使用 pgf 文档中的自定义箭头 foo 时出现错误

Question

我正在定义一个新的箭头。我通读了文档，并决定进行一些小改动，将现有的 foo 定义移至我的自定义箭头定义。

但是，我遇到了一个问题。当我尝试编译手册中的代码（3.0.1a 手册的第 1004 节、第 1020 页）时，出现以下错误：

Missing number, treated as zero.

我希望得到帮助以了解错误的原因，并获得一些关于如何自行解决此类错误的建议。

MWE 如下：

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{arrows.meta}

\pgfdeclarearrow{
  name = foo,
  parameters = { \the\pgfarrowlength },
  setup code = {
    % The different end values:
    \pgfarrowssettipend{.25\pgfarrowlength}
    \pgfarrowssetlineend{-.25\pgfarrowlength}
    \pgfarrowssetvisualbackend{-.5\pgfarrowlength}
    \pgfarrowssetbackend{-.75\pgfarrowlength}
% The hull
\pgfarrowshullpoint{.25\pgfarrowlength}{0pt} \pgfarrowshullpoint{-.75\pgfarrowlength}{.5\pgfarrowlength} \pgfarrowshullpoint{-.75\pgfarrowlength}{-.5\pgfarrowlength} % Saves: Only the length:
    \pgfarrowssavethe\pgfarrowlength
  },
  drawing code = {
    \pgfpathmoveto{.25\pgfarrowlength}{0pt}
    \pgfpathlineto{-.75\pgfarrowlength}{.5\pgfarrowlength}
    \pgfpathlineto{-.5\pgfarrowlength}{0pt}
    \pgfpathlineto{-.75\pgfarrowlength}{-.5\pgfarrowlength}
    \pgfpathclose
    \pgfusepathqfill
},
  defaults = { length = 4cm }
}

\begin{document}



\begin{tikzpicture}
\draw [-foo] (0,0) -- (8,0);
\end{tikzpicture}

\end{document}

Answer 1

我建议将此作为文档中的错误报告。我猜应该是这样的：

\documentclass[border=10pt,multi,tikz]{standalone}
\usetikzlibrary{arrows.meta}
\pgfdeclarearrow{%
  name=foo,
  parameters={%
    \the\pgfarrowlength
  },
  setup code={%
    % The different end values:
    \pgfarrowssettipend{.25\pgfarrowlength}
    \pgfarrowssetlineend{-.25\pgfarrowlength}
    \pgfarrowssetvisualbackend{-.5\pgfarrowlength}
    \pgfarrowssetbackend{-.75\pgfarrowlength}
    % The hull
    \pgfarrowshullpoint{.25\pgfarrowlength}{0pt}
    \pgfarrowshullpoint{-.75\pgfarrowlength}{.5\pgfarrowlength}
    \pgfarrowshullpoint{-.75\pgfarrowlength}{-.5\pgfarrowlength}
    % Saves: Only the length:
    \pgfarrowssavethe\pgfarrowlength
  },
  drawing code={%
    \pgfpathmoveto{\pgfqpoint{.25\pgfarrowlength}{0pt}}
    \pgfpathlineto{\pgfqpoint{-.75\pgfarrowlength}{.5\pgfarrowlength}}
    \pgfpathlineto{\pgfqpoint{-.5\pgfarrowlength}{0pt}}
    \pgfpathlineto{\pgfqpoint{-.75\pgfarrowlength}{-.5\pgfarrowlength}}
    \pgfpathclose
    \pgfusepathqfill
  },
  defaults={%
    length=4cm,
  },
}

\begin{document}
\begin{tikzpicture}
  \draw [-foo] (0,0) -- (8,0);
\end{tikzpicture}
\end{document}