Define a New Macro via a Macro Using xparse Syntax

Question

I am trying to set up a macro that defines a macro that is to be executed only once.

With what I learned from Define a Macro via Macro if Given Macro is not defined I can do that via the \expandafter\newcommand syntax. And indeed the MWE as below works and produces the desired output:

where the subsequent \HelloWorld and \HelloDude are ignored.

What I would like to do, is to learn how to use \NewDocumentCommand (instead of \newcommand) to define this new macro via a macro. My attempt (requires you to uncomment the \def at the top of the file) fails with

Illegal parameter number in definition of \reserved@a

Furthermore, am wondering if there is a way to define repeated parameters. That is instead of {m m} can I use something similar to with tabular where one can say \begin{tabular}{r*{3}{l}r} which is the identical to \begin{tabular}{rlllr}. This would save me from having to use \IfEqCasefor each possible number of parameters. I personally only need up to 3 so am ok with the slight duplication of code below, but though I'd ask in case there was some other syntax that I am not aware of.

Notes:

• You need to uncomment the \def at the top to get the MWE below to fail.

Refernces

• Define a Macro via Macro if Given Macro is not defined
• Defining Extensible macros

Code:

%% Uncomment the following \def to get the failing test case.
%\def\UseXparseForDefiningMacro{}% Works if commented out (in which case \newcommand{}{} is used to define macro)
\documentclass{article}
\usepackage{etoolbox}
\usepackage{xstring}
\usepackage{xparse}
\usepackage{xcolor}

\makeatletter
\NewDocumentCommand{\DefineMeAMacroThatExecutesOnlyOnce}{m m m}{%
    % #1 = csname to use
    % #2 = number of parameters
    % #3 = code to execute
    \newtoggle{AlreadyIssued#1}%
    \togglefalse{AlreadyIssued#1}%
    \ifdefined\UseXparseForDefiningMacro
        \IfEqCase{#2}{%
            {0}{\expandafter\NewDocumentCommand\csname#1\endcsname{     }{% No paramater version
                    \iftoggle{AlreadyIssued#1}{}{%
                        #3%                           Never executed this macro so go ahead an execute it,
                        \toggletrue{AlreadyIssued#1}% and remember that we did (so we don't do it again).
                    }%
                }}%
            {1}{\expandafter\NewDocumentCommand\csname#1\endcsname{  m  }{% 1 paramater version
                    \iftoggle{AlreadyIssued#1}{}{%
                        #3%                           Never executed this macro so go ahead an execute it,
                        \toggletrue{AlreadyIssued#1}% and remember that we did (so we don't do it again).
                    }%
                }}%
            {2}{\expandafter\NewDocumentCommand\csname#1\endcsname{ m m }{% 2 paramater version
                    \iftoggle{AlreadyIssued#1}{}{%
                        #3%                           Never executed this macro so go ahead an execute it,
                        \toggletrue{AlreadyIssued#1}% and remember that we did (so we don't do it again).
                    }%
                }}%
        }%
    \else% ---------------------------------------------  This works!!
        \expandafter\newcommand\csname#1\endcsname[#2]{%
            \iftoggle{AlreadyIssued#1}{}{%
                #3%                           Never executed this macro so go ahead an execute it,
                \toggletrue{AlreadyIssued#1}% and remember that we did (so we don't do it again).
            }%
        }%
    \fi
}
\makeatother

\DefineMeAMacroThatExecutesOnlyOnce{HelloWorld}{0}{% Does not take any parameters
    Hello World!%
}

\DefineMeAMacroThatExecutesOnlyOnce{HelloDude}{1}{% Take 1 parameter
    Hello \textcolor{red}{#1}.%
}

\begin{document}

%% Section 1: Works if \UseXparseForDefiningMacro is NOT defined
\HelloWorld
\HelloWorld

%% Section 2: Works if \UseXparseForDefiningMacro is NOT defined
\HelloDude{Peter}
\HelloDude{John}

\end{document}

Answer 1

You could do this but it's so wrong, why mix expl3 and etoolbox tests and why all the toggle stuff, if you want \foo to just execute once define it to be

 \def\foo{hello\let\foo\@empty}

No need for a separate toggle macro.

but anyway:

%% Uncomment the following \def to get the failing test case.
\def\UseXparseForDefiningMacro{}% Works if commented out (in which case \newcommand{}{} is used to define macro)
\documentclass{article}
\usepackage{etoolbox}
\usepackage{xstring}
\usepackage{xparse}
\usepackage{xcolor}

\makeatletter
\NewDocumentCommand{\DefineMeAMacroThatExecutesOnlyOnce}{m m m}{%
    % #1 = csname to use
    % #2 = number of parameters
    % #3 = code to execute
    \newtoggle{AlreadyIssued#1}%
    \togglefalse{AlreadyIssued#1}%
    \ifdefined\UseXparseForDefiningMacro
        \expandafter\NewDocumentCommand\csname#1\expandafter\endcsname
              \expandafter{\romannumeral#2000}{%
            \iftoggle{AlreadyIssued#1}{}{%
                #3%                           Never executed this macro so go ahead an execute it,
                \toggletrue{AlreadyIssued#1}% and remember that we did (so we don't do it again).
            }%
        }%
    \else% ---------------------------------------------  This works!!
        \expandafter\newcommand\csname#1\endcsname[#2]{%
            \iftoggle{AlreadyIssued#1}{}{%
                #3%                           Never executed this macro so go ahead an execute it,
                \toggletrue{AlreadyIssued#1}% and remember that we did (so we don't do it again).
            }%
        }%
    \fi
}
\makeatother

\DefineMeAMacroThatExecutesOnlyOnce{HelloWorld}{0}{% Does not take any parameters
    Hello World!%
}

\DefineMeAMacroThatExecutesOnlyOnce{HelloDude}{1}{% Take 1 parameter
    Hello \textcolor{red}{#1}.%
}

\begin{document}

%% Section 1: Works if \UseXparseForDefiningMacro is NOT defined
\HelloWorld
\HelloWorld

%% Section 2: Works if \UseXparseForDefiningMacro is NOT defined
\HelloDude{Peter}
\HelloDude{John}

\end{document}

Answer 2

Here is a solution for this problem, using (perhaps abusing) only expl3 features. I'm not really sure about the usefulness of this code. ;-)

\documentclass{article}
\usepackage{xparse}

\ExplSyntaxOn

\NewDocumentCommand{\NewOnceMacro}{m m m}
 {
  \grill_new_once_macro:Nnn #1 { #2 } { #3 }
 }

% an addition to the kernel functions
\cs_set_eq:NN \use_none: \prg_do_nothing:

\cs_new_protected:Npn \grill_new_once_macro:Nnn #1 #2 #3
 {
  \cs_set_eq:cc
   {% the corresponding "do nothing" macro
    \cs_to_str:N #1 -disabled
   }
   {% generate \use_none:<as many n's as #2>
    use_none: \prg_replicate:nn { #2 } { n }
   }
  \use:x
   {
    \exp_not:n { \NewDocumentCommand { #1 } }
      { \prg_replicate:nn { #2 } { m } } % the right number of m's
   }
   {
    % the first usage definition
    #3
    % then redefine the macro to do nothing
    \cs_gset_eq:Nc #1 { \cs_to_str:N #1 -disabled }
   }
 }

\ExplSyntaxOff

\NewOnceMacro{\HelloWorld}{0}{Hello world}
\NewOnceMacro{\Hello}{1}{Hello #1}
\NewOnceMacro{\Foo}{2}{Your #1 is full of #2}

\begin{document}

\HelloWorld
\HelloWorld

\Hello{Peter}
\Hello{Grill}

\Foo{hovercraft}{eels}
\Foo{head}{ideas}

\end{document}