\newcommand inside a \newcommand: variable already defined

Question

Consider the following code:

\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{calc}
\usepackage{etoolbox}
\begin{document}
\newcommand{\ARROW}[2]{
  \ifstrequal{#2}{left}{
    \newcommand{\coef}{-1}
  }{}
  \ifstrequal{#2}{right}{
    \newcommand{\coef}{1}
  }{};
  \coordinate(a) at (#1);
  \coordinate(b) at ($(a) + (\coef*2cm, 0)$);
  \draw[->](a) -- (b);
}
\begin{tikzpicture}
  \ARROW{1, 0}{right};
  \ARROW{2, 1}{left};
\end{tikzpicture}
\end{document}

When invoking the second time the \newcommand \ARROW (\ARROW{2, 1}{left};) the code complains that \coef is already designed.

How to optimise the code to circumvent this problem?

Answer 1

Why not using keys?

\documentclass[border=20]{standalone}
\usepackage{tikz}
\usetikzlibrary{calc}

\tikzset{
  /viesturs/right/.code=\def\ARROWCOEFF{+},
  /viesturs/left/.code=\def\ARROWCOEFF{-},
}

\newcommand{\ARROW}[2]{%
  \tikzset{/viesturs/#2}
  \coordinate(a) at (#1);
  \coordinate(b) at ($(a) + (\ARROWCOEFF 2cm, 0)$);
  \draw[->](a) -- (b);
}
\providecommand{\ARROWCOEFF}{}

\begin{document}

\begin{tikzpicture}
  \ARROW{1, 0}{right};
  \ARROW{2, 1}{left};
\end{tikzpicture}

\end{document}