為什麼這次語言測試失敗了？

Question

我的問題是以下程式碼：

\documentclass{article}
\usepackage[english]{babel}
\begin{document}
\def\suplang{english}
\ifx\languagename\suplang
    Supported language: \languagename
\else 
    Not supported language: \languagename
\fi
\end{document}

結果是“不支援的語言：英語”而不是“支援的語言：英語”。我做錯了什麼？

Answer 1

出於實施原因， 的替換文字的\languagename第一個字元為類別代碼 12；就目前\ifx而言，

e₁₁ n₁₁ g₁₁ l₁₁ i₁₁ s₁₁ h₁₁

和

e₁₂ n₁₁ g₁₁ l₁₁ i₁₁ s₁₁ h₁₁

是不同的。確實，如果你這樣做

\documentclass{article}
\usepackage[english]{babel}
\begin{document}
\edef\suplang{\string english}
\ifx\languagename\suplang
    Supported language: \languagename
\else
    Not supported language: \languagename
\fi
\end{document}

那麼輸出是

你需要做那個生意嗎？不，當然不是。

\usepackage{pdftexcmds} % to work with all engines
\makeatletter
\newcommand{\supported}[3]{%
  \ifnum\pdf@strcmp{\languagename}{#1}=\z@
     \expandafter\@firstoftwo
  \else
     \expandafter\@secondoftwo
  \fi
  {#2}{#3}%
}
\makeatother

然後

\supported{suplang}
  {Supported language: \languagename}
  {Not supported language: \languagename}

將工作。

\documentclass{article}
\usepackage[english]{babel}
\usepackage{pdftexcmds} % to work with all engines
\makeatletter
\newcommand{\supported}[3]{%
  \ifnum\pdf@strcmp{\languagename}{#1}=\z@
     \expandafter\@firstoftwo
  \else
     \expandafter\@secondoftwo
  \fi
  {#2}{#3}%
}
\makeatother

\def\suplang{english}

\begin{document}

\supported{\suplang}
  {Supported language: \languagename}
  {Not supported language: \languagename}

\end{document}

true 和 false 分支的替代定義\else：\fi

\documentclass{article}
\usepackage[english]{babel}
\usepackage{pdftexcmds} % to work with all engines
\makeatletter
\newcommand{\issupported}[1]{%
  TT\fi\ifnum\pdf@strcmp{\languagename}{#1}=\z@
}
\makeatother

\def\suplang{english}

\begin{document}

\if\issupported\suplang
  Supported language: \languagename
\else
  Not supported language: \languagename
\fi

\end{document}

Answer 2

這回答egreg 已經解釋了問題中範例出現意外結果的原因，並且還展示了解決該問題的幾種方法。

套件iflang將比較的內部內容包裝\languagename在一個簡單的巨集中\IfLanguageName。問題的例子變成：

\documentclass{article}
\usepackage[english]{babel}
\usepackage{iflang}
\begin{document}
\IfLanguageName{english}{%
    Supported language: \languagename
}{%
    Not supported language: \languagename
}
\end{document}