Frequently Asked Question List for TeX
Programming
Commands defined with * options
LaTeX commands commonly have “versions” defined with an asterisk
tagged onto their name: for example \newcommand and
\newcommand* (the former defines a \long version of the
command).
The simple-minded way for a user to write such a command involves use
of the ifthen package:
\newcommand{\mycommand}[1]{\ifthenelse{\equal{#1}{*}}%
{\mycommandStar}%
{\mycommandNoStar{#1}}%
}
\newcommand{\mycommandStar}{starred version}
\newcommand{\mycommandNoStar}[1]{normal version}
This does the trick, for sufficiently simple commands, but it has
various tiresome failure modes, and it requires \mycommandnostar
to take an argument.
The LaTeX kernel does a lot of this, and has its own command,
\@ifstar (which needs “internal command protection”, cf.
\makeatletter
\newcommand{\mycommand}{%
\@ifstar
\mycommandStar%
\mycommandNoStar%
\makeatother
}
\newcommand{\mycommandStar}{starred version}
\newcommand{\mycommandNoStar}{normal version}
(Note that arguments to \mycommandStar and \mycommandNoStar
are independent — either can have their own arguments, unconstrained
by the technique we’re using, unlike the trick described above.)
The \@ifstar trick is all very well, is fast and efficient, but
it requires that the definition be
\makeatletter protected.
A pleasing alternative is the suffix package. This elegant
piece of code allows you to define variants of your commands:
\newcommand\mycommand{normal version}
\WithSuffix\newcommand\mycommand*{starred version}
The package needs e-LaTeX, but any new enough
distribution defines LaTeX as e-LaTeX by default. Command
arguments may be specified in the normal way, in both command
definitions (after the * in the \WithSuffix
version). You can also use the TeX primitive commands, creating a
definition like:
\WithSuffix\gdef\mycommand*{starred version}
For those of an adventurous disposition, a further option is to use
the xparse package from the l3packages
distribution. The package defines a bunch of commands (such as
\NewDocumentCommand) which are somewhat analagous to
\newcommand and the like, in LaTeX2e. The big difference is
the specification of command arguments; for each argument, you have a
set of choices in the command specification. So, to create a
*-command (in LaTeX2e style), one might write:
\NewDocumentCommand \foo { s m } {%
% #1 is the star indicator
% #2 is a mandatory argument
...
}
The “star indicator” (s) argument appears as #1
and will take values \BooleanTrue (if there was a star) or
\BooleanFalse (otherwise); the other (m) argument is a
normal TeX-style mandatory argument, and appears as #2.
While xparse provides pleasing command argument
specifications, it is part of the
LaTeX 3 experimental harness.
Simply loading the package to provide \DeclareDocumentCommand
“pulls in” all of the LaTeX3 kernel (a large bunch of packages)
via the expl3 package.