Global minor mode that requires another during its lifetime

Question

Say I'm developing a minor mode A that happens to need minor mode B enabled during its lifetime.

The most straightforward way is to do something like:

(define-minor-mode A-mode
  :global t
  :lighter " A"
  (if A-mode
      (B-mode 1)
    (B-mode 0))) ; hooks here are added/removed...

Or alternatively:

(add-hook 'A-mode-hook 'B-mode)

The problem with these approaches is that the original status is not preserved. I could simply use a global variable to keep track of it but I was hoping to find a more elegant solution.

For example I can solve a similar issue involving variables like this:

(progn
  (setq foo 10)
  (print foo)     ; 10
  (let ((foo 42))
    (print foo))  ; 42
  (print foo))    ; 10

There is one more problem: the user could manually disable minor mode B while minor mode A is still on; and this shouldn't happen.

Probably is just bad practice to even require this behavior...

(FYI in my case minor mode B is temp-buffer-resize-mode.)

Answer 1

The problem with these approaches is that the original status is not preserved. I could simply use a global variable to keep track of it but I was hoping to find a more elegant solution.

Well I think you'll essentially need to do that, but it needn't actually be a defvar'd variable.

If you're using lexical binding, then you can probably just use that.

Alternatively, as it's a global value with no other use-case, you could store it as a symbol property on A-mode.

(put 'A-mode 'B-mode-enabled (bound-and-true-p B-mode))

and then when disabling A-mode:

(unless (get 'A-mode 'B-mode-enabled)
  (B-mode 0))

There is one more problem: the user could manually disable minor mode B while minor mode A is still on; and this shouldn't happen.

A-mode can always add to B-mode-hook to at least identify when that happens, even if it doesn't try to actively counter-act it.

Probably is just bad practice to even require this behavior...

Perhaps. temp-buffer-resize-mode is just acting on temp-buffer-show-hook to call resize-temp-buffer-window, which is something you could probably adapt for A-mode's specific needs.