Avoid passing an empty [optional argument] an environment (with verbatim content) using xparse?

Question

this query is related to How to pass an optional argument to an environment with verbatim content? (whose solution did not work in my case) and with Emulating @ifnextchar in expl3 (which is the one I'm trying to adapt).

I have defined an environment (listcontents) using xparse which encapsulates \filecontentsdef (which is verbatim plus \macro) using the argument !O{} and [key = val] which works well if ALWAYS pass the optional argument.

My idea is to be able to write the environment without having to use an empty optional argument [], this is where I am lost, I have read the documentation of \peek_meaning:NF but I do not understand it at all (the \catcodes is confusing for me).

It works correctly if use:

\begin{listcontents}[]
\begin{listcontents}[key=val]
\begin{listcontents}[
                    key=val
                    ]

and I would like to do it in the following way:

\begin{listcontents}
\begin{listcontents}
[not key = value, only a bracket whit text]

that is, if you do not find [key = val] right after }, insert a line ending \^^M and [not key = value, just a square bracket with text] would be recorded by the environment. The package xsim (which is written only usingexpl3) works similar to filecontents, has a code to solve this problem, but, it is very complicated to adapt for me.

This is my sample file:

\documentclass{article}
\usepackage{fancyvrb,filecontentsdef,xparse}
\pagestyle{empty}
\begin{document}
\makeatletter
\ExplSyntaxOn
\keys_define:nn { listcontents }
  {
    save-env   .tl_set:N         = \l_env_save_tl,
    save-env   .initial:n        = content,
    show-env   .bool_set:N       = \l_env_show_tl,
    show-env   .initial:n        = true,
    name-tmp   .tl_set:N         = \l_tmp_name_tl,
    name-tmp   .initial:n        = \jobname.tsc,
  }

\NewDocumentEnvironment{ listcontents }{ !O{} }
  {
  \group_begin:
  \IfNoValueF { #1 } { \keys_set:nn { listcontents } { #1 } }
  %\peek_meaning_ignore_spaces:NF [ { \char_set_catcode_active:N \^^M \char_set_catcode_end_line:N \^^M  \\ }
  %\peek_meaning:NF \c_space_tl { \char_set_catcode_active:N \^^M \char_set_catcode_end_line:N \^^M  \^^M  }
  % Acording a doc of v1.4 this its a correct line 
   \csname filecontentsdef*\endcsname{ \l_tmp_name_tl }{ \l_tmpa_tl }
   }
  {
  \endfilecontentsdef
  \group_end:
  \group_begin:
  \IfNoValueF { #1 } { \keys_set:nn { listcontents } { #1 } }
  \IfBooleanT{ \l_env_show_tl } { \filecontentsexec\l_tmpa_tl }
  \group_end:
  }
\ExplSyntaxOff
\makeatother
\section{[not key = value, just a square bracket with text]}
\begin{listcontents} 
[not key = value, just a square bracket with text]
This is correct for key=val, but space between letters in [...] disappeared
% some comented lines
\begin{verbatim*}
\begin{listcontents}
[not key = value, just a square bracket with text]
\end{verbatim*}
and \verb+[not key = value, just a square bracket with text]+ its not save 
in \verb+\jobname.tsc+ and \verb+\l_tmpa_tl+.
\end{listcontents}

The \textbf{file generate} is:
\VerbatimInput[frame=single]{\jobname.tsc}

\section{Omit a [optional argument] after begin\{listcontents\} }
\begin{listcontents}
This is what I want to happen when omit []
% some comented lines
\begin{verbatim*}
\begin{listcontents}
This is what I want to happen when omit []
% some comented lines
\end{verbatim*}
if ommit \verb*+[ ]+ the first lines and and space between 
letters disappeared, the contents of environments not save 
in \verb+\jobname.tsc+ and \verb+\l_tmpa_tl+.
\end{listcontents}

The \textbf{file generate} is:
\VerbatimInput[frame=single]{\jobname.tsc}
\end{document}

The output looks like this:

Can this be done using xparse/expl3?

regards

Answer 1

You have to delay the search for the optional argument, because otherwise the endline character after \begin{listcontent} is subject to tokenization and converted to a space before \filecontentsdef can act

\documentclass{article}
\usepackage{fancyvrb,filecontentsdef,xparse}
\pagestyle{empty}

\makeatletter
\ExplSyntaxOn
\keys_define:nn { listcontents }
  {
    save-env   .tl_set:N         = \l_env_save_tl,
    save-env   .initial:n        = content,
    show-env   .bool_set:N       = \l_env_show_bool,
    show-env   .initial:n        = true,
    name-tmp   .tl_set:N         = \l_tmp_name_tl,
    name-tmp   .initial:n        = \c_sys_jobname_str.tsc,
    save-macro .bool_set:N       = \l_save_macro_bool,
    save-macro .initial:n        = true,
  }

\NewDocumentEnvironment{ listcontents }{}
  {
   \char_set_catcode_active:N \^^M
   \pedro_start_listcontents:w
  }
  {
   \endfilecontentsdef
   \group_end:
   \IfBooleanT{ \l_env_show_bool } { \filecontentsexec\l_tmpa_tl }
  }

\cs_new_protected:Npn \pedro_listcontents_set:w [#1]
 {
  \keys_set:nn { listcontents } { #1 }
 }

\group_begin:
\char_set_catcode_active:N \^^M
\cs_new_protected:Npn \pedro_start_listcontents:w #1 ^^M
 {
  \tl_if_blank:nF { #1 } { \pedro_listcontents_set:w #1 }
  \group_begin:
  \use:c { filecontentsdef* } { \l_tmp_name_tl } { \l_tmpa_tl } ^^M
  }
\group_end:
\ExplSyntaxOff
\makeatother

\begin{document}

\section{No optional argument}
\begin{listcontents}
[text in brackets]
Some text
% a comment
\begin{verbatim*}
\foo
\end{verbatim*}
Some text
\end{listcontents}

The \textbf{generated file} is:
\VerbatimInput[frame=single]{\jobname.tsc}

\section{Optional argument}
\begin{listcontents}[name-tmp=\jobname-2.tsc]
Some text
% a comment
\begin{verbatim*}
\foo
\end{verbatim*}
Some text
\end{listcontents}

The \textbf{generated file} is:
\VerbatimInput[frame=single]{\jobname-2.tsc}
\end{document}