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Hans Aberg <[log in to unmask]>
Sun, 29 Dec 1996 13:10:45 +0100
text/plain (72 lines)
>Hans Aberg writes:
> >   Even though TeX is a single pass compiler, multiple passes can be made by
> > reading a file several times.
> >
> >   So I suggest one might consider adding this as a feature of LaTeX3:
> >
> >   For example, if one wants to automate the LaTeX standard two-pass
> > compile, the environment "document" data would be put in a special file,
> > which is read twice; if this feature is chosen, the first pass could then
> > be done without actual typesetting, making it faster.
>i fear that i don't understand what you mean. what "data" can be put
>into a special file? most of the reason why you need the typesetting
>several times (and it might be even more than two) is to gather
>information from the visual context of the document, eg \pageref can
>only report the correct number if everything up to that point is
>already correctly typeset.
>can you explain a little bit what you have in mind?

  I had this idea a few years ago, and report it now, in case it may bear
importance for the LaTeX3 project:

  If you want some code segment to be executed more than once in TeX, you
have to build that repetition into some command, or repeat the command in
question several times, but TeX does not allow you to rewind the file
actually being compiled (even though you can put some forward reading into
a command, and then do non-deterministic parsing of that read portion).
  However, if the code you want to be repeated for some reason is being put
into a separate file, then this code can be input several times, and thus
also be executed several times. This code is thus in a file, different from
the one compiled by TeX.

  Then, there there is the question in which cases this might be useful. In
the case of attempting to automate the standard LaTeX two pass compile goes
something like this:

  In order to handle forward references, LaTeX creates, in the first pass,
helping files with the information of which pages the different labels
occur. Then, in the second pass, the information in the helping files is
written into the typeset output, where this is called for.

  For LaTeX3, one might attempt reorganize the input files as follows:

  You have a special command file, which controls what is to be included in
the document; perhaps similar to a UNIX makefile.
  This is the file you excute when typesetting, but the file you now
normally LaTeX-compile is another file, included from this new command
  The command file can then execute sequences like
    % Turn off typesetting output
    \input foo.tex                    % Compute LaTeX helpfiles
    % Turn on typesetting output; reset typesetting parameters
    \input foo.tex                    % Read LaTeX helpfiles
Somehow, this has to be integrated with LaTeX, in order to make it
practical. For example, one might make a special "\include foo.tex"
command, which excutes the correct command sequence.

  There are some worries, if one attempts to do this in practise: For
example, I do recall that TeX has a feature that enables you turning off
the typesetting output, but I am not sure to what extent one is able to
reset all other typesetting parameters in a second compile.

  But the file organization described above could be of independent interest.

>and merry Xmas to everybody

  And A Happy New Year!

  Hans Aberg