Re: Calling another major mode in a major mode body

[Phil Sainty]

On 2022-11-22 11:07, Yuan Fu wrote:
> So I wonder if it’s ok to fall back to another major mode by simply
> calling that mode.

I think the following describes what that would do.


Quoting myself from https://stackoverflow.com/a/19295380 (and as a
tangent I'd be happy for some adaptation of that to live somewhere
in the elisp manual, as I think it was a decent explanation of the
processes), when we call `child-mode', the full sequence is:

(run-hooks 'change-major-mode-hook) ;; actually the first thing done by
(kill-all-local-variables)          ;; <-- this function
,@grandparent-body
,@parent-body
,@child-body
(run-hooks 'change-major-mode-after-body-hook)
(run-hooks 'grandparent-mode-hook)
(run-hooks 'parent-mode-hook)
(run-hooks 'child-mode-hook)
(run-hooks 'after-change-major-mode-hook)
;; plus the following final step, since:
;; commit 2eb6817ba971184cc109f8530f4b3b38f65650ea
;; Add :after-hook facility to define-derived-mode.
(run-hooks delayed-after-hook-functions)


`delay-mode-hooks' is still in effect until child-body has returned,
so I believe calling (fallback-mode) within child-body would result
in this sequence:


(run-hooks 'change-major-mode-hook) ;; actually the first thing done by
(kill-all-local-variables)          ;; <-- this function
,@grandparent-body
,@parent-body
,@child-body
+    (run-hooks 'change-major-mode-hook) ;; actually the first thing 
done by
+    (kill-all-local-variables)          ;; <-- this function
+    ,@fallback-parent-mode-body
+    ,@fallback-mode-body
;; The child-mode binding for `delay-mode-hooks' is now out of scope,
;; so `run-mode-hooks' finally acts...
(run-hooks 'change-major-mode-after-body-hook)
(run-hooks 'grandparent-mode-hook)
(run-hooks 'parent-mode-hook)
(run-hooks 'child-mode-hook)
+    (run-hooks 'fallback-parent-mode-hook)
+    (run-hooks 'fallback-mode-hook)
(run-hooks 'after-change-major-mode-hook)
(run-hooks delayed-after-hook-functions)


It looks like things pushed onto `delayed-after-hook-functions'
would happen in this sequence, though:

- grandparent-mode
- parent-mode
- fallback-parent-mode
- fallback-mode
- child-mode

Although `delayed-after-hook-functions' does not seem to be
permanent-local, so in fact it might be this?

- fallback-parent-mode
- fallback-mode
- child-mode




There's also this, which doesn't seem entirely appropriate, but...

** New functions 'major-mode-suspend' and 'major-mode-restore'
Used when switching temporarily to another major mode, e.g. for 
hexl-mode,
or to switch between c-mode and image-mode in XPM.