Using Expl3 token-list variables where token lists are called for

Question

The code below illustrates the trouble I’m having. It seems I cannot directly use a token-list variable where a token list is called for, but why, and how can I get the result I want?

\documentclass{article}
\usepackage{xparse}
\ExplSyntaxOn

\bool_new:N \found_bool

\cs_new_protected:Npn \find_token_in_tl:Nn #1#2
  {
    \bool_set_false:N \found_bool
    \tl_map_inline:nn { #2 }
      {
        \token_if_eq_charcode:NNT ##1 #1
          {
            \bool_set_true:N \found_bool
            \tl_map_break:
          }
      }
  }

\NewDocumentCommand { \FindTokInList } { m m }
  {
    \find_token_in_tl:Nn #1 {#2}
    \bool_if:NTF \found_bool { ! } { ? }
  }

\tl_const:Nn \punct_tl {,.;:}

\NewDocumentCommand { \FindTokInPunctList } { m }
  {
    \find_token_in_tl:Nn #1 \punct_tl
    \bool_if:NTF \found_bool { ! } { ? }
  }
\ExplSyntaxOff

\begin{document}
dot: \FindTokInList{.}{,.;:} % finds token & prints “!”

x: \FindTokInList{x}{,.;:} % doesn’t find token & prints “?”

dot: \FindTokInPunctList{.} % doesn’t find token & prints “?”, but why?

x: \FindTokInPunctList{x} % doesn’t find token & prints “?”
\end{document}

Answer 1

There are token lists and token list variables, which are n and N type, respectively. The LaTeX3 programming layer is careful not to expand anything it should not, so when you do

\tl_map_inline:nn {#2} { <code> }

with #2 = \l_JS_punct_tl you passing the single token \l_JS_punct_tl to the <code>, not the content of the variable. For that, you want

\tl_map_inline:Nn #2 { <code> }

You could also pass the value using

\cs_generate_variant:Nn \tl_map_inline:nn { V }
\tl_map_inline:Vn #2 { <code> }

but it's not necessary.

---

As an aside, LaTeX3 code variables should always be named

\<scope>_<module>_<description>_<type>

so I've used \l_JS_punct_tl in place of \punct_tl in the question.

Answer 2

There are already \tl_if_in:nnTF and \tl_if_in:NnTF:

\documentclass{article}
\usepackage{xparse}
\ExplSyntaxOn

\NewDocumentCommand { \FindTokInList } { m m }
 {
  \tl_if_in:nnTF {#2} {#1} { ! } { ? }
 }

\tl_const:Nn \c_JS_punct_tl {,.;:}

\NewDocumentCommand { \FindTokInPunctList } { m }
 {
  \tl_if_in:NnTF \c_JS_punct_tl { #1 } { ! } { ? }
  }
\ExplSyntaxOff

\begin{document}
dot: \FindTokInList{.}{,.;:} % finds token & prints !

x: \FindTokInList{x}{,.;:} % doesn't find token & prints ?

dot: \FindTokInPunctList{.} % finds token & prints !

x: \FindTokInPunctList{x} % doesn't find token & prints ?
\end{document}