Is there any way to expand the body of `NewDocumentCommand` at definition time?

Question

Is there way to expand the body passed to NewDocumentCommand at the point of definition? Ideally, just expanded the once.

Here is my (minimalised) scenario.

\documentclass{minimal}

\newcommand*{\baz}{\foo}
\newcommand*{\expandme}{good}

\ExplSyntaxOn
\exp_args:NV \NewDocumentCommand \baz { s D<>{} } {
    \expandme% Should get expanded once at definition time.
}
\ExplSyntaxOff

\renewcommand*{\expandme}{bad}

\begin{document}

    \foo<bar>% Prints "bad", but I want it to print "good".

\end{document}

Making the following modification yields an Undefined control sequence error, presumably because \exp_args is having a hard time with the { s D<>{} } argument.

\exp_args:NVNo \NewDocumentCommand \baz { s D<>{} } {
    \expandme% Should get expanded once at definition time.
}

Finally, note that the following solution, which works in the simplest case, but is problematic when \bar can take a number of values, and this code is repeated for each (as is my real scenario).

\exp_args:NNV \cs_set:Npn \tmp \expandme
% alternatively to the above line, I think: \edef\tmp\expandme
\exp_args:NV \NewDocumentCommand \baz { s D<>{} } {
    \tmp
}

Answer 1

You can use \expanded in recent tex releases:

\documentclass{minimal}

\newcommand*{\baz}{\foo}
\newcommand*{\expandme}{good}

\ExplSyntaxOn
\expanded{\noexpand\NewDocumentCommand\expandafter\noexpand\baz { s D<>{} } {
    \unexpanded\expandafter{\expandme}% Should get expanded once at definition time.
}}
\ExplSyntaxOff

\renewcommand*{\expandme}{bad}

\begin{document}

    \foo<bar>% Prints "bad", but I want it to print "good".

\end{document}

---

Or you can package it up as a custom VnV argument list

\documentclass{minimal}

\newcommand*{\baz}{\foo}
\newcommand*{\expandme}{good}

\ExplSyntaxOn
\exp_args_generate:n{VnV}
\exp_args:NVnV\NewDocumentCommand\baz { s D<>{} } \expandme
\ExplSyntaxOff

\renewcommand*{\expandme}{bad}

\begin{document}

    \foo<bar>% Prints "bad", but I want it to print "good".

\end{document}

Answer 2

If for some reason the needed \exp_args:N... variant isn't available, you can help yourself with \use:n:

\documentclass{minimal}

\newcommand*{\baz}{\foo}
\newcommand*{\expandme}{good}

\ExplSyntaxOn
\exp_args:Nno \use:n { \exp_args:NV \NewDocumentCommand \baz { s D<>{} } } {
    \expandme% Should get expanded once at definition time.
}
\ExplSyntaxOff

\renewcommand*{\expandme}{bad}

\begin{document}

    \foo<bar>% Prints "bad", but I want it to print "good".

\end{document}

Answer 3

You should define an interface for this.

\documentclass{article}

\newcommand*{\expandme}{good}

\ExplSyntaxOn
\cs_new_protected:Npn \NewDocumentCommandExpandOnce
 {
  \exp_args:NNno \NewDocumentCommand
 }
\ExplSyntaxOff

\NewDocumentCommandExpandOnce \foo { s D<>{} } {%
  \expandme% Should get expanded once at definition time.
}

\renewcommand*{\expandme}{bad}

\begin{document}

\foo<bar>% Prints "bad", but I want it to print "good".

\end{document}

You can check that the output is “good”.