Line breaks are missing in \author if it processes the output of another command

Question

In this post, I found a nice trick for dealing with lists. The code below allows to print lists with an optional separator argument.

\documentclass{article}
\usepackage{etoolbox}% http://ctan.org/pkg/etoolbox
\newcommand{\printlist}[2][,]{%
  \def\itemdelim{\def\itemdelim{#1}}% Item delimiter delayed by one cycle
  \renewcommand*{\do}[1]{\itemdelim##1}% How each item is processed
  \docsvlist{#2}}% Process CSV list
\begin{document}
$\printlist{1,2,3,4,5,6,7}$ \par
$\printlist[;]{a,b,c,d,e,f}$
\end{document}

However, if I use \printlist within the \author command, in combination with a newline \\, this won't work properly: The line break is set only for the first element of the list. For all other elements, the line break is ignored:

\documentclass{article}
\usepackage{etoolbox}% http://ctan.org/pkg/etoolbox
\newcommand{\printlist}[2][;]{%
  \def\itemdelim{\def\itemdelim{#1}}% Item delimiter delayed by one cycle
  \renewcommand*{\do}[1]{\itemdelim##1}% How each item is processed
  \docsvlist{#2}}% Process CSV list
\title{Test}
\author{\printlist[,\]{abc,def,ghi,jkl}}
\begin{document}
\maketitle
\end{document}

If I use a separator without line-break, e.g. ::, the above works as expected.

Adding additional line-breaks \\ in the \author command also leads to the expected result:

\author{\printlisti[::]{abc,def,ghi}\\123\\456\\789}

It seems that the combination of \printlist with line breaks \\ is causing a problem. How come?

Answer 1

The argument to \author will be typeset in a tabular. Each cell forms its own group thus, the internal definition of \itemdelim and the definition of \do won't survive your first row.

Essentially, this is what's happening:

\begin{tabular}{c}
  \def\itemdelim{test} \\
  \itemdelim
\end{tabular}

and the same for \do, though in the second row that will have the same definition as outside of tabular and won't lead to an error message.

---

You will need to do the definitions of \itemdelim (for the non-first entry) and \do global:

\newcommand{\printlist}[2][;]{%
  \def\itemdelim{\gdef\itemdelim{#1}}%
  \gdef\do##1{\itemdelim##1}%
  \docsvlist{#2}}
\author{\printlist[,\\]{abc,def,ghi,jkl}}

Though, you could apply the “trick” of \itemdelim directly for \do and skip \itemdelim by using #1 directly:

\newcommand{\printlist}[2][;]{%
  \def\do##1{##1\gdef\do####1{#1####1}}%
  \docsvlist{#2}}
\author{\printlist[,\\]{abc,def,ghi,jkl}}

With another auxilliary macro like \defdo we can avoid defining \do global (at the cost of another document-wide defined macro):

\newcommand*\defdo[1]{%
  \def\do##1{#1\defdo{#1}##1}}%
\newcommand{\printlist}[2][;]{%
  \def\do##1{##1\defdo{#1}}%
  \docsvlist{#2}}%
\author{\printlist[,\\]{abc,def,ghi,jkl}}

This sneaks the definition of \do from row to row behind the \\ so that it will be available in the next row.

---

Alternatively, you could pre-process the list and just forward that to \author:

\newcommand{\savelist}[3][;]{%
  \renewcommand*\do[1]{%
    \newcommand*#3{##1}%
    \renewcommand*\do[1]{%
      \appto#3{#1####1}%
    }%
  }%
  \docsvlist{#2}}
\savelist[,\\]{abc,def,ghi,jkl}\authors
\author{\authors}

In this case, the definition of \do is also “global” because it is defined on the top level of the document.

Defining \do global shouldn't create a conflict since every other code that uses \do should have defined it itself without assuming any definition. Mixing global and local assignments/definitions could be messy, though, so using \savelist might be preferable.

---

Looking beyond etoolbox, LaTeX3's l3clist package provides \clist_use:Nn which doesn't need any definitions to just place something (#1 = ,\\) between a list's entries:

\documentclass{article}
\ExplSyntaxOn
\DeclareDocumentCommand{\printlist}{ O{;} m}{
  \clist_set:Nn \l_tmpa_clist {#2}
  \clist_use:Nn \l_tmpa_clist {#1}
}
\ExplSyntaxOff
\author{\printlist[,\\]{abc,def,ghi,jkl}}
\title{Test}
\begin{document}
\maketitle
\end{document}