Passing string containing "special" catcodes (especially `_`) to custom NewDocumentCommand command

Question

How can I pass underscore to \newcommand properly? suggests the solution is to use a zero-argument command that changes the catcode of _, before expanding to the actual command.

That's kind of the opposite of why I'd want to use \NewDocumentCommand, as it's kind of awkward, and also obfuscating argument types.

Is there a way to pass strings containing underscores (file names, by the way) without leaving the comfort of my expl3/xparse/\NewDocumentCommand bubble?

I don't want to use v arguments; it should really function like a normal macro, \includechapter{foo_bar} and not like \includechapter!foo_bar!.

---

My code currently:

%%% Chapter Inclusion Macros
\RequirePackage{expl3}
\ExplSyntaxOn
% Command to include chapter files, if
% either the exclusive chapter list is empty,
% or said chapter is in there
\cs_set:Npn \cel_includechapter:n #1 {
  % Check whether list is empty
  \clist_if_empty:NTF
  \g_cel_enabled_clist % which list
  {\include{#1/#1}} % if empty, just include
  { % else
    % check whether argument in list of enabled chapters
    \clist_if_in:NnTF 
    \g_cel_enabled_clist % in which list
    {#1} % which element to look for
    {\include{#1/#1}} % if in there
    {\chapter{#1~(currently~disabled)}} %if not in there
  }
}
% user-facing command \includechapter
% includes chaptername/chaptername
% if enabled
\NewDocumentCommand{\includechapter}{m}{
  \cel_includechapter:n{#1}
}
\NewDocumentCommand{\enableChapter}{m}{
  \clist_put_right:Nn \g_cel_enabled_clist {#1}
}
\ExplSyntaxOff

The build breaks when reaching

\includechapter{foo_bar}

with

! Missing $ inserted.
<inserted text>
                $
l.147 \includechapter{foo_bar}

Answer 1

The problem is where you say

{\chapter{#1~(currently~disabled)}} %if not in there

because an underscore in the file name would trigger math mode.

Solution: change into

{\chapter{\tl_to_str:n {#1}~(currently~disabled)}} %if not in there

Answer 2

The solution in this case is to use v-type arguments:

\NewDocumentCommand{\includechapter}{v}{
  \cel_includechapter:n{#1}
}
\NewDocumentCommand{\enableChapter}{v}{
  \clist_put_right:Nn \g_cel_enabled_clist {#1}
}

This still requires more fixing, since the argument passed through needs to get processed before it can be typeset if it contains underscores.

Luckily, I don't want to use this macro as argument to another function (usrguide3.pdf):

v: Reads an argument ‘verbatim’, between the following character and its next occurrence, in a way similar to the argument of the LATEX 2ε command \verb. Thus a v-type argument is read between two identical characters, which cannot be any of %, \, #, {, } or ␣. The verbatim argument can also be enclosed between braces, { and }. A command with a verbatim argument will produce an error when it appears within an argument of another function.