在定理环境周围包裹新环境

Question

我正在尝试创建一个新环境，该环境环绕包newtheorem中定义的定理环境amsthm。但是，将可选参数传入环境，然后传入定理环境会创建一个空括号。以下是我所看到的一个非常简单的示例

\documentclass[12pt, letter]{article}

\usepackage{lipsum}
\usepackage{xcolor}

\newtheorem{thm}{Theorem}

\newenvironment{thm2}[1][]{
    % The following line doesn't work???
    %\ifx&#1&{\begin{thm}}\else{\begin{thm}}\fi
    % However, the following does work but has issues
    \begin{thm}[#1]
}
{
    \end{thm}
}

\begin{document}

\begin{thm2}[Optional Argument] 
    This one looks a little better. 
\end{thm2}

\begin{thm2}
    What's up with the empty argument?
\end{thm2}

\end{document}

在网上查找后，我发现我可以测试 #1 是否为空，但测试这个似乎没什么用，因为\ifx&#1&{\begin{thm}}\else{\begin{thm}}\fi根本无法编译。我该如何解决这个问题？

Answer 1

您可以\ifstrempty从etoolbox包中使用：

代码：

\documentclass[12pt, letter]{article}

\usepackage{lipsum}
\usepackage{xcolor}
\usepackage{etoolbox}

\newtheorem{thm}{Theorem}

\newenvironment{thm2}[1][]{%
    % The following line now works?
    \ifstrempty{#1}{\begin{thm}}{\begin{thm}[#1]}%
}
{%
    \end{thm}%
}

\begin{document}

\begin{thm2}[Optional Argument] 
    This one looks a little better. 
\end{thm2}

\begin{thm2}
    This also look good now as there is no empty argument.
\end{thm2}

\end{document}

Answer 2

只需展开\else并\fi在发布之前\begin{thm}：

\makeatletter
\newenvironment{thm2}[1][]
  {\if\relax\detokenize{#1}\relax % test for emptiness
     \expandafter\@firstoftwo
   \else
     \expandafter\@secondoftwo
   \fi
   {\begin{thm}}{\begin{thm}[1]}%
  }
  {\end{thm}}
\makeatother

另一方面，使用以下方法会更容易一些xparse：

\usepackage{xparse}

\NewDocumentEnvironment{thm2}{o}
 {\IfNoValueTF{#1}{\begin{thm}}{\begin{thm}[#1]}
 {\end{thm}}