xparse's 's' argument returns \Gamma and \Delta, instead of \BooleanFalse and \BooleanTrue

Question

The xparse documentation says,

But that's not what I get when I test s.

\documentclass{article}
\usepackage{xparse}

\NewDocumentCommand{\myfunc}{s}{#1}

\begin{document}
\myfunc    % should return \BooleanFalse; actually returns \Gamma
\myfunc*   % should return \BooleanTrue; actually returns \Delta
\end{document}

Edit: Also, \IfValueTF doesn't return 0 when * is absent.

\documentclass{article}
\usepackage{xparse}

\NewDocumentCommand{\myfuncB}{s}{
    \IfValueTF{#1}{1}{0}
}

\begin{document}
\myfuncB    % should return 0; actually returns 1
\myfuncB*   % should return 1; actually returns 1
\end{document}

Answer 1

\BooleanFalse and \BooleanTrue are boolean variables not meant for typesetting. You can only use them in a \IfBooleanTF test:

\documentclass{article}
\usepackage{xparse}

\NewDocumentCommand{\myfuncB}{s}{%
    \IfBooleanTF{#1}{1}{0}%
}

\begin{document}
\myfuncB    % returns 0
\myfuncB*   % returns 1
\end{document}

---

\IfValueTF can't be used either because it checks if the argument is -NoValue-, and neither \BooleanTrue nor \BooleanFalse are -NoValue-, so the test returns true always. \IfValueTF is supposed to be used with optional arguments like o and d.

---

Under the hood, \BooleanFalse is \char"0 and \BooleanTrue is \char"1 so they take the zeroth and first character of the current font, whatever that happens to be. In the OT1 encoding, \char"0 and \char"1 are the glyphs Γ and ∆:

\documentclass{article}
\usepackage{fonttable}
\begin{document}
\fonttable{cmr10}
\end{document}