Hook after \par, even if issued in vertical mode

[ysalmon]

Brief outline of the enhancement

Currently, there are hooks para/end and para/after that allow for putting code at the end of a paragraph, just before or just after TeX has returned to vertical mode. Due to the definition of \para_end:, these hooks are only executed when that command (for which initially \par is synonymous) is issued in horizontal mode. This makes sense most of the time because, if that is not the case, there is not an actual paragraph to end.

However, there are instances when one would want to execute a hook whenever a \para_end: command is issued. For example, doing something special when two \par are issued in a row, which can help to discriminate whether there was an empty line after an environment the code of which ends with a \par.

Minimal example showing the current behaviour

(this may not be the best use case, but is is short and very visual)

Another case is the following, where \@setpar{{\@@par}} has been eschewed in favour of a hook to retain \parshape for several paragraphs. The effect is lost on \lipsum[4] because it is following two \par, the second of which did not execute the para/after hook but did the effects of \tex_par:D.

\RequirePackage{latexbug}
\documentclass{article}
\usepackage{lipsum}

\makeatletter
\NewDocumentEnvironment{indented}{}
{\addtolength{\leftmargin}{2cm}
 \addtolength{\@totalleftmargin}{2cm}
 \addtolength{\linewidth}{-2cm}
 \parshape 1 \@totalleftmargin \linewidth
 \AddToHook{para/after}[test]{\parshape 1 \@totalleftmargin \linewidth}
}
{
  \RemoveFromHook{para/after}[test]
}

\begin{document}
\begin{indented}
\lipsum[1]\par\lipsum[2]\par\lipsum[3]\par\par\lipsum[4]\par\lipsum[5]
\end{indented}

\end{document}

The temptation is high to say \AddToHook{cmd/par/after}. This is discouraged, and the reason for this is probably that the first invokation of that command will rewrite the definition of \par to add the hook mechanism at the end. However, when \par is modified then restored at various times, it will be restored to the meaning of \@@par, that is the original definition of \para_end:, which will not use the cmd/par/after hook.

Proposal

To provide such a hook from the beginning, under the name para/afterpar, by defining

\hook_new_reversed:n {para/afterpar}  %%% NEW

\cs_new_protected:Npn \para_end: {
  \scan_stop:
  \mode_if_horizontal:TF {
    \mode_if_inner:F {
         \tex_unskip:D
         \hook_use:n{para/end}
         \@kernel@after@para@end
         \mode_if_horizontal:TF {
           \if_int_compare:w 11 = \tex_lastnodetype:D
             \tex_hskip:D \c_zero_dim
           \fi:
           \tex_par:D
           \hook_use:n{para/after}
           \@kernel@after@para@after
         }
         { \msg_error:nnnn { hooks }{ para-mode }{end}{horizontal} }
    }
  }
  \tex_par:D
  \hook_use:n{para/afterpar}  %%% NEW
}

This will allow to eschew \AddToHook{cmd/par/after} in favour of \AddToHook{para/afterpar}.

Backward compatibility should not be a problem, since for existing code, the hook will remain empty.
There is of course some danger if one puts code in the hook that creates a new paragraph, but the same problem exist for the para/after hook, and this is correctly documented.

I am not suggesting to add a similar para/beforepar hook because :

• it looks more involved to implement right,
• it is less useful, because the only novelty would be to execute code before \par when it is issued in vertical mode, but in that precise case just before and just after \par are not very different places.

[muzimuzhi]

Perhaps you can use command hook cmd/<name>/after, though patching internal functions might not be recommended. See texdoc ltcmdhooks-doc for more info.

\documentclass{article}
\ExplSyntaxOn
% this will append "\UseHookWithArguments{cmd/para_end:/after}{0}" to
% \para_end: after the "begindocument" hook
\hook_gput_code:nnn { cmd/para_end:/after } {.} {}

\hook_gput_code:nnn {begindocument/end} {.} { \cs_show:N \para_end: }
\ExplSyntaxOff

\begin{document}
\end{document}

[ysalmon]

Yes, this would be similar but not quite, because the kernel does \cs_set_eq:NN \par before the hook code is appended. The idea of the present RFE is to apply this logic very early. -- Cordialement, Yann Salmon Depuis mon téléphone ; excusez la brièveté.

[FrankMittelbach]

Perhaps you can use command hook cmd/<name>/after, though patching internal functions might not be recommended. See texdoc ltcmdhooks-doc for more info.

Patching into the internals of the paragraph handling is a real no-go (and it doesn't help if that is done with a cmd hook), it will most certainly backfire eventually. In a recent issue I already worte that we will strengthen the documentation to say this should not be done --- and if necessary test that certain commands aren't manipulated by cmd hooks.

Despite of that I'll will take a look if there is a safe way to augment the paragraph handling in that direction with a further hook but it will take some time.

[ysalmon]

Thanks you for looking into the feasibility of this. I think I was the author of the other issue you are referring to : indeed, hooking into \par is tempting (@muzimuzhi's comment also shows this is a logical temptation) but dangerous. The idea is to make it so that people no longer have a reason to try to do so.

I will be using the proposed variation of \para_end: in my own documents and will report here if I discover adverse effects.

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