Why my patch to addcontentsline is broken in \AtEndPreamble and \bool_if:NT?

Question

I want to patch the \thepage in the \addcontentsline to 2*\c@page + 1 if some bool in preamble is true. I simplify my code to this:

\documentclass{article}
\usepackage{etoolbox}
\makeatletter
\ExplSyntaxOn
\AtEndPreamble
{
  \bool_if:NT \c_true_bool
    {\patchcmd{\addcontentsline}{\thepage}{\int_eval:n {2 * \c@page + 1}}{}{\fail}}
}
% \patchcmd{\addcontentsline}{\thepage}{\int_eval:n {2 * \c@page + 1}}{}{\fail}
\ExplSyntaxOff
\makeatother
\begin{document}
  \tableofcontents
  \section{test}
\end{document}

Things in .toc are

\contentsline {section}{\numberline {1}test}{\int _eval:n {2*\c@page +1}}{}%

We can see that \int _eval:n {2*\c@page +1} is broken and wasn't calculated to 3. However if I remove either \AtEndPreamble or \bool_if:NT \c_true_bool, I get

\contentsline {section}{\numberline {1}test}{1}{}%

1 is still 1. This seems that `\patchcmd' only works in group, is there a "global" version?

If I remove both commands and remove the group, 1 changes to 3, which is what I want.

My question is why would this happen and how can I fix it?

---

EDIT: In this example, @user202729 's method works. However, if I add hyperref, \thepage in \addcontentsline is unpatchable any more.

[debug] tracing \ifpatchable on input line 23
[debug] analyzing '\addcontentsline'
[debug] ++ control sequence is defined
[debug] ++ control sequence is a macro
[debug] -- macro cannot be retokenized cleanly
[debug] -> the macro may have been defined under a category
[debug]    code regime different from the current one
[debug] -> the replacement text may contain special control
[debug]    sequence tokens formed with \csname...\endcsname;
[debug] -> the replacement text may contain carriage return,
[debug]    newline, or similar characters

Similar question: hyperref and \addcontentsline's patch in expl3 block

Answer 1

There are several problems with your code, the main one is due to category codes. What happens?

1. You need \ExplSyntaxOn in order to write the code, so TeX interprets correctly \int_eval:n.

2. The patch is performed in a different context, when \ExplSyntaxOn is no longer in force.

This is where \patchcmd fails. We need to know a bit of how it works. There is no way in TeX to access the replacement text of a macro without knowing precisely what its parameter text is and there is no way to know precisely what the parameter text is. We can just examine and save the parameter and replacement texts through \meaning, which however produces them as strings of category code 12 characters (but catcode 10 for spaces).

So \patchcmd does this and first of all it rebuilds a copy of the macro by means of \scantokens in order to compare it with the macro to be patched. If the comparison fails, \patchcmd makes @ of category code 11 and tries again the rebuild-and-compare. If also this attempt fails, \patchcmd gives up and tells you that the macro is not patchable. Otherwise it proceeds to perform the patch. Again, it has to use strings of category code 12 characters and do \scantokens. Here's where your case fails: when the patch is attempted, _ has category code 8, but your code wants it to have category code 11 and \patchcmd has no way at all to know this, unless you tell it.

\documentclass{article}
\usepackage{etoolbox}
\makeatletter
\ExplSyntaxOn
\AtEndPreamble
 {
  \bool_if:NT \c_true_bool
   {
    \ExplSyntaxOn
    \patchcmd{\addcontentsline}{\thepage}{\int_eval:n {2 * \c@page + 1}}{}{\fail}
    \ExplSyntaxOff
   }
 }
\ExplSyntaxOff
\makeatother
\begin{document}
\tableofcontents
\section{test}
\end{document}

This works because the macro to be patched hasn't spaces, _ or : in its parameter or replacement texts. Otherwise the patch would fail for the same reason as before.

A simpler strategy: do the patch in a safe place and avoid “strange” category codes.

\documentclass{article}
\usepackage{etoolbox}
\usepackage{hyperref}
\NewCommandCopy{\patchedaddcontentsline}{\addcontentsline}
\patchcmd{\patchedaddcontentsline}{\thepage}{\makeodd{page}}{}{\fail}
\ExplSyntaxOn
\NewExpandableDocumentCommand{\makeodd}{m}
 {
  \int_eval:n {2*\value{#1}+1}
 }
\AtEndPreamble
 {
  \bool_if:NT \c_true_bool
   {
    \RenewCommandCopy{\addcontentsline}{\patchedaddcontentsline}
   }
 }
\ExplSyntaxOff
\begin{document}
\tableofcontents
\section{test}
\end{document}

Answer 2

1. As the documentation states

   The assignment is local.

   You can't make it global.

   Although... you can cheat like this

   \patchcmd{\addcontentsline}{\thepage}{\int_eval:n {2 * \c@page + 1}}{}{\fail}
   \global\let\addcontentsline\addcontentsline
   

   which seems to work but I'm not sure what bad effect it has on the save stack.

   Alternatively, copy the definition of \patchcmd and add \global at appropriate places.

2. For the catcode part, if the documentation has something you don't understand, don't just skip through it. Important information are written there.

   Note that the patching process involves detokenizing the replacement text of the ⟨command⟩ and retokenizing it under the current category code regime after patching. The category code of the @ sign is temporarily set to 11. If the replacement text of the ⟨command⟩ includes any tokens with non-standard category codes, the respective category codes must be adjusted prior to patching.

   In particular

   • here the "replacement text" consist of \int_eval:n {2 * \c@page + 1}.
   • which includes the token \int_eval:n with non-standard category code (\ExplSyntaxOn region).

   So one way is to do it like this.

   \documentclass{article}
   \usepackage{etoolbox}
   \makeatletter
   \ExplSyntaxOn
   \AtEndPreamble
   {
     \bool_if:NT \c_true_bool
       {
       \ExplSyntaxOn
       \patchcmd{\addcontentsline}{\thepage}{\int_eval:n {2 * \c@page + 1}}{}{\fail}
       \ExplSyntaxOff
   }
   }
   % \patchcmd{\addcontentsline}{\thepage}{\int_eval:n {2 * \c@page + 1}}{}{\fail}
   \ExplSyntaxOff
   \makeatother
   \begin{document}
     \tableofcontents
     \section{test}
   \end{document}
   

   For \makeatletter and \makeatother, you happen-to not need because it's automatic, as mentioned in the documentation above. But for expl3 code, you do need it.

   Side note, you still need the outer \ExplSyntaxOn, you can't change catcode inside a command argument, see macros - Why doesn't \makeatletter work inside \newcommand? - TeX - LaTeX Stack Exchange.

   Alternatively, make the ⟨replacement text⟩ safe under normal catcode with a wrapper command like this.

   \documentclass{article}
   \usepackage{etoolbox}
   \makeatletter
   \ExplSyntaxOn
   \cs_new:Npn \myExpandableCommandSafeUnderNormalCatcode {
       \int_eval:n {2 * \c@page + 1}
   }
   \AtEndPreamble
   {
     \bool_if:NT \c_true_bool
       {
       \patchcmd{\addcontentsline}{\thepage}{\myExpandableCommandSafeUnderNormalCatcode}{}{\fail}
   }
   }
   % \patchcmd{\addcontentsline}{\thepage}{\int_eval:n {2 * \c@page + 1}}{}{\fail}
   \ExplSyntaxOff
   \makeatother
   \begin{document}
     \tableofcontents
     \section{test}
   \end{document}
   

Answer 3

Use \inteval instead of \int_eval:n. You can also replace \c@page by \value{page}:

\documentclass{article}
\usepackage{etoolbox}
\usepackage{hyperref}
\makeatletter
\ExplSyntaxOn
\AtEndPreamble
{
  \bool_if:NT \c_true_bool
    {    
    \patchcmd{\addcontentsline}{\thepage}{\inteval {2 *  \value{page} + 1}}{}{\fail}    
}
}
\ExplSyntaxOff
\makeatother
\begin{document}
  \tableofcontents
  \section{test}
\end{document}