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RE: A vision for multiple major modes: some design notes

From: Drew Adams
Subject: RE: A vision for multiple major modes: some design notes
Date: Thu, 21 Apr 2016 13:26:53 -0700 (PDT)

> > But just what does "the parts that affect redisplay" mean?
> > If we mean parts that need to do something particular wrt
> > redisplay, then yes, that makes sense.
> I mean the part that is needed for redisplay to behave in each island
> according to user expectations.  For example, imagine that a mode that
> is relevant to a certain island chain sets up face-remapping-alist in
> some particular way -- when redisplay does its job, it repeatedly
> consults this variable when it needs to compute faces.  I'm saying
> that the part of the changes for this feature that affects redisplay
> will have to arrange for recalculation of the value of
> face-remapping-alist when the display engine gets to examining the
> portion of buffer text that belongs to this island chain.  Since the
> position where the display engine processes is not visible to Lisp,
> this arrangement will have to be in C.  And similarly with any other
> variable whose value the display engine accesses from its C code, like
> standard-display-table, for example.

Thanks for the example.  That's the kind of thing I thought
you had in mind.

> > You mentioned earlier that redisplay needs to access
> > buffer-local variables as it moves through the buffer.
> > And you said that redisplay needs to get the right values
> > of such variables.
> >
> > But for some island-chain operations, e.g. some that I'm
> > thinking of that do not care about the mode of a chain
> > or whether it even has a mode, I don't see why redisplay
> > would need to do anything special.
> This could be so in some particular use cases, but it's not
> so in general.

Depends on what one means by "in general". ;-) To me, having
a different mode associated with a chain is a special case of
either having such a mode or not having one.  Likewise, for
having chain-local variables or not.  Both having and not
having are special cases of "in general".

> Modes do affect the way text is displayed.

Yes.  But if a chain does not use a mode that is different
from the buffer's mode, then there should be no special
mode-specific handling needed for it.

> Besides, Alan says that "most" buffer-local variables will
> become island-chain local.  If we believe him, then your
> use cases you mention above are lucky exceptions rather
> than the rule.

I don't see them as either lucky exceptions or the rule.
I imagine that there are lots of possible uses of a chain
of islands of text, some of which involve a different mode
or in some other way involve different display possibilities,
and some of which do not.

>From the point of view of C code (e.g. redisplay) modification,
the latter use cases would I guess be lucky (little or nothing
new to do).  That doesn't mean they would be exceptional (rare)
in terms of user use cases.  (Dunno know whether they would be.)

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