Emacs Per File Customization

Question

I have an Emacs Lisp file with custom macros I want fontified and indented differently. The code looks like:

(defmacro* when-let ((var value) &rest body)
  `(let ((,var ,value))
     (when ,var ,@body)))

(defun func ()
  (when-let (a 1)
    a))

I want when-let fontified as a font-lock-keyword and indented as above. I know I can do this in my .emacs file but I'd prefer to make it a directory local or file local customization. The problem is that directory local and file local customizations seem to be limited to setting variables. In my .emacs file I have the following.

(add-hook 'emacs-lisp-mode-hook
   (lambda ()
             (put 'when-let 'lisp-indent-function 1)
             (font-lock-add-keywords nil
                                     '(("(\\(when-let\\)\\>" 1
                                        font-lock-keyword-face)))))

I want this in .dir-locals.el because it applies only to one file.

Answer 1

You can specify elisp for evaluation in file local variables¹ by specifying an eval: value (the documentation says 'Eval:' but only lower-case 'eval:' seems to work). e.g.:

;;; Local Variables:
;;; mode: outline-minor
;;; eval: (hide-body)
;;; End:

As a security measure, Emacs will ask you for confirmation whenever it sees a value it does not already recognise as safe. If you tell it to remember it permanently, it writes the value to safe-local-variable-values in the (custom-set-variables) section of your init file.

Note that the above example of enabling a minor mode is deprecated (the mode local variable is for major modes only), so we need to re-write it as another evaluated form, in which we call the minor mode function.

If you need to evaluate multiple forms, you can either specify multiple eval values, which will be evaluated in order:

;;; Local Variables:
;;; eval: (outline-minor-mode 1)
;;; eval: (hide-body)
;;; End:

Or alternatively, just use progn:

;;; Local Variables:
;;; eval: (progn (outline-minor-mode 1) (hide-body))
;;; End:

The difference is that the latter would be considered a single value for the purposes of safe-local-variable-values, whereas with multiple eval values each one is considered independently.

¹C-hig (elisp) File Local Variables RET