From \newcommand to \NewDocumentCommand

Following on from my last post, I thought it might be useful to give some simple example of how the xparse package works and why it’s useful. I want to do this to show end users of LaTeX how it can replace \newcommand, so the example will not involve anything too complex, code-wise.


I’m going to illustrate moving from \newcommand to \NewDocumentCommand with a series of simple examples. For all of them, you need to load the xparse package:

\usepackage{xparse}

Macros with no arguments

The simplest type of macro is one with no arguments at all. This isn’t going to show off xparse very much but is is a starting point. The traditional method to do this is

\newcommand\NoArgs{Text to insert}

which becomes

\NewDocumentCommand\NoArgs{}{Text to insert}

That does not look too bad, I hope. Notice that I’ve got an empty argument in the xparse case: this is where the arguments are listed, and with \NewDocumentCommand there always has to be a list of arguments, even if it is empty. That’s a contrast with the \newcommand approach, where we only need to mention arguments when there are any.

One or more mandatory arguments

The most common type of argument for a macro is a mandatory one. With \newcommand, we’d give a number of arguments to use:

\newcommand\OneArg[1]{Text using #1}
\newcommand\TwoArgs[2]{Text using #1 and #2}

\NewDocumentCommand is a bit different. Since it can work with different types of argument, each one is give separately as a letter. A mandatory argument is ‘m’, so we’d need

\NewDocumentCommand\OneArg{m}{Text using #1}
\NewDocumentCommand\TwoArgs{mm}{Text using #1 and #2}

This is still pretty similar to \newcommand: the useful stuff starts when life gets a little more complicated.

One of more optional (square brackets) arguments

To really get something clever out of xparse, the arguments need to be a little more varied than I’ve show so far. Let’s look at optional arguments, which LaTeX puts in square brackets. If I want the first argument to be optional, then LaTeX can help me

\newcomand\OneOptOfTwo[2][]{Text with #2 and perhaps #1}
\newcomand\OneOptOfThree[3][]{Text with #2, #3 and perhaps #1}


\NewDocumentCommand\OneOptOfTwo{O{}m}%
{Text with #2 and perhaps #1}
\NewDocumentCommand\OneOptOfTwo{O{}mm}%
{Text with #2, #3 and perhaps #1}

How about two optional arguments? It’s pretty obvious:

\NewDocumentCommand\TwoOptOfThree{O{}O{}m}%
{Text with #3 and perhaps #1 and #2}

What if we want something as a default value for the optional argument? With \newcommand, that would be

\newcommand\OneOptWithDefault[2][default]%
{Text using #1 (could be the default) and #2}

which would become

\NewDocumentCommand\OneOptWithDefault{O{default}m}%
{Text using #1 (could be the default) and #2}

The same idea applies to each optional argument: whatever is in the braces after the O is the default value.

More complex optional arguments

You might be wondering why we need the ‘{}c after ‘O’ when there is no default value: why not just ‘o’? Well, there is ‘o’ as well. Unlike \newcommand, \NewDocumentCommand can tell the difference between an option argument that is not given and one that is empty. To do that, it provides a test to see if the argument is empty:

\NewDocumentCommand\OneOptOfTwoWithTest{om}{%
\IfNoValueTF{#1}
{Do stuff with #2 only}
{Do stuff with #1 and #2}%
}

Don’t worry if you forget to do the test: the special marker that is used here will simply print ‘-NoValue-’ as a reminder!

Two types of optional argument

Sometimes you might want two different optional arguments, and be able to tell which is which. This can be done by using something other than square brackets, often using angle brackets (‘<’ and ‘>’). We can do that using the letter ‘d’ (or ‘D’ if we give a default).

\NewDocumentCommand\TwoTypesOfOpt{D<>{}O{}m}%
{Text using #1, #2 and #3}

What input syntax does this make? Let’s look at some examples

\TwoTypesOfOpt{text}             % One mandatory
\TwoTypesOfOpt[text]{text}       % A normal optional
\TwoTypesOfOpt<text>{text}       % A special optional
\TwoTypesOfOpt<text>[text]{text} % Both optionals

How did that work? The first two characters after the ‘D’ are used to find the optional argument, so in this case ‘<’ and ‘>’.

Finding stars or other special markers

Another common idea in LaTeX is to use a star to indicate some special version of a macro. Creating those with \newcommand is difficult, but it is easy with \NewDocumentCommand

\NewDocumentCommand\StarThenArg{sm}{%
\IfBooleanTF#1
{Use #2 with a star}
{Use #2 without a star}%
}

Here, ‘s’ represents a star argument. You’ll see that it ends up as #1, while the mandatory argument is #2. You’ll also see that there needs to be a test to see if there is a star (\IfBooleanTF). This doesn’t mention stars as the test can be used for other things.

Summing up

There is more to xparse than I’ve mentioned here, but I hope that this is a useful flavour of what it can be used for. To get more flexibility there is a bit more to think about compared to \newcommand, but the overall consistency is hopefully worth it.

4 thoughts on “From \newcommand to \NewDocumentCommand”

1. Vishnu

Thanks for this article on xparse. Suppose that I want to modify the frac command so that frac{x} will produce 1/x, and frac{y}{x} will produce y/x. (How) Can I do it with xparse?

Thanks,
–Vishnu

2. Joseph Wright

Two things need to happen here. First, you need to save the existing definition of frac, then create a new definition using xparse. The second argument has to be the optional one, so there is a little bit of thought required.

documentclass{article}
usepackage{xparse}
letRealFracfrac
RenewDocumentCommandfrac{mg}{%
IfNoValueTF{#2}
{RealFrac{1}{#1}}
{RealFrac{#1}{#2}}%
}
begin{document}
One arg: \$frac{10}\$.
Two args: \$frac{2}{30}\$.
end{document}

3. Rainer Schnaack

Cool stuff;-)
Im just surprised that the somewhat obvious alternate
documentclass{article}
usepackage{xparse}
letRealFracfrac
RenewDocumentCommandfrac{G{1}m}{%
RealFrac{#1}{#2}%
}
begin{document}
One arg: \$frac{10}\$.
Two args: \$frac{2}{30}\$.
end{document}
doesn’t work—why not?
I mean, what’s the big difference to sth like
documentclass{article}
usepackage{xparse}
letRealFracfrac
RenewDocumentCommandfrac{O{1}m}{%
RealFrac{#1}{#2}%
}
begin{document}
One arg: \$frac{10}\$.
Two args: \$frac[2]{30}\$.
end{document}

Now, the xparse-doc led me to believe, the G-specifier does the same thing as the O-specifier, just with braces instead of brackets—then again, I’d stick to brackets for optional arguments, so `no harm done’ in my case;-)

KR
Rainer

4. Joseph Wright

The challenge is that TeX doesn’t really let us ‘look ahead’ to see whether the first group is followed by a second. By hand, I think you could code a first optional argument in braces (you’d test if there was a second one then do some shuffling). However, xparse needs to work in a general way and that is achieved by dealing with each argument separately.

When I get a moment I’ll revise the documentation to try to make this clearer.