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There seems to be a bug in clojure-mode (mentioned here) because it does things in a mode hook (for paredit-mode) but doesn't undo them when the mode is turned off.

I understand that add-hook can register a function to run when a mode is turned on, but is there a way to run a hook when a mode is turned off?

Here's a github link see the clojure-mode code that adds the hook.

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  • 1
    To further clarify, minor modes are entered and exited (and so hooks run on both occasions). Major modes are never 'exited' at all, so they cannot run a hook before a different major mode is called. Instead the new major mode effectively just clobbers all the buffer-local settings and starts over -- but the standard way that they do that means that change-major-mode-hook runs (with the original major mode still enabled), before anything else happens.
    – phils
    Jan 22, 2019 at 19:39
  • stackoverflow.com/a/19295380/324105 has more details about major modes and hooks, if you're interested.
    – phils
    Jan 22, 2019 at 19:46

2 Answers 2

Reset to default
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You can use the hook variable change-major-mode-hook for cleaning up major mode settings. It is called right before killing the local variables of the old major mode.
Example code:

(defun clojure-leave-clojure-mode-function ()
  (when (eq major-mode 'clojure-mode)
    (message "Leaving clojure-mode.")))

(add-hook 'change-major-mode-hook #'clojure-leave-clojure-mode-function)
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  • This is a great suggestion, but in general I think users shouldn't need to run any cleanup code manually, the major-mode should take care of this by making changes buffer locally.
    – clemera
    Jan 22, 2019 at 17:12
  • @clemera If there is a bug in clojure-mode then that bug must be fixed. That is clear. Beside that take a look at the value of change-major-mode-hook. If you have cc-mode loaded then there is at least c-leave-cc-mode-mode. It is not so uncommon to use it for cleanup tasks.
    – Tobias
    Jan 22, 2019 at 17:16
  • Ok, but the major-mode should set it.
    – clemera
    Jan 22, 2019 at 17:17
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Not for major-modes, for minor-modes the hook runs on exit as well (See here). The major mode is responsible to make changes which affect other packages buffer locally. You probably should send a PR to change the add-hook code to install the hooks locally for the clojure-mode buffer:

(add-hook 'paredit-mode-hook #'clojure-paredit-setup nil 'local)

EDIT:

Looking at the code, this will not suffice. clojure-paredit-setup needs to be changed to do its changes buffer locally, too.

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  • This won't solve the problem consistently, because clojure-mode adds the hook to paredit-mode-hook every time it runs. Your code removes the offending hook, but it will be replaced every time you open a new file in clojure-mode.
    – Tyler
    Jan 22, 2019 at 19:53
  • Hm, but I suggested to add it buffer locally, the idea is that when activating clojure-mode it changes paredit-mode-hook only locally, for every buffer where it's activated. This way the global paredit-mode-hook is not effected, so there is nothing that need to be undone for non clojure buffers. BUT as I pointed out in the edit, clojure-paredit-setup needs to be changed, too because it changes other global variables.
    – clemera
    Jan 22, 2019 at 22:43
  • Worth noting though, apparently that doesn't work if the buffer (or emacsclient) gets killed. So, if you want to undo something done when setting a minor mode up, apparently you need to come up with an ad-hock hack. The closest I've found to work is the delete-frame-functions hook.
    – Hi-Angel
    May 19 at 9:52

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