Treating the contents of a TeX-style group ({\blah ...}) as macro arguments

Question

I am trying to make adjustments to a large document that uses TeX-style macros, like this:

{\defun SomeFunctionName arg1 arg2}

The \defun macro is defined as \newcommand{\defun}{\tt}.

Can I adjust this definition to retrieve the SomeFunctionName arg1 arg2 part as an argument, to allow for a more complex definition? Fox example, if defun was a proper LaTeX macro, I could put a box around the text, or put something before and after, etc.

Minimal example:

\documentclass{article}
\newcommand{\defun}{\tt}

\begin{document}
\section{\defun SomeFunctionName arg1 arg2}
Call {\defun SomeFunctionName} to foo the bar.
\end{document}

How could I (for example) add some text before and after every invocation of defun? What is the general approach to handle the contents of {\defun ...} as an argument, short of editing all occurences?

To clarify: this is a large, two decades old document. I didn't write it.

Answer 1

\documentclass{article}

\protected\def\defun{\expandafter\zdefun\expandafter{\iffalse}\fi}

\def\zdefun#1{A \fbox{#1} B\egroup}

\begin{document}
\section{\defun SomeFunctionName arg1 arg2}
Call {\defun SomeFunctionName} to foo the bar.
\end{document}

---

Here

\iffalse}\fi

expands to nothing so

\expandafter{\iffalse}\fi

expands to a single unmatched { but is a matching {} so can be included in a definition.

So given

 {\defun SomeFunctionName}

The { starts a group and

\defun SomeFunctionName}

expands in one step to

\expandafter\zdefun\expandafter{\iffalse}\fi SomeFunctionName}

and in the next step to

\zdefun{SomeFunctionName}

\zdefun has a normal #1 argument which is delimited by the { we just inserted and the } that was originally in the file. (Note this } was originally a group close delimiter, it is now used as an argument delimiter so doesn't close a group.)

So this expands in one step to

A \fbox{SomeFunctionName} B\egroup

and the

A \fbox{SomeFunctionName} B

gets typeset, and finally the

\egroup

closes the group started by the { in the original file.

Answer 2

My preferred approach would be: use the find-and-replace function of your editor of choice and replace {\defun with \textdefun{.

EDIT: As @Clément points out, this does not work in cases like \section{\defun abc def}. I am afraid those cases would elude even a more sophisticated regexp-based approach, because one would have to add an additional closing brace.

Answer 3

You can do it with \aftergroup:

\documentclass{article}

\protected\def\defun{\aftergroup\newdefun\aftergroup{}}
\newcommand\newdefun[1]{\fbox{#1}}

\begin{document}

\tableofcontents

\section{Here {\defun SomeFunctionName arg1 arg2}}
Call {\defun SomeFunctionName} to foo the bar.

\end{document}

The same limitation as David Carlisle's answer holds: calls of \defun that don't appear inside a pair of braces will break. In the case of

\section{\defun Whatever}

the error will be when processing \tableofcontents. But \section{{\defun Whatever}} will behave well.