How do you compact a code that expands \macro and store it in an expl3 sequence?

Question

I'm trying to rewrite some of the codes I have by following the correct expl3 rules. With the following code I define the scontents environment that encapsulates the filecontentsdefmacro environment of the filecontentsdef package using xparse/expl3. Everything works OK, I get to record the content of scontents in a sequence and then I can access it, the theme is the way I get it. The original package has the \filecontentsexec macro to run the content recorded by the filecontentsdefmacro environment, but, I haven't been able to make this work for me and have had to copy almost directly the definition directly from the package and write it in terms of expl3. Like this:

% Expand \l_scontents_macro_tmp_tl and pass to seq
\cs_new_protected:Nn \_scontents_macro_to_seq:
  {
    \tl_put_right:Nx \l_tmpa_tl
      {
         \tex_newlinechar:D = 13
         \tex_everyeof:D = { \exp_not:N }
         \exp_not:N \scantokens \exp_after:wN { \tl_use:c { l_scontents_macro_tmp_tl } }
         \tex_newlinechar:D = 10 \scan_stop:
       }
    \tl_put_right:Nx \l_tmpb_tl
      {
        \exp_not:N \_scontents_append_contents:nn 
          { 
            \exp_not:V \l_scontents_name_seq_tl 
          } 
          { \exp_not:V \l_tmpa_tl } 
      }
    \l_tmpb_tl % add to seq
  }

The problem is that I use (and abuse) the :D argument that as far as I've read, it shouldn't be occupied, my idea was to be able to use something shorter, similar to this:

% Expand \l_scontents_macro_tmp_tl and pass to seq
\cs_new_protected:Nn \_scontents_macro_to_seq:
  {
    \_scontents_append_contents:nn { \l_scontents_name_seq_tl } 
      { 
        \filecontentsexec \exp_after:wN { \tl_use:c { l_scontents_macro_tmp_tl } }
      }
  }

This is the complete example file:

\documentclass{article}
\usepackage{filecontentsdef}[2019/04/20]
\usepackage{xparse}
\ExplSyntaxOn
\tl_new:c { l_scontents_macro_tmp_tl }

\keys_define:nn { scontents }
  {
    save-env  .tl_set:N   = \l_scontents_name_seq_tl,
    save-env  .initial:n  = contents,
    show-env  .bool_set:N = \l_scontents_show_env_tl,
    show-env  .initial:n  = false  
  }

% Adapted from https://tex.stackexchange.com/a/215571/7832
\cs_new_protected:Npn \_scontents_append_contents:nn #1 #2
  {
    \seq_if_exist:cF { g_scontents_seq_name_#1_seq }
      {
        \seq_new:c { g_scontents_seq_name_#1_seq }
      }
    \seq_gput_right:cn { g_scontents_seq_name_#1_seq } { #2 }
  }

\cs_new_protected:Npn \_scontents_getfrom_seq:nn #1 #2
  {
    \seq_item:cn { g_scontents_seq_name_#2_seq } { #1 }
  }

\ProvideExpandableDocumentCommand{\getstored}{ O{1} m }
  {
    \_scontents_getfrom_seq:nn { #1 } { #2 }
  }

% Define environment ( wrap \filecontentsdefmacro ) 
% Adapted from https://tex.stackexchange.com/a/487746/7832
\ProvideDocumentEnvironment{ scontents }{}
  {
    \char_set_catcode_active:N \^^M
    \scontents_start_environment:w
  }
  {
    \scontents_stop_environment:
    \scontents_atend_environment:
  }

% Delaying [key=val] for environment
\cs_new_protected:Npn \scontents_environment_keys:w [#1]
  {
    \keys_set:nn { scontents } { #1 }
  }

% Star environment
\group_begin:
\char_set_catcode_active:N \^^M
\cs_new_protected:Npn \scontents_start_environment:w #1 ^^M
  {
    \tl_if_blank:nF { #1 } { \scontents_environment_keys:w #1 }
    \group_begin: % open a group for environment
    \use:c { filecontentsdefmacro } { \l_scontents_macro_tmp_tl } ^^M 
  }
\group_end:

% Stop environment
\cs_new_protected:Nn \scontents_stop_environment:
  {
    \endfilecontentsdefmacro 
    \group_end:  % close a group for environment
  }

% Expand \l_scontents_macro_tmp_tl and pass to seq
\cs_new_protected:Nn \_scontents_macro_to_seq:
  {
    \tl_put_right:Nx \l_tmpa_tl
      {
         \tex_newlinechar:D = 13
         \tex_everyeof:D = { \exp_not:N }
         \exp_not:N \scantokens \exp_after:wN { \tl_use:c { l_scontents_macro_tmp_tl } }
         \tex_newlinechar:D = 10 \scan_stop:
       }
    \tl_put_right:Nx \l_tmpb_tl
      {
        \exp_not:N \_scontents_append_contents:nn 
          { 
            \exp_not:V \l_scontents_name_seq_tl 
          } 
          { \exp_not:V \l_tmpa_tl } 
      }
    \l_tmpb_tl % add to seq
  }

% Execution of the code after finishing the scontent environment 
\cs_new_protected:Nn \scontents_atend_environment:
  {
    \_scontents_macro_to_seq:
    \bool_if:NT \l_scontents_show_env_tl
      {
        \_scontents_getfrom_seq:nn { -1 }{ \l_scontents_name_seq_tl }
      }
    \cs_undefine:N \l_scontents_macro_tmp_tl
  }
\ExplSyntaxOff
\setlength{\parindent}{0pt} % just for the example
\begin{document}
\section*{Test environment}
Test \verb+\begin{scontents}+ no \verb+[key=val]+\par

\begin{scontents}
Using \verb+scontents+ env no \verb+[key=val]+, save in 
\verb+contents+ with index 1. \footnote{AND footnotes !!}

\begin{verbatim}
      (A) verbatim environment
\end{verbatim}
\end{scontents}

Test \verb+\begin{scontents}[save-env=other]+\par

\begin{scontents}[save-env=other]
Using \verb+scontents+ env with \verb+[save-env=other]+, save in  
\verb+other+ with index 1.

\begin{verbatim*}
      (B) verbatim environment
\end{verbatim*}
\end{scontents}

\section*{Show stored contents}
\getstored[1]{other}
\getstored[1]{contents}

\end{document}

I have tried to write the code following the "rules" (and what I have learned in the forum), if something is badly written (related to expansion or cs), please correct me accordingly.

Greetings.

Answer 1

As far as I understand, you can't use \filecontentsexec the way you do. What you want is to pass the processed result of that macro to \_scontents_append_contents:nn. As \filecontentsexec is not fully expandable, this is not going to work. You should only use this macro in the normal execution mode, not as part of token list manipulations.

However, you can shorten your \_scontents_macro_to_seq: macro a bit, because the material inside \l_tmpa_tl of your first version is expandable. Using temporary variables is not necessary then, you can just pass the expanded result to \_scontents_append_contents:nn directly:

% Expand \l_scontents_macro_tmp_tl and pass to seq
\cs_gset_protected:Nn \_scontents_macro_to_seq:
  {
    \exp_args:NNx \_scontents_append_contents:nn \l_scontents_name_seq_tl 
      {
        \tex_newlinechar:D = 13
        \tex_everyeof:D = { \exp_not:N }
        \exp_not:N \scantokens \exp_after:wN { \tl_use:c { l_scontents_macro_tmp_tl } }
        \tex_newlinechar:D = 10 \scan_stop:
      }
  }

(If you are not familiar with the set of \exp_args: functions \exp_args:NNx \A \B \C is equivalent to \A \B {<full expansion of \C>}.)