Convert token to string in LaTeX3

Question

While trying to answer another question on this site, I was experimenting with \peek_after:Nw and \l_peek_token.

I can set \l_peek_token OK, but then I want to test if this token is one of several characters, rather than just one character. (Just one character is easy and I could use \peek_meaning:NTF.)

I thought that I would be able to easily convert l_peek_token to a string and then use \str_if_in:nnTF for the test. But, I don't know how to do this.

I know I could use a bunch of nested \token_if_eq_meaning:NNTF, but I find this syntax clumsy.

How can I do this?

MWE

\documentclass{article}
\usepackage{xparse}
\ExplSyntaxOn
\cs_new:Nn \__dcp_test_punct:
  {
    % What goes here to convert \l_peek_token to the right format?
    \str_if_in:nnTF {.,} { \l_peek_token }
      { ~Punctuation~found:~ }
      { ~Punctuation~not~found~ }
    ---
    % This works, but I find the syntax clumsy.
    \token_if_eq_meaning:NNTF \l_peek_token .
      { ~Punctuation~found:~ }
      {
        \token_if_eq_meaning:NNTF \l_peek_token ,
          { ~Punctuation~found:~ }
          { ~Punctuation~not~found~ }
      }
  }
\cs_new:Nn \__dcp_test_punct:n
  {
    \peek_after:Nw \__dcp_test_punct:
  }
\DeclareDocumentCommand {\testpunct} { m }
  {
    \__dcp_test_punct:n {#1}
  }
\ExplSyntaxOff
\begin{document}
\testpunct{A}.

\testpunct{A},

\testpunct{A}
\end{document}

Answer 1

Further thinking along with @egreg's comments have led me to do things in a slightly different way and use an inline map. It's not quite as compact as I might have hoped to achieve with \str_if_in:nnTF, but at least it doesn't involve nested boolean checks.

\documentclass{article}
\usepackage{xparse}
\ExplSyntaxOn
\bool_new:N \l__dcp_punct_found_bool
\cs_new_protected:Nn \__dcp_test_punct:
  {
    \bool_set_false:N \l__dcp_punct_found_bool
    \str_map_inline:nn { .,:;!? }
      {
        \token_if_eq_meaning:NNT \l_peek_token ##1
          {
            ~Punctuation~found:~
            \bool_set_true:N \l__dcp_punct_found_bool
            \str_map_break:
          }
      }
    \bool_if:NF \l__dcp_punct_found_bool
      { ~Punctuation~not~found~ }
  }
\cs_new_protected:Nn \__dcp_test_punct:n
  {
    \peek_after:Nw \__dcp_test_punct:
  }
\DeclareDocumentCommand {\testpunct} { +m }
  {
    \__dcp_test_punct:n {#1}
  }
\ExplSyntaxOff
\begin{document}
\testpunct{A}.

\testpunct{A},

\testpunct{A}:

\testpunct{A};

\testpunct{A}!

\testpunct{A}?

\testpunct{A}
\end{document}

Answer 2

Sorry, I was just playing around with \futurelet. The idea of turning the resulting implicit-character token (related: A Question About \futurelet) into something explicit with the use of \meaning was suggested to me by Christian Tellechea.

Christian actually provided (under separate cover) a macro \implicittomacro that converts catcode 11/12 implicit-character tokens into a macro. You can go to my answer A Question About \futurelet, to see his macro.

My goal was to be able to use the features of the listofitems package to search the implicit token's value for multiple punctuation values all at once, as detailed in the specification \setsepchar{.||,||:||;||!||?}. Success!

\documentclass{article}
\usepackage{listofitems,xcolor}
\newcommand\testpunct[1]{#1\futurelet\xfl\pdecide}
\def\pdecide{\bgroup\tiny\color{cyan}%
  \edef\ximp{\meaning\xfl}%
  \edef\tmp{\detokenize{the character }}%
  \expandafter\setsepchar\expandafter{\tmp}%
  \readlist\mylist{\ximp}%
  \ifnum\listlen\mylist[]<2\relax (Non catcode=12 follows)\else%
    \setsepchar{.||,||:||;||!||?}%
    \itemtomacro\mylist[2]\thecharacter%
    \readlist*\mychar{\thecharacter}%
    \ifnum\listlen\mychar[]=1\relax (Non-punctuation character follows \mychar[1])\else%
      (This Punctuation found: \mycharsep[1])%
    \fi%
  \fi%
\egroup}
\begin{document}
\testpunct{A}\relax

\testpunct{A}B

\testpunct{A}@

\testpunct{A}.

\testpunct{A},

\testpunct{A}:

\testpunct{A};

\testpunct{A}!

\testpunct{A}?

\testpunct{A}

\end{document}