Using foreachitem inside a \draw command

Question

I'm trying to dynamically construct a \draw command inside a tikz picture like this:

\documentclass[]{scrartcl}

\usepackage{tikz}
\usepackage{listofitems}

\begin{document}

\begin{tikzpicture}
    \draw[black,thick] (0,0)--(0,-10)--(0,10)--cycle;

    \readlist*\mylist{-10,10}

    % This line attempts to mimic the above \draw dynamically, but it fails
    %\draw[black,thick] (0,0)--{\foreachitem\x\in\mylist[]{\ifnum\xcnt>1\ -- \else\ \fi(0,\x)}}--cycle;
\end{tikzpicture}

\end{document}

However, this fails to compile with a slew of errors, including this one:

tikz Error: Cannot parse this coordinate

How can I dynamically construct a draw like this that ends with a cycle? I know this MWE is kinda dumb right now because it's just a straight line, but if I can solve this then I can achieve my actual goal.

Update

I made some progress using @marmot's suggestion of using foreach, but unfortunately I hit the same problem again when trying to apply it to my actual use case, which is this:

% args:
% 1 = draw style
% 2 = value count
% 3 = list of values (comma-separated)
\newcommand{\band}[3]{
    % I want to achieve something like this (assuming arg #2 = 3, and arg #3 = 1,2,3)
    \draw[#1] (360/3:1) -- (360/3*2:2) -- (360/3*3:3) --cycle;

    % But this fails    
    %\draw[#1] foreach [count=\xi] \X in {#3} { (360/3*\xi:\X) -- }cycle;
}

Answer 1

Your updated question is very closely related to this one, where nearly (?) identical difficulties have been reported. The upshot is, as explained by Paul Gaborit in this comment that

Each iteration of foreach is a (TeX) group. You can't write \draw {(0,0) --} (0,1); but you can write \draw (0,0) { -- (0,1)};

There are various ways to make it work. The essential trick is that you expand the foreach before \draw sees it. (pgfplots has a \pgfplotsinvokeforeachungrouped for precisely this purpose and there other means to achieve that.) Here is a primitive yet short and working way, which does not require any packages beyond TikZ, which you are using anyway.

\documentclass[tikz,border=3.14mm]{standalone}
% args:
% 1 = draw style
% 2 = value count
% 3 = list of values (comma-separated)
\tikzset{my foreach code/.code={\xdef\Lst{}\foreach #1 }}
\newcommand{\band}[3][]{
    % I want to achieve something like this (assuming arg #2 = 3, and arg #3 = 1,2,3)
    %\draw[#1] (360/3:1) -- (360/3*2:2) -- (360/3*3:3) --cycle;

    \draw[#1,my foreach code={\X [count=\xi] in {#3} {\xdef\Lst{\Lst (360/3*\xi:\X) -- }}}] 
      \Lst   cycle;;
}
\begin{document}
\begin{tikzpicture}
    \band{3}{1,2,3}
    \band[red]{3}{5,7,9}
\end{tikzpicture}
\end{document}

Let me stress that if you just want to draw these triangles and do insist on having a loop over atoms of the form (coordinate) -- then there are much simpler ways. Here is an example.

\documentclass[tikz,border=3.14mm]{standalone}
\begin{document}
\begin{tikzpicture}[band/.style={insert path={plot[samples at={0,1,2}]
    ({360/(\x+1)}:{#1}[\x]) -- cycle}}]
  \draw[band={1,2,3}];
  \draw[red,band={5,4,3}];
\end{tikzpicture}
\end{document}

This is supposed to illustrate two points:

• You do not need to do all these \newcommands, TikZ allows you to do pretty much everything with styles.
• You seem to re-invent plot handlers.

Answer 2

For whatever reason, the \draw command doesn't like the listofitem calculations inside of its arguments. The alternative here is to save the desired tokens in a token list and then out put the token list clean, after all the calculations are done.

I introduce two macros, \addpathtoks{} to add literal tokens to the path, and \xaddpathtoks, to expand (once) the first token in argument and add the result to the path. The expand part is needed to turn the \x into an actual number.

UPDATED ANSWER to updated question

Note the black triangle is done via \band macro, the dashed yellow (overlaying) triangle is done manually.

\documentclass[]{scrartcl}

\usepackage{tikz}
\usepackage{listofitems}
\newtoks\pathtoks
\newcommand\addpathtoks[1]{%
  \pathtoks\expandafter{\the\pathtoks#1}}
\newcommand\xaddpathtoks[1]{%
  \expandafter\addpathtoks\expandafter{#1}}

\newcommand{\band}[3]{
\readlist*\mylist{#3}
\pathtoks{}
\foreachitem\x\in\mylist[]{%
  \ifnum\xcnt=1\relax \addpathtoks{\draw#1}\else
    \addpathtoks{--}
  \fi
  \addpathtoks{(360/}\addpathtoks{#2*}%
  \xaddpathtoks{\x:}\xaddpathtoks{\x)}
  \ifnum\xcnt=\listlen\mylist[]\relax\addpathtoks{--cycle;}\fi
}
\the\pathtoks
}
\begin{document}
\begin{tikzpicture}

\band{[black,thick]}{3}{1,2,3}
\draw[yellow,thin,dashed] (360/3:1) -- (360/3*2:2) -- (360/3*3:3) --cycle;
\end{tikzpicture}

\end{document}

See SUPPLEMENT for possible improvement.

ORIGINAL ANSWER to original question

\documentclass[]{scrartcl}

\usepackage{tikz}
\usepackage{listofitems}
\newtoks\pathtoks
\newcommand\addpathtoks[1]{%
  \pathtoks\expandafter{\the\pathtoks#1}}
\newcommand\xaddpathtoks[1]{%
  \expandafter\addpathtoks\expandafter{#1}}
\begin{document}
   \readlist*\mylist{-10,10}
\begin{tikzpicture}
    \draw[black,thick] (0,0)--(0,-10)--(0,10)--cycle;

\pathtoks{}
\foreachitem\x\in\mylist[]{%
  \ifnum\xcnt=1\relax \addpathtoks{\draw[black,thick] (1,0)}\fi
  \addpathtoks{--(1,}\xaddpathtoks{\x)}%
  \ifnum\xcnt=\listlen\mylist[]\relax\addpathtoks{--cycle;}\fi
}
\the\pathtoks

\end{tikzpicture}

\end{document}

SUPPLEMENT

If I understand what is actually being done here, one may be able to get away with two, rather than three, arguments to \band, if it is always the case that the original #2 will always be the list length of #3.

\documentclass[]{scrartcl}

\usepackage{tikz}
\usepackage{listofitems}
\newtoks\pathtoks
\newcommand\addpathtoks[1]{%
  \pathtoks\expandafter{\the\pathtoks#1}}
\newcommand\xaddpathtoks[1]{%
  \expandafter\addpathtoks\expandafter{#1}}
\newcommand\xxaddpathtoks[1]{%
  \expandafter\xaddpathtoks\expandafter{#1}}

\newcommand{\band}[2]{
\readlist*\mylist{#2}
\pathtoks{}
\foreachitem\x\in\mylist[]{%
  \ifnum\xcnt=1\relax \addpathtoks{\draw#1}\else
    \addpathtoks{--}
  \fi
  \addpathtoks{(360/}\xxaddpathtoks{\listlen\mylist[]*}%
  \xaddpathtoks{\x:}\xaddpathtoks{\x)}
  \ifnum\xcnt=\listlen\mylist[]\relax\addpathtoks{--cycle;}\fi
}
\the\pathtoks
}
\begin{document}
\begin{tikzpicture}

\band{[black,thick]}{1,2,3}
\draw[yellow,thin,dashed] (360/3:1) -- (360/3*2:2) -- (360/3*3:3) --cycle;
\end{tikzpicture}

\end{document}