Call command based on comma-separated list

Question

I have several commands, called \cata to \catd. Now I need a command which loops over a given, comma-separated list, and calls the respective command. My current approach is:

\documentclass[10pt,a4paper]{article}
\usepackage[latin1]{inputenc}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{graphicx}
\usepackage{tikz}
\newcommand{\cata}{Hello a\newline}
\newcommand{\catb}{Hello b\newline}
\newcommand{\catc}{Hello c\newline}
\newcommand{\catd}{Hello d\newline}
\newcommand{\cat}[1]{%
\expandafter\newcommand\csname cat#1\endcsname%
}
\newcommand{\loopover}[1]{
\foreach \i in {#1}{
    \cat\i}
}
\begin{document}

Hello World\\
\loopover{a, b, c, d}
\end{document}

My expected output would be:

Hello World
Hello a
Hello b
Hello c
Hello d

If I change \loopover{a,b,c,d} to \loopover{a,a,c}, I want to get this

Hello World
Hello a
Hello a
Hello c

Up till now I get the error LaTeX Error: Command \cata already defined., which makes sense. Is there another way to achieve my intention?
Edit: I would like to expand the commands afterwards, so the current case is maybe a bit misleading. I would like to get the command \cata called itself, and not rewrite it.

The final application looks like:

\documentclass[10pt,a4paper]{article}
\usepackage[latin1]{inputenc}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{graphicx}
\usepackage{tikz}
\usepackage{tocloft}
\newcommand{\lista}{List a}
\newcommand{\listb}{List b}
\newcommand{\listc}{List c}
\newcommand{\listd}{List d}
\newlistof{cata}{expa}{\lista}
\newlistof{catb}{expb}{\listb}
\newlistof{catc}{expc}{\listc}
\newlistof{catd}{expd}{\listd}
\newcommand{\cata}[1]{%
    \refstepcounter{cata}
    \par\noindent\textbf{cata \thecata. #1}
    \addcontentsline{expa}{cata}
    {\protect\numberline{\thechapter.\thecata}#1}\par
}
\newcommand{\catb}[1]{%
    \refstepcounter{catb}
    \par\noindent\textbf{catb \thecatb. #1}
    \addcontentsline{expb}{catb}
    {\protect\numberline{\thechapter.\thecatb}#1}\par
}
\newcommand{\catc}[1]{%
    \refstepcounter{catc}
    \par\noindent\textbf{catc \thecatc. #1}
    \addcontentsline{expc}{catc}
    {\protect\numberline{\thechapter.\thecatc}#1}\par
}
\newcommand{\catd}[1]{%
    \refstepcounter{catd}
    \par\noindent\textbf{catd \thecatd. #1}
    \addcontentsline{expd}{catd}
    {\protect\numberline{\thechapter.\thecatd}#1}\par
}

\newcommand{\cat}[2]{%
\expandafter\newcommand\csname cat#1\endcsname{#2}%
}
\newcommand{\loopover}[2]{
\foreach \i in {#1}{
    \cat\i{#2}}
}
\begin{document}
\tableofcontents
\newpage
\lista\newline
\listb\newline
\listc\newline
\listd\newline
\chapter{TestA}
\cat{a,b}{Test}
\end{document}

I would like to loop over the list {a,b} given to \cat, and call the respective command in order to add {Test} to this table of content.

Answer 1

It's not clear why redefining \cat<x> each time:

\documentclass[10pt,a4paper]{article}

\usepackage{tikz} % \foreach
\newcommand{\cata}{Hello a\newline}
\newcommand{\catb}{Hello b\newline}
\newcommand{\catc}{Hello c\newline}
\newcommand{\catd}{Hello d\newline}
\newcommand{\loopover}[1]{%
  \foreach \i in {#1}{\csname cat\i\endcsname}%
}
\begin{document}

\noindent
Hello World\\
\loopover{a, b, c, d}
\loopover{a, a, c}

\end{document}

A simple implementation with xparse:

\documentclass[10pt,a4paper]{article}

\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentCommand{\loopover}{m}
 {
  \clist_map_inline:nn { #1 } { \use:c { cat ##1 } }
 }
\ExplSyntaxOff

\newcommand{\cata}{Hello a\newline}
\newcommand{\catb}{Hello b\newline}
\newcommand{\catc}{Hello c\newline}
\newcommand{\catd}{Hello d\newline}

\begin{document}

\noindent
Hello World\\
\loopover{a, b, c, d}
\loopover{a, a, c}

\end{document}

This has several advantages over \foreach: for instance it automatically trims spaces around the items in the comma separated list and it doesn't hide the loop cycles in a group.

Here \clist_map_inline:nn is the loop making function and \use:c is expl3 lingo for \csname...\endcsname.

---

Update

Your more complicated code falls within the same solution. Just define

 \newcommand{\loopover}[2]{%
   \foreach \x in {#1}{\csname cat\x\endcsname{#2}}%
 }

There is no issue with this: if \x is a, the \cata token is formed and it absorbs the argument {#2} as usual.

Answer 2

Not sure if your example is just that, an example for a much more complex use case. But as far as your question is concerned, you can use xparse and a description environment.

Output

Code

\documentclass{article}
\usepackage{enumitem}
\usepackage{xparse}

\setlist[description]{itemsep=.3\baselineskip, font=\normalfont}

\NewDocumentCommand\mylist{>{\SplitList{,}}m}
  {
    \begin{description}
      \ProcessList{#1}{ \insertl }
    \end{description}
  }
\newcommand\insertl[1]{\item[Hello] #1}

\begin{document}
\noindent 
Hello world \par

\mylist{
    a, b, c, d
}%

\end{document}