Error with length of list of listofitems and ... in \foreach

Question

While I was answering another question, I found the error

! Illegal unit of measure (pt inserted).

only when I use ... in a \foreach with a simple operation of the list length:

\documentclass{article}
\usepackage{tikz}
\usepackage{listofitems}
\setsepchar{;}
\begin{document}
\readlist\points{(1,2); (2,3); (3,2); (1,1); (2,2)}
This works:
\begin{tikzpicture}
    \foreach \i  in {1,2,3,4}
        {\draw \points[\i] -- \points[\i+1];}
\end{tikzpicture}
This works:
\begin{tikzpicture}
    \foreach \i  in {1,...,4}
        {\draw \points[\i] -- \points[\i+1];}
\end{tikzpicture}
This works:
\begin{tikzpicture}
    \foreach \i  in {1,2,3,\pointslen-1}
        {\draw \points[\i] -- \points[\i+1];}
\end{tikzpicture}
This gives the error 
\texttt{! Illegal unit of measure (pt inserted).}
\begin{tikzpicture}
    \foreach \i  in {1,...,\pointslen-1}
        {\draw \points[\i] -- \points[\i+1];}
\end{tikzpicture}
\end{document}

Answer 1

Not sure where the actual problem is (though my guess is in the \foreach limits evaluator), but enclosing the suspect term in \numexpr...\relax (to evaluate an integer) fixes it.

\documentclass{article}
\usepackage{tikz}
\usepackage{listofitems}
\setsepchar{;}
\begin{document}
\readlist\points{(1,2); (2,3); (3,2); (1,1); (2,2)}
This works:
\begin{tikzpicture}
    \foreach \i  in {1,2,3,4}
        {\draw \points[\i] -- \points[\i+1];}
\end{tikzpicture}
This works:
\begin{tikzpicture}
    \foreach \i  in {1,...,4}
        {\draw \points[\i] -- \points[\i+1];}
\end{tikzpicture}
This works:
\begin{tikzpicture}
    \foreach \i  in {1,2,3,\pointslen-1}
        {\draw \points[\i] -- \points[\i+1];}
\end{tikzpicture}
%This gives the error 
%\texttt{! Illegal unit of measure (pt inserted).}
\begin{tikzpicture}
    \foreach \i  in {1,...,\numexpr\pointslen-1\relax}
        {\draw \points[\i] -- \points[\i+1];}
\end{tikzpicture}
\end{document}

Answer 2

The last item in the specification of the range for \foreach has to be an integer. Also

\foreach \i in {1,...,5-1} {...}

would fail with the same error. To the contrary, \numexpr\pointslen-1\relax will work.

An expl3 implementation:

\documentclass{article}
\usepackage{tikz}
\usepackage{xfp}
\ExplSyntaxOn
\NewDocumentCommand{\definelist}{mm}
 {
  \seq_clear_new:c { l_carlatex_list_#1_seq }
  \seq_set_split:cnn { l_carlatex_list_#1_seq } { ; } { #2 }
 }
\cs_generate_variant:Nn \seq_set_split:Nnn { c }
\NewExpandableDocumentCommand{\getlistitem}{mm}
 {
  \seq_item:cn { l_carlatex_list_#1_seq } { #2 }
 }
\NewExpandableDocumentCommand{\getlistlen}{m}
 {
  \seq_count:c { l_carlatex_list_#1_seq }
 }
\ExplSyntaxOff
\begin{document}
\definelist{points}{(1,2); (2,3); (3,2); (1,1); (2,2)}
This works:
\begin{tikzpicture}
  \foreach \i  in {1,...,\inteval{\getlistlen{points}-1}} {
    \draw \getlistitem{points}{\i} -- \getlistitem{points}{\i+1};
  }
\end{tikzpicture}
\end{document}