How to re-load a package?

Question

The situation I have is this. First, I run

(require 'prolog)

...which loads the system's default version of prolog.el, and makes the variable prolog-mode-version available. It has value "1.22".

Now, I know that my personal copy of this package, in /path/to/my/own/private/prolog.el, starts with

(defvar prolog-mode-version "1.25"
  "Prolog mode version number")

So I run

(load "/path/to/my/own/private/prolog.el")

...expecting that, after doing so, prolog-mode-version will now have value "1.25".

This is not what happens, though: according to describe-variable at least, the value of prolog-mode-version is still "1.22".

What must I need to do to (re-)load 'prolog from /path/to/my/own/private/prolog.el?

`defvar`s (and `defcustom`s, which use `defvar`) can only be [defined once](https://www.gnu.org/software/emacs/manual/html_node/elisp/Defining-Variables.html). To re-evaluate a `defvar`, place point over it and `eval-defun` (`C-M-x`). — Tianxiang Xiong, Feb 27 '17 at 17:22
@TianxiangXiong: thanks! How can I do what you describe programmatically? — kjo, Feb 27 '17 at 17:23
Not easily. See: http://emacs.stackexchange.com/questions/2298/how-do-i-force-re-evaluation-of-a-defvar — Tianxiang Xiong, Feb 27 '17 at 17:26
@TianxiangXiong: Thanks again. Come to think of it, I need to `eval` the whole file, since there's a ton of other `defvar`s in there... I never expected this to be such a nightmare... — kjo, Feb 27 '17 at 17:31
If you're running your own version of the package, why not stop using the system-provided one? — Tianxiang Xiong, Feb 27 '17 at 17:31
Why are you loading the built-in version of the package in the first place? — Tyler, Feb 27 '17 at 17:31
@TianxiangXiong: I explain why here: http://emacs.stackexchange.com/q/31086/882 — kjo, Feb 27 '17 at 17:34
Is there ever a situation where you have a personal version of the package installed but *don't* want to use it? It would be easier to check for the presence of your custom version, and load it if present, and if it isn't present *then* load the built-in. — Tyler, Feb 27 '17 at 17:39
@Tyler: If the system version is more recent than my version, I'd prefer to use the system version. — kjo, Feb 27 '17 at 17:48
Just use your local version. "Use system version if it's newer" sets you up for surprises--what if the system version breaks backward compatibility in some unexpected way? — Tianxiang Xiong, Feb 27 '17 at 20:32

npostavs · Accepted Answer · 2017-03-01T14:45:31.563

7

You can unload the system prolog mode first, this unbinds all its variables, so that the new defvar init-forms will take effect:

(require 'prolog)
(when (version< prolog-mode-version "1.25")
  (unload-feature 'prolog)
  (load "/path/to/my/own/private/prolog.el"))

Note that unload-feature is not a commonly used function, so it's possible you may hit some bugs. Most packages are not written with unloading in mind. Glancing at prolog.el, I don't see anything that should be a problem though (examples of potentially "problematic" forms would be side-effecting top-level calls, apart from the usual defun, defvar, defconst, defcustom, etc).

edited Mar 01 '17 at 14:45

answered Feb 27 '17 at 18:02

npostavs

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+1 for mentioning the caveat "*Most packages are not written with unloading in mind.*" – Drew Feb 27 '17 at 21:21
Thanks! I noticed that, although I had used `(require 'prolog)` in my original question, in your answer you used `(require 'prolog-mode)` instead. Was there a particular reason for that change? – kjo Mar 01 '17 at 12:18
1

@kjo no, that was a mistake on my part, fixed now. – npostavs Mar 01 '17 at 14:46

score 3 · Answer 2 · answered Feb 27 '17 at 18:15

As an alternative to unload-feature, you could use this to check the version of the default package without actually loading the code:

(let ((version (with-temp-buffer
                 (insert-file (find-library-name "prolog"))
                 (re-search-forward "(defvar prolog-mode-version \"\\([^\"]+\\)")
                 (string-to-number (match-string 1)))))
  (if (< version 1.25)
      (load "/path/to/my/own/private/prolog.el")
    (require 'prolog)))

How to re-load a package?

2 Answers2