When defining a new macro inside a new macro (in xparse) how do I make the internal map long?

Question

I would like to use a DeclareDocumentCommand in order to be able to define a different command (using arguments from the first). I would like to make the internal arguments long. Here is the code I have as well as the output.

\documentclass{article}
\usepackage{xparse}
\begin{document}


\DeclareDocumentCommand{\DC}{m}{%
\DeclareDocumentCommand{#1}{+m}
{%
##1
}
}

\DC{test}

\test{Will this

break?}
\end{document}

and I get an "unknown argument type e" error.

However if I do

\documentclass{article}
\usepackage{xparse}
\begin{document}


\DeclareDocumentCommand{\test}{+m}{%
#1
}

\test{Will this

break?}
\end{document}

It works fine.

Answer 1

Your code produces

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!
! LaTeX error: "xparse/not-one-token"
! 
! First argument of '\DeclareDocumentCommand' must be a command.
! 
! See the LaTeX3 documentation for further information.
! 
! For immediate help type H <return>.
!...............................................  

which is due to the fact that you should call

\DC{\test}

---

Older versions of xparse would produce the error message

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!
! LaTeX error: "xparse/unknown-argument-type"
! 
! Unknown argument type 'e' replaced by 'm'.
! 
! See the LaTeX3 documentation for further information.
! 
! For immediate help type H <return>.
!...............................................  

but it's due to the same problem.

Answer 2

Without knowing more about what you are up to, I can't say I endorse this solution, though I often do find it useful to create a 'command maker' in order to create a series of commands that do the same general thing without actually writing out the full \newcommand sequence every time.

So maybe you have in mind this:

\documentclass{article}
\usepackage{xparse}

\DeclareDocumentCommand{\DC}{m}
{%
 \expandafter\DeclareDocumentCommand\expandafter{\csname #1\endcsname}{+m}
%  compare:
%  \expandafter\long\expandafter\def\csname #1\endcsname##1%
  {%
    {\bfseries ##1}% for example
  }%
}

\begin{document}

\DC{test}

\test{Will this

break?}

Again:
\DC{xtest}

\xtest{Will this

break?}

\end{document}

I hope you are being careful with \DeclareDocumentCommand!