What does `NewDocumentCommand \sidenote { o o +m }` mean?

Question

I have obviously missed some updated information about specifying LaTeX commands What I am used to is along the lines of

\newcommand{<command-name>}[<numargs>][<default>]{<command definition>}

and now I have seen that there is a definition like

\NewDocumentCommand \sidenote { o o +m }

which means nothing to me and which has no obvious relationship to the way I have specified new commands for the last 3 or so decades.

Where can I find out about this complete change in the command definition facilities and, possibly, why weren't there any obvious announcements about it?

I am on the comp.text.tex newgroup and I run TeXlive 2021 but am waiting for the DVD to arrive before updating to 2022.

you find the documentation with texdoc xparse. o o +m means three argument, two optional and one mandatory which can be long. — Ulrike Fischer, Jul 01 '22 at 18:43
@UlrikeFischer You want to answer (perhaps including usrguide(3))? — Joseph Wright, Jul 01 '22 at 18:49
it's newish in the format but been available as the xparse package for a decade or so and some versions of this form have been available for longer than that: xparse.dtx has copyright dates going back as far as 1999 — David Carlisle, Jul 01 '22 at 19:04
I think perhaps what you missied is that one no longer needs to explicitly include the line \usepackage{xparse} to make use of \NewDocumentCommand{}{}. So, if you see this in code you need to look at the xparse documentation. It is actually much easier to control parameters with xparse, once you learn it, and only need to read a few pages of documentation. The older \newcommand still works. — Peter Grill, Jul 01 '22 at 21:15
Special case of macros - Where do I find out how a command/environment is defined? - TeX - LaTeX Stack Exchange, I think. — user202729, Jul 03 '22 at 01:09

David Carlisle · Accepted Answer · 2022-07-02T21:30:22.143

It's newish in the format (October 2020) but has been available as the xparse package for a decade or so and some versions of this form have been available for longer than that: xparse.dtx has copyright dates going back as far as 1999.

The new code in the format is documented in texdoc source2e chapter g (ltcmd) although more convenient is the updated user guide (currently texdoc usrguide3 although we will probably merge this back with usrguidesoon)

\NewDocumentCommand \sidenote { o o +m }{...#1 ... #2 ... #3}

defines \sidenote with two optional arguments and one mandatory argument. The + here denotes a \long argument that may accept blank lines (\par) like using \newcommand rather than \newcommand* except that \NewDocumentCommand allows this to be separately specified for each argument.

So
\sidenote[a][b]{main}
which can not easily be defined using \newcommand, which only allows declaration of a single optional argument.

In general the new features of each release are described in ltnews

texdoc ltnews

shows a pdf with the full series, you want LaTeX News 32 (2020/10) which documents this.

Wow! texdoc ltnews produced over 70 page detailing LaTeX improvements. — Peter Wilson, Jul 03 '22 at 17:52

What does `NewDocumentCommand \sidenote { o o +m }` mean?

1 Answers1