Is it possible to set major mode from buffer name, not filename?
reddit / emacs

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arensb · 2022-09-29 18:36

If I visit file `foo.py`, Emacs opens it in python-mode because the
extension ".py" appears in `auto-mode-alist` This is true even if the
file doesn't yet exist.

However, if I simply create a new buffer called `foo.py`, using `M-x
switch-to-buffer`, without tying it to a file, the new buffer is
opened in fundamental-mode.

Is there a way to set a new buffer's major mode based on its buffer
name, rather than (or in addition to) its filename? I haven't found
any hooks that would do the trick. Conversely, is there any reason why
I *don't* want to do this?

Thanks,

(ETAA: this is slightly different from [this
question](https://emacs.stackexchange.com/questions/14013/change-the-
default-mode-used-when-opening-a-new-buffer). That one is about
setting the one mode to use in a new buffer. Mine is about calling a
function to choose a mode dynamically. But that question seems to
imply that there's no hook to do what I want.)

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nv-elisp · 2022-09-29 20:33

https://www.gnu.org/software/emacs/manual/html_node/elisp/Auto-Major-
Mode.html

https://www.gnu.org/software/emacs/manual/html_node/emacs/Choosing-
Modes.html#Choosing-Modes

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arensb · 2022-09-29 20:56

Can you please be more specific? I know how to choose a mode based on
the filename or on the contents of the buffer, but I don't see
anything above on how to set the mode of a new, empty buffer.

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nv-elisp · 2022-09-29 21:09

You can use the knowledge above in combination with `buffer-list-
update-hook` to achieve what you're after. There will probably be
corner cases that need to be considered, though. The easier solution
is just to use `find-file` and not save the file.

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nv-elisp · 2022-09-29 21:25

This should get you started. Haven't tested; not interested in
supporting.

    (defun +set-new-buffer-mode ()
      "Set new buffer's major-mode."
      (when-let ((buffer (car (buffer-list)))
                 (name (buffer-name buffer))
                 (mode (cdr (cl-find-if
                             (lambda (regexp) (string-match regexp
name))
                             auto-mode-alist
                             :key #'car))))
        (with-current-buffer buffer
          (unless (eq mode major-mode) (funcall mode)))))

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arensb · 2022-09-29 22:13

Thanks. After a little bit of digging, it looks as though `buffer-
list-update-hook`'s functions aren't told which buffer just got
created.

Yes, I could try to keep track of what `(buffer-list)` returns, and
try to guess which buffer just appeared, but that seems difficult and
prone to error.

Maybe a better approach would be to write a wrapper around `switch-to-
buffer`, that sets the mode in the buffer, if it was created.