2018-09-10 Persisting Emacs variables

Some time ago I coded a certain Elisp tool (for internal use at Wiadomości Matematyczne) and I needed to preserve the value of some variable between Emacs sessions. I asked how to do this on the Emacs mailing list, and I received a whole lot of very interesting and worthwhile responses. I decided to stay simple, though, and used the snippet of code I mentioned there to just add some variable and its value to init.el. For your convenience, here is the code – although I strongly suggest reading the mentioned thread before using it. My use-case is quite specific, and for most cases the solution described there will be better. Use this at your own risk!

(defun make-variable-persistent (variable-name)
  "Save VARIABLE-NAME to `custom-file' or `init-file'."
  (when (y-or-n-p (format "I am going to save variable `%s' in %s.  Should I proceed? " variable-name user-init-file))
    (with-temp-file user-init-file
      (let ((sentinel-text (format "\n;; Persistent variable `%s'.  Do not edit manually!\n" variable-name)))
	(insert-file-contents user-init-file)
	(if (search-forward sentinel-text nil t)
	    (if (not (looking-at-p (format "^(setq %s" variable-name)))
		(error "Broken init file")
	      (when (eq (char-after (point)) ?\n)
		(delete-char 1)))
	  (goto-char (point-max))
	  (insert sentinel-text)))
      (insert (format "(setq %s %S)\n" variable-name (symbol-value variable-name))))))

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