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;;; /tmp/test/.dir-locals.el ((auto-mode-alist . (("/\\.dir-locals\\(-2\\)?\\.el\\'" . lisp-data-mode) ("/[^/-][^/]*\\.el\\'" . emacs-lisp-mode))))

I use emacs -Q to visit file /tmp/test/.dir-locals.el, then Emacs chooses emacs-lisp-mode for this file.

What goes wrong? Why not lisp-data-mode?

GNU Emacs Lisp Reference Manual says that the variable auto-mode-alist uses the first match, so I think the pseudo-variable (I don't what is a pseudo-variable; I heard it from phils, seems like a good name) auto-mode-alist in ".dir-locals.el" should also do that, for consistency.

GNU Emacs Manual, 49.2.5 Per-Directory Local Variables:

The special key auto-mode-alist in a .dir-locals.el ... works much like the variable auto-mode-alist (see Choosing File Modes).

GNU Emacs Lisp Reference Manual, 24.2.2 How Emacs Chooses a Major Mode:

If auto-mode-alist has more than one element whose regexp matches the file name, Emacs will use the first match.

However, it seems that the current behavior of dir local auto-mode-alist is to choose a mode from back to front.
The reverse order is so contrary to perception.

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  • Please say why you might expect lisp-data-mode. IOW, say what you say in your bug report, vis a vis the behavior difference between auto-mode-alist and entry auto-mode-alist in dir-locals. Commented Jul 2, 2023 at 14:15

2 Answers 2

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I've reported this as bug#64415; seems like it will be rejected...

To sum up

This bug is about different behavior between the variable auto-mode-alist and the directory variable auto-mode-alist.

The original behavior of how Emacs handles that dir var is to scan the alist from the back to the front, which is very counter-intuitive.

I suggest that we should make that behavior consistent with how Emacs handles the variable auto-mode-alist.

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  • Please summarize the bug report. Commented Jul 2, 2023 at 14:14
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    1. Please don't ask questions in answers (except posed to the OP, to clarify the question). 2. I meant to summarize what the bug is, e.g., why you think something is wrong. I didn't mean summarizing why it was closed (but that's OK also, if you think it's helpful). Commented Jul 2, 2023 at 14:36
  • What I meant was please describe what the bug is, not just link to it and then summarize opinions about whether the bug should be closed. Otherwise, this is essentially a link-only answer. E.g., "The bug is about different/conflicting behavior between...." Commented Jul 2, 2023 at 20:30
  • @Drew: hope I understand what you mean this time Commented Jul 3, 2023 at 2:04
  • Yup; you got it. Thx. Commented Jul 3, 2023 at 4:30
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Edit: This answer applies to Emacs 27.x and earlier. See the note at the end for why.

Firstly, that's not a valid syntax for a dir-locals file -- the top level of the form is not a list of variables/values.

Secondly, what you're attempting is nonsensical -- dir-locals are (can be) per-mode, and so obviously the dir-locals are not checked until the buffer's mode has been set. Changing auto-mode-alist for a buffer after its automatic mode has already been selected is pointless.

Edit: The above applies to Emacs version 27.x and earlier. Emacs 28.1 added support for a new special top-level key auto-mode-alist. Quoting (emacs)Directory Variables in 28.2:

 The special key ‘auto-mode-alist’ in a ‘.dir-locals.el’ lets you set a file’s major mode. It works much like the variable ‘auto-mode-alist’ (*note Choosing Modes::). For example, here is how you can tell Emacs that ‘.def’ source files in this directory should be in C mode: ((auto-mode-alist . (("\\.def\\'" . c-mode))))
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  • The basic syntax is ((mode . ((var . value) [(var . value) ...])) [(mode...) (mode...)]) -- so your version is trying to target a major mode symbol auto-mode-alist. Commented Jul 2, 2023 at 9:38
  • I just followed the example in the emacs manual. Commented Jul 2, 2023 at 9:39
  • For which version of Emacs? Commented Jul 2, 2023 at 9:40
  • 28.2 and 30.0.50 Commented Jul 2, 2023 at 9:40
  • Ok, I see that Emacs 28 added support for a new pseudo-variable auto-mode-alist in dir-locals forms. My answer applies to older versions of Emacs. I'll add a note... Commented Jul 2, 2023 at 9:43

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