Why the ⟨code⟩ of \NewDocumentCommand{⟨cmd⟩}{⟨arg spec⟩}{⟨code⟩} isn't displayed in the terminal?

Question

Consider the following MCE:

\documentclass{article}
\newcommand{\mynewcommand}{foo}
\NewDocumentCommand{\MyNewDocumentCommand}{}{bar}
\ExplSyntaxOn
\iow_term:x {mynewcommand=\mynewcommand}
\iow_term:x {MyNewDocumentCommand=\MyNewDocumentCommand}
\ExplSyntaxOff
\begin{document}
\begin{itemize}
\item \verb|\mynewcommand|=\mynewcommand
\item \verb|\MyNewDocumentCommand|=\MyNewDocumentCommand
\end{itemize}
\end{document}

When it is compiled, what it is displayed:

in the resulting PDF is expected:

  • \mynewcommand=foo
  • \MyNewDocumentCommand=bar

in the terminal is expected for \mynewcommand but is unexpected for \MyNewDocumentCommand:
```
  mynewcommand=foo
  MyNewDocumentCommand=\MyNewDocumentCommand
```

Why, in the terminal, the ⟨code⟩ of \NewDocumentCommand{⟨cmd⟩}{⟨arg spec⟩}{⟨code⟩} isn't displayed, whereas the ⟨code⟩ of \newcommand{⟨cmd⟩}[⟨num⟩][⟨default⟩]{⟨code⟩} is displayed?

commands defined with \NewDocumentCommand are protected (robust). — Ulrike Fischer, May 09 '23 at 13:13
try \newcommand{\mynewcommand}[1][]{foo} you will see why \protected is used here — David Carlisle, May 09 '23 at 13:16
@UlrikeFischer OK, thanks. Is it possible to force this ⟨code⟩ to be displayed? — Denis Bitouzé, May 09 '23 at 13:18
@DavidCarlisle OK (with \ShowCommand not inside \iow_term:x). — Denis Bitouzé, May 09 '23 at 13:23
In the shown example you could use \NewExpandableDocumentCommand. But in other cases, this isn't a good idea. So it depends on what you do and want. — cabohah, May 09 '23 at 13:51

score 4 · Accepted Answer · answered May 09 '23 at 18:54

If you use \newcommand\macro then the \macro is defined using TeX primitives \long\def\macro. The macro is expanded inside \edef and inside \write and \message primitives, i.e. when you write an information into the terminal or log file.

On the other hand, if you use \NewDocumentCommand\macro then the macro is defined by TeX primitives \protected\def\macro (and the macro body is more complicated in this case). When the \macro is defined as \protected then it is expanded in typesetting context but it isn't expanded in \edef, \write, \message.

score 1 · Answer 2 · answered May 09 '23 at 20:40

1

I'm not sure why you'd expect that output. For instance

\iow_term:x { LaTeX=\LaTeX }

will stop with a weird error

! Undefined control sequence.
\S@10 ->\gdef \tf@size
                       {10}\gdef \sf@size {7}\gdef \ssf@size {5}

If you want to use \NewDocumentCommand to define a container for some text (but you shouldn't), then do

\NewExpandableDocumentCommand{\MyNewDocumentCommand}{}{bar}

and now

\iow_term:x {MyNewDocumentCommand=\MyNewDocumentCommand}

will indeed produce

MyNewDocumentCommand=bar

I'm not sure what would this be useful for.

answered May 09 '23 at 20:40

egreg

1,121,712

Why do you recommend to not use the \NewDocumentCommand to define a container for a text ? – Denis Bitouzé May 10 '23 at 20:30
Are you advising \NewExpandableDocumentCommand instead of \NewDocumentCommand to define a container for a text? – Denis Bitouzé May 13 '23 at 15:57
@DenisBitouzé I'm advising \newcommand – egreg May 13 '23 at 16:17

Why the ⟨code⟩ of \NewDocumentCommand{⟨cmd⟩}{⟨arg spec⟩}{⟨code⟩} isn't displayed in the terminal?

2 Answers2

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