\AddToHook unexpected side effect

[jlaurens]

Brief outline of the bug

Assume \FOO is not defined.
After \AddToHook{cmd/FOO/before}{} \FOO is \relax and no longer breaks when used.

This should not be considered a feature.

Minimal example showing the bug

\RequirePackage{latexbug}       % <--should be always the first line (see CONTRIBUTING)!
\documentclass{article}
\AddToHook{cmd/FOO/before}{}
\begin{document}
\show\FOO
\end{document}

Log file (required) and possibly PDF file

This is pdfTeX, Version 3.141592653-2.6-1.40.26 (TeX Live 2024) (preloaded format=pdflatex)
 restricted \write18 enabled.
entering extended mode
(./trashme.tex
LaTeX2e <2024-06-01> patch level 1
L3 programming layer <2024-05-27>
(/usr/local/texlive/2024/texmf-dist/tex/latex/latexbug/latexbug.sty)
(/usr/local/texlive/2024/texmf-dist/tex/latex/base/article.cls
Document Class: article 2024/02/08 v1.4n Standard LaTeX document class
(/usr/local/texlive/2024/texmf-dist/tex/latex/base/size10.clo))
(/usr/local/texlive/2024/texmf-dist/tex/latex/l3backend/l3backend-pdftex.def)
(./trashme.aux)
> \FOO=\relax.
l.7 \show\FOO
             
? 

[muzimuzhi]

The \relax assignment happens at the c-type expansion of \exp_args:Nc \__hook_patch_cmd_or_delay:Nnn inside \__hook_try_put_cmd_hook:w:

latex2e/base/ltcmdhooks.dtx

Lines 584 to 589 in 7d4ec54

	\cs_new_protected:Npn \@@_try_put_cmd_hook:w
	#1 / #2 / #3 / #4 \s_@@_mark #5
	{
	\@@_debug:n { \iow_term:n { ->~Adding~cmd~hook~to~'#2'~(#3): } }
	\exp_args:Nc \@@_patch_cmd_or_delay:Nnn {#2} {#2} {#3}
	}

Luckyly, as \__hook_patch_cmd_or_delay:Nnn only do global assignment(s), wrapping it in an extra group does the trick:

\RequirePackage{latexbug}       % <--should be always the first line (see CONTRIBUTING)!
\documentclass{article}

\ExplSyntaxOn
\cs_undefine:N \__hook_try_put_cmd_hook:w
\cs_new_protected:Npn \__hook_try_put_cmd_hook:w
    #1 / #2 / #3 / #4 \s__hook_mark #5
  {
    \__hook_debug:n { \iow_term:n { ->~Adding~cmd~hook~to~'#2'~(#3): } }
    \group_begin: % <<< added
    \exp_args:Nc \__hook_patch_cmd_or_delay:Nnn {#2} {#2} {#3}
    \group_end: % <<< added
  }
\ExplSyntaxOff

\AddToHook{cmd/FOO/before}{}

\begin{document}
\show\FOO
\end{document}

[FrankMittelbach]

@muzimuzhi I was thinking along the same lines, but I don't like to introduce a dependency of relying on \__hook_patch_cmd_or_delay:Nnn using only gloal assignments, so I do it slightly differently. The alternative is to fully take out the first argument and generate it later (but that requires more surgery.

[Rimole]

@FrankMittelbach

ef35caf#diff-a37b1fd2a3d174eb379905a10f50d81a694fa77b3f8252390a53076ff6bb7933

base/doc/ltnews41.tex line 378:

it no longer raised and \enquote{Undefined csname} error but silently

Replace and by an , please.

[FrankMittelbach]

already done a few hours ago 76ecf9d