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Date:         Thu, 23 Jul 2020 18:32:04 +0200
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From:         Frank Mittelbach <frank.mittelbach@LATEX-PROJECT.ORG>
Subject: Re: Guidelines on the new hook management -- tex4ht
To:           LATEX-L@LISTSERV.UNI-HEIDELBERG.DE
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Status: R

Am 23.07.20 um 09:07 schrieb Michal Hoftich:
> Hi all,
> 
>> Inspired by issue #900 on the PGF/TikZ repo [1], I'd like to kick off
>> the discussion on how to correctly integrate the new hook management
>> system into existing LaTeX package codebases.
> 
> I've just tested latex-dev with TeX4ht and I can confirm that it
> fatally crashes with latex-dev.
> 

so it does, but then this isn't really surprising when looking at what 
tex4ht patches and at what level.

I can't claim that I understand the details of what it does, I only 
looked very briefly now.

But basically the main issue is that in the old LaTeX \document started 
out with \endgroup to cancel the \begingroup issued by \begin

As \begin now offers hooks this \endgroup has moved into \begin to avoid 
that the hook code is not canceled by the end of  the group.

The second issue is that the tex4ht calls \begin{document} inside 
\begin{document} again. There is also a call to \end{document} inside 
\document which doesn't do much other than breaking now.

Furthermore LaTeX's enddocument hook is directly modified (and not 
through \AtEndDocument and that fails because it is below the official 
interfaces.

------------

So to get tex4ht back up and running again only a small change to 
\document is necessary:

%FMi drop those lines from \document
%
%   \expandafter\let\csname enddocument\endcsname\empty
%   \end{document}%

Because of the double call to \begin{document} that leaves us with an 
open group at the end of the document. So either we need to explicitly 
close that group at end document, e.g.

    \AddToHook{enddocument/end}{\endgroup}

or probably better make a second change in the \document redefinition 
and add it there:

%FMi close the group started by \begin{document} on second call
%   \let\document\o:document
    \def\document{\endgroup\o:document}%


------------

Concerning the other issue in latex.4ht there is

\let\o:enddocumenthook\@enddocumenthook
\def\@enddocumenthook{\HtmlEnv
    \Configure{newpage}{}%
    \o:enddocumenthook
    \at:docend  \csname export:hook\endcsname  }

and \@enddocumenthook  isn't used any longer. So the redef goes to /dev/null

The idea here seems to be that

   \HtmlEnv\Configure{newpage}{}%

should be before the content of \AtEndDocument and

   \at:docend  \csname export:hook\endcsname

A possible replacement therefore using the hook management would be

  \AddToHook{env/document/end}
            {\HtmlEnv\Configure{newpage}{}}
  \AddToHook{enddocument/afterlastpage}
            {\at:docend  \csname export:hook\endcsname}

However this fails because tex4ht also overwrites the code for \begin 
and \end and therefore the generic hooks env/... lost

So without correcting/updating the redefinitions for \begin \end it has 
to be

  \AddToHook{enddocument}
            {\HtmlEnv\Configure{newpage}{}}
  \AddToHook{enddocument/afterlastpage}
            {\at:docend  \csname export:hook\endcsname}


------------------

with that everything seems to be working.

However, a lot of the lowlevel patching code inside tex4ht seems to be 
only necessary because environment hooks are not available, so after 
adjusting \begin and \end to use the kernel definitions it should be 
possible to simplify a lot by simply using appropriate "env/foo/before" 
and "env/foo/after" hooks.

frank