automatically change input method based on context

[charles@aurox.ch (Charles A. Roelli)]

Hi,

I'd like Emacs to switch input method automatically based on the
font-lock face property of point.  As an example, if point is in a
comment, Emacs should run `(set-input-method "french-prefix")', and then
when point leaves the comment, `(set-input-method nil)' should run
again.  I managed to do it with a timer (not really elegant), but it
seems like this is something that would be possible with
`cursor-sensor-mode'.

My idea is to:
1. Add a `cursor-sensor-functions' property to all the text that is
   fontified as a comment (and somehow keep this dynamically updated).
2. Use this function in `cursor-sensor-functions' to handle changing the
   input method:

   (defun motion-set-input-method (window oldpos dir)
     (pcase dir
       ('entered (set-input-method "french-prefix"))
       ('left (set-input-method nil))))

But I don't know how to go about (1).  Any advice?

Cheers,
Charles