, the function change-to-foobar-directory should be called.

But I get the same file another-file.c instead editing the file/buffer C:\foobar\foorbar.el. So it seems with the hook, change-to-foobar-directory will not work properly. I'm not sure what caused this. When I call that command interactively, it will work properly.

Any idea what caused this?

Answer 1

Dunno what Evil might change in the picture (try without Evil, to see if there is a difference).

But if you are interested in the buffer name of a buffer visiting a file then what you should use is buffer-file-name, not just buffer-name. Specifically, (file-name-nondirectory (buffer-file-name)), if you want only the non-dir part. (buffer-name is for any buffer; buffer-file-name returns nil if the buffer is not visiting a file. This lets you conditionally not try to, for example, use the name of a non-file buffer as a file name.)

Similarly, do not use concat for file names. Instead, use expand-file-name with the directory you want to use as the second arg: (expand-file-name buffer-temp-name "C:/foobar/") (and no need to use MS Windows directory separators.

Also, use let to bind the relative file name, instead of just setq (unless, for some reason, you have a global variable of that name and you want to change its value).

Finally, if you want to kill the buffer and then find a file to replace it, just use find-alternate-file. This will take care of the problem you are experiencing, and that @nanny and @Stefan have pointed to.

(defun change-to-foobar-directory ()
  (interactive)
  "..."
  (let* ((filebuf               (buffer-file-name))
         (relname               (and filebuf  (file-name-nondirectory filebuf)))
         (emacs-lisp-mode-hook  ()))
    (when filebuf
     (find-alternate-file (expand-file-name relname "C:/foobar/")))))

Suggestion: Spend a little time with the Elisp manual, node File Names and its subnodes. You'll appreciate it.

Answer 2

In your problematic case, when you're visiting foobar.el, Emacs does the following in this order:

• fetch foobar.el into a new buffer.
• initialize the major mode (including running the hooks).
• display the buffer in the selected window.

So your hook will kill the foobar.el buffer, create a new foobar.el buffer at the new location. At this point, you should get an "infinite-recursion error" here since that find-file will also run the emacs-lisp-mode-hook. If you somehow avoid that error, then you bump into another error which is that right after your hook has finished, the third step ("display the buffer in the selected window") may cause yet more grief (either signalling an error because that buffer has been killed in the mean time, or displaying some other buffer in its stead).

Answer 3

Hooks get called as the last step in a mode's initialization.

And when I'm in the file another-file.c, I create a new buffer foobar.el

So you create that buffer, foobar.el, emacs-lisp-mode gets enabled in the buffer, and then its hooks are run. One of its hooks is your function change-to-foobar-directory. Your function calls kill-this-buffer. But what's this-buffer at this time? foobar.el.

The function is working perfectly fine, it simply has a logical error. Instead, try:

(defun change-to-foobar-directory ()
    (interactive)
    (setq buffer-temp-name (buffer-name))
    (kill-buffer (other-buffer))
    (find-file (concat "C:\\foobar\\" buffer-temp-name)))

Or see Drew's answer for a better alternative.