Re: enriched-mode and switching major modes.

[Oliver Scholz <alkibiades@gmx.de>]

Richard Stallman <rms@gnu.org> writes:

>     I mean "the document's character data" here.  The important point is
>     that formats suitable for WP (RTF, HTML ...) separate character data
>     from formatting information entirely.
>
> My point is that this is exactly what we must not do in Emacs, lest it
> ruin everything in a subtle way.

[I moved that quote to the top, because the issue it adresses seems to
me to be at the root of our disagreement.  Everything I am going to
write below needs to refer to the distinction between the abstract
document and its visual rendering.  I am sorry if I am stealing your
time by flooding you with unsolicited information; but since we
discuss this prematurely, i.e. without me showing the code
implementing it, I want at least to clearly express my points.]

The distinction between character data and formatting information,
mentioned in the paragraph you quoted, is inherent in the type of
documents we discuss here.  You can enter as many newline characters
in the source of an RTF document as you want, an RTF reader will
simply remove them.  You can also put as many newline characters and
spaces into a HTML document as you want, a browser will just treat
each sequence of such whitespace control characters as a single space.

But probably you challenge my statement that it is important to keep
the distinction between the abstract document and its visual
representation in mind when discussion the implementation of WP
functionality.

This distinction is also inherent in the types of documents we discuss
here.  But first I should make clear what I mean by those terms.  I
say that we need to distinguish, at least conceptually, between: the
abstract document, the data structure that represents the abstract
document, the encoded file and the visual representation of the
abstract document.

1) the abstract document

   The abstract document is what the user of a word processor wants.
   She doesn't specify: "I want four space characters before each line
   in this area." nor "I want an <indent> tag before and an </indent>
   tag after this piece of text."  The former would refer to the way
   the visual representation could happen to be implemented in Emacs,
   the latter would refer to the document file format.  The user may
   be---and users of word processors typically are---blissfully
   ignorant of both.

   Instead the user says: "This piece of text is a paragraph, because
   I hit RET when I finished writing it.  I want it to be a paragraph
   of the type "Standard Text".  I want "Standard Text" paragraphs to
   be indented on the left by 1 cm and to have a font size of 10 pt.
   Exept here, I want this paragraph to have an indentation of 2 cm."

   I call the ensemble of character data and this latter specification
   of how the character data should "be like" the abstract document.
   It is what the user has expressed by typing text and interacting
   with the UI.  (It is thus important to design a good UI so that a
   user may express her wishes most clearly.)


2) the data structure used to represent that abstract document
   internally

   I list this explicetly, because there are quite some glitches here
   when implementing WP functionality in Emacs.  Among other things I
   suspect that I have failed to make one or two requirements for
   implementing the data structure in Emacs clear, if that data
   structure is meant to express abstract documents that are suitable
   for processing RTF (HTML ...).  More on this below.


3) the encoded document file

   This is the document as it would be written to a file: the concrete
   RTF or HTML file.  Not every file format is capable of expressing
   every abstract document.  The expressiveness of the available file
   formats would limit---or even shape---the possible abstract
   documents that a user may specify in a word processor.


4) the visual (aural) representation of the abstract document

   This is how the document is rendered on screen (or by a speech
   synthesizer; I won't further discuss aural rendering for now.)

   It is important to keep two things in mind:

       a) For a given word processor application, two different
          documents could look exactly the same when rendered on the
          display.

          For instance, in the document there could be specified as a
          paragraph property that the first line of a paragraph should
          be indented by 1 cm.  While in another document a user could
          get exactly the same visual effect by entering a number of
          space characters.  (Notabene: for a given application; when
          transfered to another application or to another machine,
          the appearances to those two documents could -- and probably
          would -- differ.)


       b) For a given document, two different applications or the
          same application on two different machines/operating
          systems might render two different visual representations.

          For example the font for a paragraph could be "Times New
          Roman" on MS Windows, but "Times" or even "New Century
          Schoolbook" on XFree.  Or the rendering device could be a
          tty. In that case word wrapping would happen at different
          places.  Or the user might have not specified a width for
          paragraphs at all, in that case the rendering could depend
          on application specific defaults or even on the size the
          application window happens to have.



             +---------------------+
             |  abstract document  |
             +---------------------+
                          ^
                          |
                          v
          +-----------------------------+
          |  data structure             |
          +-----------------------------+
            ^             |           |
            |             |           |
            |             v           v
+-------------+ +-----------------+ +------------+
|user commands| |visual appearance| |encoded file|
+-------------+ +-----------------+ +------------+



Even if we put these things together in Emacs, for one reason or
another, we need to keep said distinctions in mind---again: at least
conceptually---or else confusion will arise.  Any implementation of WP
functionality will have to account for how it deals with each of these
aspects or else Emacs would become infamous as the "so-called word
processor that provides a lot of surprises for its users."

The implementation that I suggested in my last posting (that one which
would work without a box model supported by the display engine),
strives to preserve the distinction by a) implementing a data
structure that stores those parts of the abstract document which
affect the formatting properties of paragraphs in a special text
property of the paragraph text, b) using font-lock and filling to
make these formatting properties visible.  Since filling would add
characters to the buffer content, my proposed implementation would
distinguish those characters added for visual appearance from the
characters which are part of the abstract document by means of
another special text property.

This is very important: If a user enters space characters into an
Emacs buffers, she wants there to be space characters.  Those
characters would have to become part of the character data in the
encoded file.  But if a user just specifies: I want this paragraph to
be indented, then the space characters used to display the left
margin _must_not_ become part of the encoded file.

I can not put too much emphasis on this.  Imagine the space characters
and the newlines as they appear in the buffer would be preserved.  A
encoded RTF file could look like this:

{\rtf\ansi
    This is just a paragraph.  It contains pointless example text.
    Habe nun ach, Philosophie, Juristerei und Medizin und leider auch
    Theologie durchaus studiert mit heissem Bemuehn. Da steh' ich nun
    ich armer Tor und bin so klug als wie zuvor.  To be or not to be
    this is the question.  Whether it is nobler in the mind ...}

If the user visits that file again with Emacs on the same system and
has not changed any of her customisations, then it would look the
same again.  So she would be content.

But try it!  Write that text above to a file and open it with
OpenOffice or AbiWord or whatever.  If I open it in OpenOffice on my
system, it looks like this (copied & pasted line by line, adding a
newline after each line):


    This is just a paragraph.  It contains pointless example text.    Habe nun ach, Philosophie, 
Juristerei und Medizin und leider auch    Theologie durchaus studiert mit heissem Bemuehn. Da 
steh' ich nun    ich armer Tor und bin so klug als wie zuvor.  To be or not to be    this is the question.  
Whether it is nobler in the mind ...


Obviously this is not an option.  `enriched-mode' has a nice UI bug
here.  If a range of text has a `left-margin' text property, say with
a value of 4, it removes the spaces on the left when saving and puts
according "<indent> ... </indent>" tags around it.  But the fill
function of text-mode also allows the user to insert just four spaces
at the beginning of a paragraph, auto-fill will then fill it with four
spaces indentation.  In the file, this will result exactly in what I
described for RTF above. This is by no means distinguishable from the
case where there is a `left-margin' text property, except by carefully
examining several pieces of the buffer text with `C-u C-x ='.

This bug has a great change to go unnoticed for a while or even
forever, because a) text/enriched is an extremely simple markup
format, b) there is probably not other application that deals with
text/enriched files (contrary to text/enriched e-mail) and c) the
text/enriched files of Emacs specify the value of fill-column in their
"header".  So even if the document is transfered to another user who
has a different default value for fill-column, the buffer's
fill-column will be the same as in the Emacs were the document was
created.  The circumstances under which the bug could show up are
probably rather rare.

We can by absolutely no means rely on things like this for RTF or HTML
or DocBook or TEI XML.  Those formats are far too complex.  Those
formats are widely used.  Such inconsitencies in the user interface
would inevitably result in more bugs of this kind which would
inevitably show up.  We do have to design WP functionality very
carefully in order to distinguish between the abstract document and
its visual appearance.  And the user interface must at each point
provide enough feedback about what is part of which.

This bug is due to an ambiguous user interface in enriched-mode. There
is an easy fix here: let the fill function indent a paragraph if and
only if there is a `left-margin' text property.

Thank goodness, text/enriched has no concept of character formatting
properties applying to a paragraph (a block box), and thus the
abstract document that can be produced in enriched-mode does not
provide that either; which fits well to Emacs, because the display
engine also does not know that concept.  In an Emacs buffer as well as
in a text/enriched file paragraph boundaries are defined by the
newline characters that precede and follow it; character formatting
properties in an Emacs buffer are specified with text properties and
in a text/enriched file with tags.  So these fit together.

Not so RTF.  Not so HTML.  A paragraph is specified not by newline
characters before or after it, but by syntactical markup.  Character
formatting properties may apply not only to a range of characters (an
"inline box") but also to a whole paragraph (a "block box").

So where am I supposed to store those character formatting properties
applying to the whole paragraph?  The only option I have are text
properties.  But since text properties do not define a paragraph
visually (the display engine does not support this), I have to make
sure that a range of text defined to be a paragraph by means of text
properties, does /appear/ as a paragraph in the buffer.  That means
that I have to define a fill-function as described in my last posting.

Perhaps I did not make one point clear enough: this fill function must
have the _total_control_ over the whitespace formatting in the buffer.
There must not any other Lisp function interfere here.  Or else the
abstract document/the data structure and the appearance on the screen
would get out of sync.  If the user wrongly thinks that the visual
appearance matches the abstract document as specified in the data
structure and saves the file, she will be painfully surprised the next
time she visits it.  In other words: we would have introduced an
indirect way of corrupting the user's files.  We would have introduced
a myriad of possibilities for bugs like the one in enriched-mode
described above.

>     The hairy part is whitespace formatting. The problems arise from the
>     fact that I can't tell Emacs: "Display this text from position POS1 to
>     POS2 as a paragraph with a left margin of 20 pt and a right margin of
>     40 pt with 20 pt above and below -- *without* adding any character to
>     the buffer."
>
> The idea of Emacs is that the appearance is represented by text in the
> buffer.  We designed text properties so that the text in th ebuffer
> could be something more than a sequence of characters.
>
> Any extensions have to preserve this principle, or we will lose
> the benefits of Emacs.

I obviously disagree, since I argued for an enhancement that goes
well beyond this principle.  I fail too see at all how we would lose
the benefits of Emacs this way.  As I see it, it would only add to
Emacs capabilities, extend the domain of documents it is suitable for
and enlarge its benefits this way.

>     If Emacs' display engine would support this, e.g. as a `block' text
>     property, then I could write:
>
>     (progn (switch-to-buffer (generate-new-buffer "*tmp*"))
>          (insert "Example text. Example paragraph. Example text.")
>          (put-text-property 15 33
>                             'block
>                             '(:margin (4 1 1 1) :border nil :padding nil)))
>
> If the block parameters are specified as a text property on the entire
> contents of the block, that might solve the problem.  However, there
> are some details here that are nontrivial problems.
>
> 1. How to distinguish between two similar boxes with the same specs
> and a single longer box.

I have not spent much thought on this;  I have only spent quite some
time trying to grok the display engine (in vain, so far) and have not
come to a stage where I could make some reasonably funded proposals
for the design of that feature.  Maybe they could programatically be
distinguished by checking for `eq'uality rather than `equal'ity.
Maybe for the user this wouldn't suffice, because they would look
the same after `C-x =' which could or could not be confusing.

I have no idea how to specify /nested/ block boxes with text
properties, either.  This is also a problem.

> 2. How to represent line breaks.  Saying "break one long line at
> display time" would work ok for display, but all the commands that
> operate on lines would see just one long line there.

`C-a', `C-p' and the like would need to be enhanced to deal with this.
Since it should of course be possible to check from Lisp whether some
buffer text is the content of a box, I don't see any problems here
which would be greater then the problem of implementing the display
feature itself.  In fact, there are at least three packages that
already implement this for wrapped long lines:  screen-lines.el,
screenline.el and window-lines.el. (Ouch!  They stopped to work with
the current CVS version ...)

> 3. How to represent indentation.  If the indentation appears only
> in redisplay, Lisp code that looks at the text will think it is not
> indented at all.

I actually regard this as a feature.  In WP documents the left margin
has no more significance than the right margin, which is not currently
implemented by adding space characters, either.  Functions that, for
instance, match on the space characters used to display a left margin,
would match on something which is not part of the character data of
the document: if the user sends that document to somebody else using
another word processor or uses another word processor herself, they
simply won't be there.

And with the implementation that I proposed in my last mail: if an
arbitrary Lisp function not part of the WP mode actually changes the
contents of a buffer containing a WP document, they better change only
the character data, not any spaces or newlines added programatically
for whitespace formatting or else they could wreak havoc in an
intolerable way.  

But the display feature I proposed would greatly increase the chance
that existing Emacs Lisp functions would seamlessly do the right thing
in a buffer containing a such a WP document.  Right now they work on
character data only and they should continue to work on character data
only in a WP document.

> I think we need to look for a hybrid solution where there could be a
> text property saying how this box is supposed to look, but that works
> by inserting newlines, indentation, etc., so that the text can be seen
> in Lisp without need to decode the box.

You mean so that those characters are added and updated
automagically?   Hmm, I seem to recall that I have experimented a bit
with using jit-lock to do the whitespace formatting, when I first
started to think about WP in Emacs.  But I don't remember how well
that worked.  I did not follow that approach further, because it
seemed unorthodox to me at that time.

However, something like this could at least deal with some of the
problems of keeping visual representation and the abstract document in
sync, so it would be at any rate a big improvement.

However, the implementation strategy that I described is meant as a
temporary solution to implement a box model in Lisp without support
from the display engine.  If you see it as a final state, moreover,
if you think that it is fundamentally different from a box model,
then I have failed to explain it.

>     If Emacs display engine would support a block model, we would just
>     tell the display engine how to render the paragraphs. There is not a
>     single newline chars and no space between paragraphs that would be
>     part of the character data.  I.e.
>     `(buffer-substring-no-properties (point-min) (point-max))' would
>     return:
>
>     "Lirum larum (A headline)\"Mariage is the chief cause of divorce.\"\
>     This is just ordinary paragraph text. Nothing special here. This is\
>      a list item. It contains two subitems:One and Two This is another \
>     list item."
>
> This model fails to address those problems.  It would work as a way
> of grafting a separate word processing facility into Emacs, but it
> would not integrate well with the existing Emacs Lisp world.

I don't understand why you say that.  And I don't know which parts of
the existing Lisp world you mean.

`query-replace' and isearch, for example, would do the right thing.
Not so with the replacement for a real box model that I described.
`query-replace' could even indirectly lead to file corruption, as
explained above.

Please note also, that a real box model supported by the display
engine is the only way to get tables with the capabilities that
people will expect (I am talking about columns with different font
heights here and about table borders).  It is not possible to mimic
it without.

> However, later you talk about an implementation more like what I have
> in mind, where the boxes and lists would be rendered by changing the
> buffer text; therefore, the buffer text would show what's really
> there.

Erm, what does the concept of "what's really there" in that context
mean?  In the buffer, or more generally spoken: in the data structure
a containing block box, or a text property storing formatting
information is, of course, no less there than any space or newline
character added for whitespace formatting.  But when Emacs writes the
WP document out to a file a again, say an RTF file, then it would
/remove/ those space and newline characters.  It definitely has to;
everything else would be a most serious bug.  So, spoken from the point
of view of the file format, those space and newline characters are not
"there" at all.

>     However, about one thing I am positiv: there is absolutely no room
>     for a minor mode here.  That's why I say that enriched-mode (as a
>     minor mode) is a dead end.
>
> I don't see that that follows from the rest of your points.

I don't understand why you say that.  I described an implementation
that needs to be in full control of every aspect of whitespace
formatting (at the very least).  I don't see how that could
justifiedly be implemented as a minor mode.  And if it were, I don't
see with what major modes it should be able to work together.

I can think of a third implementation strategy for editing RTF files
that /could/ be implemented as a minor mode; and if enriched-mode
would be slightly changed, it could even be part of
enriched-the-minor-mode.  I don't want to undermine my own position,
but here it is:

It would be an heuristical approach.  The encoding function (when
saving to a file) could determin whether a paragraph (defined by its
preceding and following newline characters) is indented by space
characters.  It could then write the nessary markup to the file
accordingly.  The problem of character formatting properties applying
to a paragraph could be solved in a similar, heuristical way:
determine whether---let's say---a `paragraph-stylesheet' with a
particular property predominates a paragraph; if so, write the
according markup again.

There are two important consequences implied in this approach: 

About the first one I am not 100% sure. But it is quite possible that
We would not even try to come close to the visual appearance of the
document in other word processing applications. One might say: we
would try to preserve only the syntactial properties of the document
when reading it.

The other consequence is that this approach will invariable fail
sometimes; this is implied in the fact that this approach is
heuristical.  If it is carefully implemented, it might work well
enough in common cases to be useful.  I don't know; that area would
need to be explored.  Bulleted and numbered lists would probably have
to be implemented as an exeption to the heuristical approach in a way
that is similar to what table.el does for tables.

However: it would be an incredible exaggeration to call Emacs a "word
processor" if it followed such an heuristical approach. And personally
I am not interested in pursuing it.  Not the least reason is that some
particular aspects of what I personally want for WP in Emacs would not
be feasible this way.  This is not the time to discuss the whole abyss
of my ideas.  Yet I want, for instance, an API that lets people access
the contents of a WP document syntactically by a query language (to
name the devil: XPath); so that people could specify in Lisp: give me
all text areas that are of the type "headline 2" etc.  I mention this
approach only for completeness.

    Oliver
--
Oliver Scholz               30 Fructidor an 212 de la Révolution
Ostendstr. 61               Liberté, Egalité, Fraternité!
60314 Frankfurt a. M.