bug#64415: [PATCH] Use first match in dir-local `auto-mode-alist'

bug#64415: [PATCH] Use first match in dir-local `auto-mode-alist'

[Shynur Xie]

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lispref says var `auto-mode-alist' uses first match, so pseudo
var `auto-mode-alist' in _dir-locals.el should also do that, for
consistency.

Here is a problem caused by the original behavior when Emacs handles
_dir-locals file: <https://emacs.stackexchange.com/questions/77841>.

--
shynur

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From 6cb2063a807b7dc2c3da9378db1c134b2e82a0a3 Mon Sep 17 00:00:00 2001
From: Shynur <one.last.kiss@outlook.com>
Date: Sun, 2 Jul 2023 18:15:02 +0800
Subject: [PATCH] Use first match in dir-local `auto-mode-alist'

* lisp/files.el (set-auto-mode): Reverse the reversed alist.
---
 lisp/files.el | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/lisp/files.el b/lisp/files.el
index 148f47cbc97..fc3b47082ea 100644
--- a/lisp/files.el
+++ b/lisp/files.el
@@ -3396,8 +3396,9 @@ we don't actually set it to the same mode the buffer already has."
     (unless done
       (with-demoted-errors "Directory-local variables error: %s"
 	;; Note this is a no-op if enable-local-variables is nil.
-        (let* ((mode-alist (cdr (hack-dir-local--get-variables
-                                 (lambda (key) (eq key 'auto-mode-alist))))))
+        (let* ((mode-alist (reverse (cdr (hack-dir-local--get-variables
+                                          (lambda (key)
+                                            (eq key 'auto-mode-alist)))))))
           (setq done (set-auto-mode--apply-alist mode-alist
                                                  keep-mode-if-same t)))))
     (and (not done)
-- 
2.39.1

bug#64415: [PATCH] Use first match in dir-local `auto-mode-alist'

[Eli Zaretskii]

> From: Shynur Xie <one.last.kiss@outlook.com>
> Date: Sun, 2 Jul 2023 10:29:13 +0000
> msip_labels: 
> 
> lispref says var `auto-mode-alist' uses first match, so pseudo
> var `auto-mode-alist' in _dir-locals.el should also do that, for
> consistency.

Isn't that an incompatible change in long-time behavior?  Your
stack-exchange correspondent might be pleased, but someone else, who
relies on the current behavior, will not be.

bug#64415: [PATCH] Use first match in dir-local `auto-mode-alist'

[Shynur Xie]

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.

___

> Eli> Isn't that an incompatible change in long-time behavior?

That is a bug fix.  What do you mean by ‘long-time behavior’?  To be
honest, it is a long-time bug IMO.

> Eli> someone else, who relies on the current behavior, will not be.

Yes, so just let them know that this behavior has changed in NEWS.
They just need to reverse their original alist, is that very
difficult?  Not to mention that the original behavior was not
guaranteed at all.

You can of course refuse that patch, and the inconsistency between
‘auto-mode-alist’ and dir-local ‘auto-mode-alist’ will last forever.

bug#64415: [PATCH] Use first match in dir-local `auto-mode-alist'

[Shynur Xie]

I agree that compatibility should be considered, but this dir-local
var was introduced since 28, it’s not too late to change it.

The reverse order is so contrary to perception.

bug#64415: [PATCH] Use first match in dir-local `auto-mode-alist'

[Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors]

> * lisp/files.el (set-auto-mode): Reverse the reversed alist.
> ---
>  lisp/files.el | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/lisp/files.el b/lisp/files.el
> index 148f47cbc97..fc3b47082ea 100644
> --- a/lisp/files.el
> +++ b/lisp/files.el
> @@ -3396,8 +3396,9 @@ we don't actually set it to the same mode the buffer already has."
>      (unless done
>        (with-demoted-errors "Directory-local variables error: %s"
>  	;; Note this is a no-op if enable-local-variables is nil.
> -        (let* ((mode-alist (cdr (hack-dir-local--get-variables
> -                                 (lambda (key) (eq key 'auto-mode-alist))))))
> +        (let* ((mode-alist (reverse (cdr (hack-dir-local--get-variables
> +                                          (lambda (key)
> +                                            (eq key 'auto-mode-alist)))))))
>            (setq done (set-auto-mode--apply-alist mode-alist
>                                                   keep-mode-if-same t)))))
>      (and (not done)

AFAICT the reversal happens within `dir-locals-collect-mode-variables`.
I think this is a bug in `hack-dir-local--get-variables`: it should return the
vars in the same order as found in the file.


        Stefan