How to set major mode for a file in .dir-locals.el?

Question

I would like to set the major mode for a file as you would using file-local variables

Local Variables:
mode: text
End:

but in .dir-locals.el in order not to “pollute” the file in question. Is this possible?

Answer 1

It's possible to specify a string instead of a mode, but this only works for subdirectories. So I'll use nil instead to match all modes, then the eval key to change the major mode conditionally:

((nil
  (eval
   (lambda ()
     (when (string= (file-name-nondirectory buffer-file-name)
                    "file-name.extension")
       (my-mode))))))

A downside of this approach is that the eval key is unsafe, so you'll need to confirm the variable permanently for this to have an effect.

Answer 2

The following .dir-locals.el works just fine for me to set the major mode of all files in a directory to shell-script-mode:

((nil . ((mode . shell-script))))

The obvious downside is that you can't specify the major mode for only a given subset of files in the directory.

Addendum: Amusingly, it also has the side effect of setting the major mode of .dir-locals.el itself to shell-script-mode.

Addendum 2: Rather more amusingly, it also has the side effect of setting the major mode of Dired buffers to shell-script-mode, making it impossible to open any (as an error is thrown during the initialization).

Answer 3

For me accepted answer give error:

(error "Lisp nesting exceeds ‘max-lisp-eval-depth’")

Because it tried to apply the mode recursively, when it open a file it executed a mode and if the mode was changed it executed the eval again recursively:

For me this works:

((nil (eval
       (lambda ()
         (when (and (string-match-p "\\.js\\'" buffer-file-name)
                    (not (string= (major-mode) "rjsx-mode")))
           (rjsx-mode))))))

For rjsx-mode that I wanted to apply for all js file in project directory.