LaTeX3 how to use content/value of predefined command in token list/string?

Question

First of all, I guess that will be a very easy answer for people who are experienced in expl3. But for me its new and I'm very willing to learn that stuff to understand Latex3 better. So I hope for some pro help here.

What i want to achieve is the following:

First an userdefined command which has a simple sentence as content. Let's say \NewDocumentCommand{\testcommand}{}{A Test Sentence for expl3}

Now I want to store the content of the command in a token list or string. So that it can be printed/used or counted. The latter is needed, because I want to use the number of characters in an if-condition which activates different commands depending on the total count.

My MWE looks like this:

\documentclass[%
]{article}
\usepackage[T1]{fontenc}

\NewDocumentCommand{\testcommand}{}{A Test Sentence for expl3}

\ExplSyntaxOn
\tl_new:N \test_tl

\tl_set:Nn \test_tl {\testcommand}

\NewDocumentCommand{\showteststring}{}{
    \tl_use:N \test_tl \par
    \tl_count:N \test_tl
}

\NewDocumentCommand{\countteststring}{}{
    \tl_to_str:N \test_tl \par
    \str_count:N \test_tl
}
\ExplSyntaxOff

\begin{document}

    \showteststring
    
    \countteststring
    
\end{document}

\showteststring prints the content of \testcommand correctly using \tl_use:N, but \tl_count:N only counts one token; of course this token is \testcommand.

The same for the for \countteststring. It converts only the command-sequence \testcommand into a string and counts the 13 characters of the command-sequence itself.

So I need to expand \testcommand first. But my tries so far didn't offer any success. For instance, I tried:

\tl_set:Nx \test_tl {\testcommand} (with x) instead of \tl_set:Nn \test_tl {\testcommand}. It compiles fine, but the output remains the same.

I also tried some variants with \exp_args:N, but didn't get it to work, because I don't fully understand where to place it and which argument specifier to use in expl3 syntax.

I also thought about the v/V argument specifier, but couldn't get it to work.

I looked into the interface3 doc. But its overwhelming in case of informations and I couldn't figure out the right commands so far. Further, I searched Tex.SE and read the blog by Ziyue Xiang; also very informative, but not the specific solution I'm looking for (or, of course, I may have overlooked something).

Thus, I am already thankful in advance for any help or hint! For an explanation I would also be very grateful, so I understand it better next time!

Answer 1

You have declared \testcommand as a document command: these are protected from expansion. That means that \tl_set:Nx \l_test_tl { \testcommand } doesn't change the result. I would declare \testcommand as an expandable command and use V-type expansion:

\newcommand*\testcommand{A Test Sentence for expl3}
\tl_set:NV \l_test_tl \testcommand

etc.

Answer 2

The \NewDocumentCommand facility is aimed at offering more powerful argument parsing than the legacy \newcommand. It is not at all suitable for defining “text containers”.

If you want text containers, define an infrastructure for them.

\documentclass{article}

\ExplSyntaxOn

\NewDocumentCommand{\definecontainer}{mm}
 {
  \tl_clear_new:c { g_lukeflo_cont_#1_tl }
  \tl_gset:cn { g_lukeflo_cont_#1_tl } { #2 }
 }

\NewExpandableDocumentCommand{\usecontainer}{m}
 {
  \tl_use:c { g_lukeflo_cont_#1_tl }
 }

\NewExpandableDocumentCommand{\countcontainer}{sm}
 {
  \IfBooleanTF { #1 }
   {% count as string
    \str_count:e { \tl_to_str:v { g_lukeflo_cont_#2_tl } }
   }
   {% count as tl
    \tl_count:c { g_lukeflo_cont_#2_tl }
   }
 }

\ExplSyntaxOff

\definecontainer{test}{A Test Sentence for expl3}
\definecontainer{test2}{abc}

\begin{document}

`\usecontainer{test}' has \countcontainer{test} items

`\usecontainer{test}' has \countcontainer*{test} tokens

`\usecontainer{test2}' has \countcontainer{test2} items

`\usecontainer{test2}' has \countcontainer*{test2} tokens

\end{document}

Note that \tl_count:N will not count spaces, whereas they're counted when the token list is first converted into a string.