Re: Combining face and map stuff

[Stefan Monnier <monnier@iro.umontreal.ca>]

> Which reminds me of another thing.  I would like to define some commands
> local to a region, which can be done with 'local-map in overlays or
> 'keymap in text properties.  But can I combine them, too, in possibly
> overlapping regions, and get the aggregate keymap?

Here's an idea with which I've beeing toying:

- rather than have just one set of text-properties per char, make it
  possible to have several.  E.g. font-lock would use its own set of
  text properties.  One way to do that is that a `property' can now be
  a cons whose car is a "plane" and whose cdr is a property.
  So for example, the special font-lock-face (which currently gets
  "mapped" to face' via char-property-alias-alist) would just be `face'
  because font-lock would use (font-lock . face).
  A property like `face' would really be equivalent to (nil . face).

- then you add property-specific merge functions.  I.e. when looking up
  `face' you'd get the merge of all the `face' properties of the various
  text-property planes.  You'd probably want those merge functions to be
  written in Elisp and customizable.  So merging `keymap' properties
  becomes easy (well, it may benefit from multiple-keymap inheritance,
  which is a completely different topic).

- now font-lock can erase all the properties of the `font-lock' plane
  without worrying about erasing properties installed by other packages.

- to avoid calling Elisp code to merge things during text-property
  lookups, the merge would take place in put-text-property (i.e. no need
  to change the redisplay code at all).

- modifying a plane other than the default (nil) one could be considered
  as "not modifying the buffer" (just like adding/removing overlays).

- we could obsolete char-property-alias-alist which is only ever used
  by/for font-lock-face.

- maybe outline-minor-mode could use text-properties rather than
  overlays to make text invisible (tho it would make it impossible(?) to
  use outline-minor-mode in an indirect buffer without affecting the
  base buffer and the other indirect buffers).  So it might reduce the
  need for overlays (which have the disadvantage of being
  algorithmically slow/costly).


        Stefan