I need to use a version of prolog.el not older than 1.25, so I defined the function maybe-reload-prolog-el:

(defun maybe-reload-prolog-el ()
  (when (version< prolog-mode-version "1.25")
     (file-name-as-directory (concat my-lib-path "prolog-el")))
    (load-library "prolog"))))

...which adds the path to my personal copy of prolog.el to load-path (and then reloads prolog) if it detects that prolog-mode-version is too old 1.

Then I put a call to maybe-reload-prolog-el in a hook I already had for prolog-mode:

(add-hook 'prolog-mode-hook
  (lambda ()
    ;; ;; some keyboard settings, like
    ;; (local-set-key [f10] ...)
    ;; (local-set-key  [f9] ...)
    ;; ;; etc

The main problem with this approach is that prolog.el is responsible for more than prolog-mode. For example, it has the code for the autoloaded function run-prolog, which I use to run a Prolog sub-process in an Emacs buffer. AFAICT, prolog-mode-hook does not get triggered when I run run-prolog, so I end up running an outdated version of it.

Is there some way to trigger maybe-reload-prolog-el right after prolog.el is loaded, before any of its code gets executed?

BTW, as currently written, maybe-reload-prolog-el can run only after prolog.el is loaded, because it relies on the variable prolog-mode-version, which is defined in prolog.el, to decide whether to load my local copy of prolog.el. IOW, if I simply put the following code in my init file

(if (version< prolog-mode-version "1.25")
   (file-name-as-directory (concat my-lib-path "prolog-el"))))

...it fails with:

Symbol’s value as variable is void: prolog-mode-version

If there were some way to test the version of the default prolog.el's version before loading it (let's call this method PROLOG-OLDER-THAN-1.25-p, then I would just put this in my init file:

(if (PROLOG-OLDER-THAN-1.25-p)
   (file-name-as-directory (concat my-lib-path "prolog-el"))))

Please let me know if there's a way to implement a PROLOG-OLDER-THAN-1.25-p that does not require prolog.el to be loaded.

Of course, I know that I can always use my copy of prolog.el, unconditionally. IOW, I can just put this in my init file:

 (file-name-as-directory (concat my-lib-path "prolog-el"))))

This approach works in the short-term, of course, but it requires more maintenance long-term.

1 Please let me know if what my maybe-reload-prolog-el function does is the best way to conditionally reload prolog.el. I'm not too confident that (load-library "prolog") is the right thing to do here.

  • 1
    "a way to implement PROLOG-OLDER-THAN-1.25-p that does not require prolog.el to be loaded" - the only way would be to search the source code text.
    – npostavs
    Commented Feb 27, 2017 at 16:06
  • 1
    You could try and inspect the data from the package manager package.el
    – YoungFrog
    Commented Feb 27, 2017 at 19:01
  • 1
    @YoungFrog prolog.el is not (yet) an package.el package.
    – npostavs
    Commented Mar 1, 2017 at 14:51

2 Answers 2


Did you try using eval-after-load or with-eval-after-load? E.g. something like this?

(eval-after-load "prolog" '(maybe-reload-prolog-el))

I'd check emacs-major-version instead of the version of prolog.el.

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