弧线问题

Question

我需要根据变量构造一个弧。有些代码行可以工作，有些则不行。有什么想法吗？

\documentclass[tikz]{standalone}
\begin{document}

\def\BLS{\baselineskip}

\begin{tikzpicture}

% works
\draw[->] (0,0) arc (-90:90:.5*\BLS) ;

% works
\draw[->] (0,0) arc (-90:90:\BLS{} and 2cm) ;

% don't work
\draw[->] (0,0) arc (-90:90:.5*\BLS{} and 2cm) ;

% don't work
\draw[->] (0,0) arc (-90:90:.5*\BLS and \BLS) ;

\end{tikzpicture}

\end{document}  

Answer 1

使用

\draw[->] (0,0) arc (-90:90:{.5*\BLS} and 2cm);   
\draw[->] (0,0) arc (-90:90:{.5*\BLS} and \BLS);

Answer 2

或者使用：

\draw[->] (0,0) arc (-90:90:\BLS{}/2 and 2cm) ;
\draw[->] (0,0) arc (-90:90:\BLS/2 and \BLS) ;

Answer 3

另一种方法：

\draw[->] (0,0) arc (-90:90:\dimexpr.5\BLS\relax\space and 2cm) ;