[twg-tds] Extensions

[Thomas Esser]

I am about to suggest a rewrite of one section of the TDS which is
about "Extensions" (e.g. pdftex, etex):

The old text is like this:
    New programs that are extensions of old ones shall use a new top-level
    directory name for their extension-specific input files
    ...
    Input files that use these new commands cannot be placed in the
    top-level \path|tex| directory, since the original \TeX{} program
    cannot read them.  So they must go in a new directory,

I think that we have leared two things in the last few years:
    Some package authors want their package to be placed into the
    search path of the original program to be able to provide a
    warning, if the user runs with the wrong engine (rather than
    a "file not found" error). One such example was the thumbpdf
    package in the early days when it only supported pdftex.

    Most package authors who make use of extensions try to detect the
    engine and use the features that are available for the actual run. So,
    the package needs to be placed into the search path of the original
    program. Examples are context (which runs faster if it detects the
    etex engine by making use of advanced features), today's thumbpdf
    (which also supports tex->dvips->pdf), texinfo (which supports pdf
    features) and cwebmac.tex.

    Packages evolve and eventually support more than one "engine", e.g.
    thumbpdf.

    Users want to be able to switch between engines. If one decides to
    use pdfetex and on other engine, lots of search paths have to be
    changed, because otherwise pdftex.cfg (which exists in texmf/pdftex)
    will not be found and things stop working.

So, in short, I suggest that everything that is searched along the
TEXINPUTS search path be placed into texmf/tex, no matter if some
extension is used or not. We are not talking about many files, because
lots of packages are already placed the way I suggest. It would simplify
a lot of things and after I have suggested something like this elsewhere
(pdftex devel list, Olaf Weber, Fabrice Popineau, Sebastian Rahtz
and later passed to tex-live list), all responses have been positive
(e.g. Hans Hagen, Han The Thanh, Staszek Wawrykiewicz, Sebastian Rahtz).

Thomas