Transforming a list with xparse

Question

I would like to be able to turn a list A,B,C into A \\ B \\ C, where A, B, C are LaTeX expressions. It would be quite simple to do with a package like xstring, but for the possibility that A, B or C contain a comma. So I thought of using xparse,writing some code like {>{\SplitList{,}}m}.

Unfortunately not only it didn't work, but even trying such a simple thing as:

\DeclareDocumentCommand\foo%
{>{\SplitList{,}}m}%
{\ProcessList{#1}{#1 X}
.......................
\foo{a,b,c}

just produces:

I expected aXbXcX. I really don't understand what's happening, nor what ProcessList does.

Another problem is that up to now, I've no idea how to indicate the replacement of the separating commas with newline commands.

Answer 1

\ProcessList doesn't allow inline functions like you've written; the interface expects a single macro to process each subsequent item of the list:

\documentclass[a4paper]{article}
\usepackage{xparse}
\begin{document}
\DeclareDocumentCommand \foo { >{\SplitList{,}} m }
 {
  \ProcessList{#1}{\processfoo}
 }
\newcommand\processfoo[1]{#1X}

\foo{a,b,c}
\end{document} 

The way that you have written it, \ProcessList{#1}{#1 X}, results in receiving arguments like:

\ProcessList{a,b,c}{a,b,c X}

and (for whatever reason) this is how you end up with the unexpected output in your question.

Answer 2

The stringstrings package is set up to conveniently manipulate character stings. It has a limited ability to manipulate macro strings like \\. In this MWE, I first show (ABC) the conversion involving \\. For the second example (DEF), it shows the one-line equivalent if the characters involved are not macros. The third output (cde) merely shows that \\ has been properly restored by the \decodetoken.

\documentclass{article}
\usepackage{stringstrings}
\begin{document}
\encodetoken{\\}
\convertchar[e]{A,B,C}{,}{\\}
\retokenize{\thestring}
\thestring
\decodetoken{\\}

\convertchar{D,E,F}{,}{X}

c\\d\\e
\end{document}

---

In response to the OP's comment about the need to stack inside the matrix environment, I post this addendum to see if a tabstackengine alternative would suffice for stacking matrix-like matter? Here, it is shown using commas as the end-of-line delimiter.

\documentclass{article}
\usepackage{tabstackengine}
\stackMath
\begin{document}
\setstackEOL{,}
\setstacktabulargap{0pt}
\setstackgap{L}{1.3\baselineskip}
\tabularLongstack{rl}{
x =&u^2, y=&v^2, z=&w^2
}
\end{document}