Read command with varying input arguments

Question

Out of curiosity I'd like to define a LaTeX command that looks like:

\newcommand{\trigger}{out1,out2,...,outn}

with a rather strange function (at least, I couldn't find anything like this elsewhere):

First of all, the number of arguments out1,out2,... etc. are variables. Then, every time the \trigger command is used in the document's content, a different string is printed: first out1, then out2, etc. and finally outn. After outn is printed, the whole thing should be reset such that the next string will be out1 again.

So the first problem I have is to make a command that can read the comma separated list between {} and the second problem is how to print just one them every time the newly defined command is called.

Answer 1

It's quite easy with expl3:

\documentclass{article}
\usepackage{xparse}
\ExplSyntaxOn
\NewDocumentCommand{\newsetcommand}{mm}
  {
   \seq_new:c { g_setcommand_ \cs_to_str:N #1 _seq }
   \seq_gset_split:cnn { g_setcommand_ \cs_to_str:N #1 _seq } {,} {#2}
   \cs_new:Npn #1
     {
      \seq_gpop_left:cN { g_setcommand_ \cs_to_str:N #1 _seq } \l_setcommand_temp_tl
      \seq_gput_right:cV { g_setcommand_ \cs_to_str:N #1 _seq } \l_setcommand_temp_tl
      \tl_use:N \l_setcommand_temp_tl
     }
  }
\tl_new:N \l_setcommand_temp_tl
\cs_generate_variant:Nn \seq_gset_split:Nnn { c }
\ExplSyntaxOff

\newsetcommand{\trigger}{out1,out2,out3,out4}

\begin{document}

\trigger

\trigger

\trigger

\trigger

\trigger

\end{document}

You see that \newsetcommand defines a new macro that expands to the successive elements of the list and as many commands of the same type can be defined.

Here's the output:

---

How it works

\newsetcommand{\trigger}{<list>} defines

1. A new sequence called \g_setcommand_trigger_seq, containing the ordered list got from the comma separated <list>

2. A new macro \trigger that at each call pops the leftmost element from the associated sequence, adds it on the right of the sequence and prints it.

Since the popped element is added back at the other end of the sequence, when the last original element is printed, the next will be again the first.

---

"Classical" implementation

\makeatletter
\newcommand{\newsetcommand}[2]{%
  \toks@{}%
  \@for\next:=#2\do{%
    \toks@=\expandafter{\the\expandafter\toks@\expandafter{\next}}%
  }
  \expandafter\gdef\csname setcommand\string#1\expandafter\endcsname\expandafter{\the\toks@}%
  \edef#1{\noexpand\@usecommand{\string#1}}%
}
\def\@usecommand#1{%
  \expandafter\expandafter\expandafter\@@usecommand\csname setcommand#1\endcsname\@nil{#1}\@nil}
\def\@@usecommand#1#2\@nil#3\@nil{%
  #1%
  \expandafter\gdef\csname setcommand#3\endcsname{#2{#1}}%
}
\makeatother

The syntax is the same as before.

EDIT

Thanks to David Carlisle's answer, this can be simplified:

\makeatletter
\newcommand{\newsetcommand}[2]{%
  \global\@namedef{@setcommand\string#1}{#2}%
  \edef#1{\noexpand\@usecommand{\string#1}}%
}
\def\@usecommand#1{%
  \expandafter\expandafter\expandafter\@@usecommand\csname @setcommand#1\endcsname\@nil#1\@nil
}
\def\@@usecommand#1,#2\@nil#3\@nil{%
  \global\@namedef{@setcommand#3}{#2,#1}%
}
\makeatother

Answer 2

Nothing wrong with the earlier answers, but here's another one

\documentclass{article}

\newcommand{\triggerlist}{out1,out2,...,outn}
\def\trigger{\expandafter\xtrigger\triggerlist\xtrigger}
\def\xtrigger#1,#2\xtrigger{#1\def\triggerlist{#2,#1}}

\begin{document}

\trigger

\trigger

\trigger

\trigger

\trigger

\trigger

\trigger

\trigger

\end{document}

Answer 3

My humble attempt with etoolbox, thanks to one of Leo Liu's answers. I used \csdef and \csuse to store and use the values, and \DeclareListParser to iterate through the items in the comma-separated list. Finally, the modulo feature is provided by the intcalc package.

\documentclass{article}

\usepackage{etoolbox}
\usepackage{intcalc}

\DeclareListParser*{\myforeach}{,}

\newcounter{helpercounter}
\newcommand{\savethistext}[1]{%
\stepcounter{helpercounter}%
\csdef{triggerval\thehelpercounter}{#1}}

\newcounter{progresscounter}
\newcommand{\trigger}[1]{%
\setcounter{progresscounter}{\intcalcMod{\value{progresscounter} + 1}{\value{helpercounter} + 1}}%
\csuse{triggerval\theprogresscounter}\par%
}

\myforeach{\savethistext}{out1, out2, out3, out4}

\begin{document}

\trigger

\trigger

\trigger

\trigger

\trigger

\trigger

\trigger

\trigger

\trigger

\trigger

\end{document}

Answer 4

Here is an implementation using high-level ConTeXt features: counters and conversion.

\defineconversion[trigger][out1, out2, out3, out4]
\definestructurecounter[trigger][numberconversion=trigger]
\setstructurecounter[trigger]{0}

\def\trigger%
    {\incrementstructurecounter[trigger]%
     \convertedstructurecounter[trigger]}

\starttext
\startlines
\trigger
\trigger
\trigger
\trigger
\trigger
\stoplines
\stoptext