How do you write a macro that takes arguments containing paragraphs?

Question

I'm trying to write a macro that takes arguments containing paragraphs. If you write a normal macro and one of its arguments contains a paragraph, it will break:

\documentclass{article}
\usepackage[utf8]{inputenc}

\title{test}

\begin{document}

\maketitle

\section{Introduction}

\def\mymacro#1{#1}

\mymacro{This

contains a paragraph}

\end{document}

This produces an error:

Runaway argument?
{This
! Paragraph ended before \mymacro was complete.
<to be read again> 
                   \par
l.15

So I tried to redefine \par for expanding the arguments of the macro, but now it won't stop compiling:

\documentclass{article}
\usepackage[utf8]{inputenc}

\title{test}

\begin{document}

\maketitle

\section{Introduction}

\gdef\oldpar{\par}

\def\mymacro{\gdef\par{}\mymacroi}

\def\mymacroi#1{#1\gdef\par{\oldpar}}

\mymacro{foo

bar}

\end{document}

Answer 1

At the time TeX was written, one page of a document would take several minutes to be processed, and syntax highlighting was not a thing, so it was a good thing to have some mechanism to detect if you forgot a }. A \def, by default, doesn't allow a \par token unless you explicitly say it's a \long\def:

\def\mymacro#1{#1}

LaTeX, on the other hand, uses that by default, so if you use proper LaTeX commands (\def shouldn't be used in LaTeX documents), \newcommand makes a \long\def by default. If you want a “short” \def then you use \newcommand*.

xparse returns the short argument default, but lets you define a \long macro using the + argument modifier:

\NewDocumentCommand\mymacro{ m}{#1}% \def
\NewDocumentCommand\mymacro{+m}{#1}% \long\def

---

Your second attempt is clever, and it could have worked except for two things.

First is that you are using \gdef\oldpar{\par} and then \gdef\par{\oldpar}. Once you expand \par you get \oldpar which, when expanded, yields \par which, when expanded, yields \oldpar which, when expanded, yields \par which, when expanded, yields \oldpar... Running forever :/

You need to use \let (or \global\let to have global effect) in this case: \let\oldpar\par. This creates an exact copy of \par named \oldpar which does not depend on what is \par.

Second, the runaway argument checking is implemented in a lower level, independent of the definition of \par, so this would fail with the same error:

\let\par\relax
\def\mymacro#1{#1}
\mymacro{foo

bar}

because when TeX sees two \endlinechar tokens (which is a space by default) TeX inserts an implicit \par token, which raises the Runaway argument error. Knowing that, then:

\newcount\oldELchar
\oldELchar=\endlinechar
\def\mymacro{\endlinechar=-1\relax\mymacroi}
\def\mymacroi#1{#1\endlinechar=\oldELchar}
\mymacro{foo

bar}

won't raise an error, but a new line won't be a space anymore.

Answer 2

The argument of a macro defined with \def does not allow \par tokens. Neither it allows blank lines, because they're transformed to \par during the phase in which TeX processes text input into tokens. Note that redefining \par is useless in this respect, because it's precisely the token \par that's disallowed, independently of its meaning.

Solution: make your macro \long.

\long\def\mymacro#1{#1}

Better solution:

\newcommand{\mymacro}[1]{#1}

because \newcommand uses \long\def internally. The variant \newcommand* instead uses \def without the prefix.