xparse empty arguments

Question

I have defined a command using xparse as

\DeclareDocumentCommand\mymacro{o o o}

and I want to call it with various numbers of arguments (up to 3)

\mymacro, \mymacro[test][two], \mymacro[][][test]

etc...

The issue is that [] doesn't seem to return a \NoValue but an empty token as the tests I use \IfNoValueTF never return false on it.

Is there a way to make an "empty" argument like \mymacro[] set #1 to \NoValue when it is an empty string?

Essentially I need "empty" arguments to be set to \NoValue for my code to work.

I could always do

\mymacro[\NoValue][test][\NoValue]

but of course that defeats the purpose of making it simple.

In my code I use \IfNoValueTF{#n} a lot but for it to work on empty arguments I would need something like \IfNoValueOrEmptyTF.

Answer 1

Using a mixture of xparse and \ifx conditionals you can test for empty arguments:

\documentclass{article}
\usepackage{xparse}% http://ctan.org/pkg/xparse
\makeatletter
\DeclareDocumentCommand\mymacro{o o o}{
  \def\@tempa{#1}Arg 1 is \ifx\@tempa\@empty\relax empty\else #1\fi. \par
  \def\@tempa{#2}Arg 2 is \ifx\@tempa\@empty\relax empty\else #2\fi. \par
  \def\@tempa{#3}Arg 3 is \ifx\@tempa\@empty\relax empty\else #3\fi.
}
\makeatother
\begin{document}
\verb|\mymacro|:\par\mymacro

\verb|\mymacro[test][two]|: \par\mymacro[test][two]

\verb|\mymacro[][][test]|: \par\mymacro[][][test]

\verb|\mymacro[][][]|: \par\mymacro[][][]

\verb|\mymacro[one][][three]|: \par\mymacro[one][][three]
\end{document}​

Answer 2

Here is a \IfNoValueOrEmpty macro:

\documentclass{article}
\usepackage{xparse}
\ExplSyntaxOn
\DeclareExpandableDocumentCommand{\IfNoValueOrEmptyTF}{mmm}
 {
  \IfNoValueTF{#1}{#2}
   {
    \tl_if_empty:nTF {#1} {#2} {#3}
   }
 }
\ExplSyntaxOff

\NewDocumentCommand{\mymacro}{ooo}{%
  \IfNoValueOrEmptyTF{#1}{\typeout{\#1 is empty}}{\typeout{\string#1 is `#1'}}%
  \IfNoValueOrEmptyTF{#2}{\typeout{\#2 is empty}}{\typeout{\string#2 is `#2'}}%
  \IfNoValueOrEmptyTF{#3}{\typeout{\#3 is empty}}{\typeout{\string#3 is `#3'}}%
}

\mymacro
\mymacro[a]
\mymacro[a][b]
\mymacro[a][b][c]
\mymacro[][b]
\mymacro[][][c]
\mymacro[][][]

However, a command with three optional arguments is difficult to manage, because one needs to remember which is which. A key-value syntax might be easier to employ.

Answer 3

Empty arguments and NoValue ones are not the same (and indeed using \NoValue directly is not supported as we will soon change the definition). Unlike LaTeX2e's \newcommand, xparse differentiates between an argument which is given but is empty and one which is not given. Thus if you want to be able to have input of the form

\foo[][][baz]

you don't want a NoValue test at all, but rather

\ExplSyntaxOn
\DeclareDocumentCommand{\foo}{ O{} O{} O{} }
  {
    \tl_if_empty:nTF {#1}
    ...
  }

testing for an empty value using the code-level test. (We do not have a design-level empty test at the present time.)