listings package and latex3 variable

Question

I'm trying to change a custom environment from a lot of optional parameters to a ;-separated list (using this answer as a template and writing its main functionality as a latex3 macro).

The macro should put each of the tokens in the list on a separate line, formatted with a monospace font (it's source code). I previously used \lstinline from listings, but it seems to be incompatible with the latex3-variable. Using \texttt instead works.

I'd rather stay with \lstinline than with \texttt, because I need the postbreak feature it provides for marking breaks in long lines.

To reproduce the problem:

\documentclass{minimal}
\usepackage{listings,xparse}

\lstset{basicstyle=\ttfamily,breaklines=true,columns=fullflexible}

\NewDocumentCommand{\code}{m}{\textbf{\texttt{#1}}}  % works

% uncomment for error
%\NewDocumentCommand{\code}{m}{\lstinline{#1}}       % results in "Improper
                                                     % alphabetic constant."

\ExplSyntaxOn
\NewDocumentCommand{\methods}{m}{\methods_main:n {#1}}

\seq_new:N \l_methods_body_seq
\cs_new:Npn \methods_main:n #1
{
    \noindent
    \seq_set_split:Nnn \l_methods_body_seq {;} { #1 }
    \seq_pop_right:NN \l_methods_body_seq \l_tmpa_tl
    \seq_map_inline:Nn \l_methods_body_seq { \code{##1} \\ }
    \code{\tl_use:N \l_tmpa_tl} %<---- error occurs in this line
}
\ExplSyntaxOff

\begin{document}

\methods{monoSpacedLine1();int methodSignature();const int ANSWER 42}

\end{document}

The error occurs with every argument to the \methods macro I tried. I also already tried using \tl_to_str:N instead of \tl_use:N, or putting extra braces around it.

Is it possible to expand the variable in a way that \lstinline accepts it?

Answer 1

You have to expand the token list before \code can see it; the cheapest method is with \exp_args:NV:

\documentclass{article}
\usepackage{listings,xparse}

\lstset{basicstyle=\ttfamily,breaklines=true,columns=fullflexible}
\NewDocumentCommand{\code}{m}{\lstinline{#1}}

\ExplSyntaxOn
\NewDocumentCommand{\methods}{m}{\methods_main:n {#1}}

\seq_new:N \l_methods_body_seq
\tl_new:N \l_methods_last_tl
\cs_new_protected:Npn \methods_main:n #1
 {
  \noindent
  \seq_set_split:Nnn \l_methods_body_seq {;} { #1 }
  \seq_pop_right:NN \l_methods_body_seq \l_methods_last_tl
  \seq_map_inline:Nn \l_methods_body_seq { \code{##1} \\ }
  \exp_args:NV \code \l_methods_last_tl
 }
\ExplSyntaxOff

\begin{document}

\methods{monoSpacedLine1();int methodSignature();const int ANSWER 42}

\end{document}

This will produce the same as

\code{<value of \l_methods_last_tl>}

which is what's desired.

Note that \methods_main:n is defined with \cs_new_protected:Npn because it does "unexpandable things" such as setting variables.

However, a preferred method would be to use an "inner" function also for \code:

\documentclass{article}
\usepackage{listings,xparse}

\lstset{basicstyle=\ttfamily,breaklines=true,columns=fullflexible}

\ExplSyntaxOn
\NewDocumentCommand{\methods}{m}{\methods_main:n {#1}}
\NewDocumentCommand{\code}{m}{\methods_code:n {#1}}

\cs_new_protected:Npn \methods_code:n #1
 { \lstinline{#1} }
\cs_generate_variant:Nn \methods_code:n { V }

\seq_new:N \l_methods_body_seq
\tl_new:N \l_methods_last_tl
\cs_new_protected:Npn \methods_main:n #1
 {
  \noindent
  \seq_set_split:Nnn \l_methods_body_seq {;} { #1 }
  \seq_pop_right:NN \l_methods_body_seq \l_methods_last_tl
  \seq_map_inline:Nn \l_methods_body_seq { \methods_code:n {##1} \\ }
  \methods_code:V \l_methods_last_tl
 }

\ExplSyntaxOff

\begin{document}

\methods{monoSpacedLine1();int methodSignature();const int ANSWER 42}

\end{document}