Defining starred versions of commands (* macro)

Question

I would like to define a *'d version of a command, something like

\newcommand{\foo}{blah}
\newcommand{\foo*}{blahblah}

If I try to do this, LaTeX complains that I'm trying to redefine \foo, so I guess I need to do something special to handle *s. But what?

Answer 1

If you look at source2e you might see a lot of lines that look like

\def\foo{\@ifstar\@foo\@@foo}
\def\@foo#1{...}
\def\@@foo#1{...}

This makes \foo a one-argument command that has regular and starred versions. The starred version is the expansion of \@foo while the nonstarred version is that of \@@foo. Using the @ sign in the auxiliary macros is a TeX convention which some authors embrace and some avoid.

There are higher-level ways to do it (as lockstep points out) but once you learn this pattern it's not too hard to use. Just make sure it's between \makeatletter...\makeatother or in a .sty file.

Edits removed some inaccuracies and editorializing.

Answer 2

See this entry in the TeX FAQ.

The "elegant" way is to use the suffix package (which requires eTeX):

\documentclass{article}
\usepackage{suffix}
\newcommand\foo{blah}
\WithSuffix\newcommand\foo*{blahblah}
\begin{document}
\foo
\foo*
\end{document}

Answer 3

LaTeX3 solution:

\documentclass{article}
\usepackage{xparse}
\NewDocumentCommand\foo{s}{%
  \IfBooleanTF#1%
    {blahblah}% If a star is seen
    {blah}%     If no star is seen
}