Can I bind a key as buffer local but not in the active keymap?

Question

So I'm writing some custom functions to help me editing some guitar chords using LaTeX's songs package. I find convenient to rebind isearch-exit, so that when I press RET, the cursor is left at the beginning of the matched text (this speeds up in inserting chords in the right position). Since I do this only in some specific files, I thought to create a custom minor mode, and active that in those files using an eval in .dir-locals. The point is, I need to bind RET in isearch-mode-map (like with define-key), but only in a buffer local way (like with local-set-key). Can this be done?

Ideally, given this function:

(defun isearch-exit-other-end ()
  "Exit isearch, at the opposite end of the string."
  (interactive)
  (goto-char isearch-other-end)
  (isearch-exit))
;; (define-key isearch-mode-map (kbd "M-<return>") #'isearch-exit-other-end)

(define-minor-mode chord-insertion-mode
  "Mode to help inserting chords with the LaTeX \"songs\" package, at the right spot."
  :lighter " chord"
  :keymap `((,(kbd "C-c C-c") . #'my--insert-chord)
        (,(kbd "C-c c") . #'my--insert-chord)
        )
  :after-hook (???)) ;; <----- WHAT TO PLACE HERE?

I would like to do something similar to

(define-local-key isearch-mode-map (kbd "RET") #'isearch-exit-other-end)

How can I set a key buffer-locally, but in a keymap different from the active one? Thanks!

Answer 1

A couple possibilities I can think of. Both involve doing everything that's conditional in a command that's bound to what isearch-exit is normally bound to. Either modify isearch-exit itself or replace it with your own command.

1. Just remap isearch-exit to a command you write that tests whether something is true in the current major mode. E.g.:

   (define-key [remap isearch-exit] 'my-isearch-exit)
   

   That "something is true" test could test a buffer local Boolean variable, or it could test whether some minor mode is enabled. You can use the mode hook for a major mode to set the local variable value or turn on the minor mode.

   In other words: just change RET and <return> to use your command, which does what you want and then does what isearch-exit does (or vice versa) - it can invoke isearch-exit.

2. Just advise isearch-exit, to conditionally do what you want before or after it does its thing. The advice would test the value of some buffer-local variable or minor mode -- see #1 above.

Answer 2

Here's a way to do it by making isearch-mode-map buffer-local:

(make-local-variable 'isearch-mode-map)
(setq isearch-mode-map (copy-keymap isearch-mode-map))
(define-key isearch-mode-map (kbd "RET") #'isearch-exit-other-end)

It's a little heavy-weight in that it ends up creating a copy of isearch-mode-map for every buffer, which you could possibly avoid by using keymap-inheritance, but I've not tested that.