A Command works fine when (defined) outside of a LaTeX3 Environment, but yields "Undefined control sequence" when (defined) inside the Environment

Question

A Command, e.g. \Subentry, works fine when defined outside a LaTeX3 Environment, e.g. {mainentry}, but as soon as one would like to put its definition inside the (definition of the) mentioned Environment a Undefined control sequence-error for that mentioned Command is yielded, meaning somehow LaTeX can not read its definition.

In MWE 1, no error is produced, apparently because the lines containing the \Subentry command, namely the following lines:

\def\Subentry{\def\SubentryS{\def\SubentryS{ \endgraf }}\futurelet\next\SubentryA}
\def\SubentryA{\ifx\bgroup\next\expandafter\SubentryB\fi}
\def\SubentryB#1#2{\SubentryS { #1 #2 {\bfseries #1} } \futurelet\next\SubentryA}

are defined outside of the Environment {mainentry}.

• In MWE 1, no Undefined control sequence is produced.
• In MWE 2, those lines will be placed inside the Environment {mainentry}, at the place of % ??? inside MWE 1. We also put the issuing of the Command, namely \Subentry{C}{D} within the issuing of the Environment. Then, we shift the definitions of the Command to the beginning of the Environment, instead of putting them at the end (following User Wipet's comment).

MWE 1 (no error)

% !TEX TS-program = lualatexmk

\documentclass[a4paper]{scrreprt}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Declare packages and commands
%
\usepackage{xparse}
\ExplSyntaxOn
\cs_set_eq:NN
\IfEmptyTF
\tl_if_blank:nTF
\ExplSyntaxOff
%
\NewDocumentEnvironment {mainentry} { m m m } {%
% ???
  \IfEmptyTF{#3}{%
#1 #2
  }{%
#1 #2 \endgraf #3
  }
  }{%
}
%
%
%
\def\Subentry{\def\SubentryS{\def\SubentryS{ \endgraf }}\futurelet\next\SubentryA}
\def\SubentryA{\ifx\bgroup\next\expandafter\SubentryB\fi}
\def\SubentryB#1#2{\SubentryS { #1 #2 {\bfseries #1} } \futurelet\next\SubentryA}
%
%
%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\begin{document}

\begin{mainentry}{A}{B}{}
\end{mainentry}

Some text.

\Subentry{C}{D}

\end{document}

MWE 2 (error)

Please note that I would like to put the (definition of the) Command \Subentry inside the (definition of the) Environment {mainentry} because I would like {\textbf #1} (which occurs inside that Command), to be replaced with {\textbf ##1} (or whatever code necessary ... I don't know exactly what syntax to use) so that it actually takes in the first argument of the Environment (so then {mainentry}) where it is placed in (speaking about definitions of), not just the Commands' own first argument.

Currently, the instance of {\textbf #1} yields C (in casu, the first argument of the Command it finds itself in), whereas I would like it to yield A (the first argument of the encompassing Environment, one level up in hierarchy, in which that Command may find itself) (speaking about definitions of).

Therefor, MWE 2 uses {\textbf ##1} instead of {\textbf #1}. (following User David Carlisle's comment). ---> PLEASE NOTE THAT I MISINTERPRETED THIS COMMENT ... NOTE BY OP AFTER FURTHER UNDERSTANDING ... ##1 actually refers to the inner argument, not the outer argument <---

The error will look something as follows:

./test.tex:69: Use of \SubentryB doesn't match its definition.

% !TEX TS-program = lualatexmk

\documentclass[a4paper]{scrreprt}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Declare packages and commands
%
\usepackage{xparse}
\ExplSyntaxOn
\cs_set_eq:NN
\IfEmptyTF
\tl_if_blank:nTF
\ExplSyntaxOff
%
\NewDocumentEnvironment {mainentry} { m m m } {%

\def\Subentry{\def\SubentryS{\def\SubentryS{ \endgraf }}\futurelet\next\SubentryA}
\def\SubentryA{\ifx\bgroup\next\expandafter\SubentryB\fi}
\def\SubentryB#1#2##1{\SubentryS { #1 #2 {\bfseries ##1} } \futurelet\next\SubentryA}

  \IfEmptyTF{#3}{%
#1 #2
  }{%
#1 #2 \endgraf #3
  }
  }{%

}
%
%
%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\begin{document}

\begin{mainentry}{A}{B}{}

\Subentry{C}{D}

\end{mainentry}

Some text.

\end{document}

Answer 1

You can be experimenting and thinking about the following code:

\documentclass[a4paper]{scrreprt}

\newenvironment {mainentry} {\mainentryA}{}
\def\mainentryA#1#2#3{%
   \def\SubentryC##1##2{%
      Here is external parameter (#1) and second external (#2).
      And internal parameter is (##1) and second internal (##2).
   }%
   \ifx\mainentryA#3\mainentryA  % if #3 is empty
   #1 #2
   \else
   #1 #2\endgraf #3
   \fi
}
\def\Subentry{\def\SubentryS{\def\SubentryS{\endgraf}}\futurelet\next\SubentryA}
\def\SubentryA{\ifx\bgroup\next\expandafter\SubentryB\fi}
\def\SubentryB#1#2{\SubentryS\SubentryC{#1}{#2}\futurelet\next\SubentryA}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\begin{document}
\begin{mainentry}{A}{B}{}
\Subentry{C}{D}{E}{F}
NEXT
\Subentry{U}{V}{W}{X}{Y}{Z}
\end{mainentry}
Some text.
\end{document}

You can use LaTeX and see the output. The code is derived from your code but I didn't use redundant packages (specially, I use none packages) and redundant code in order to focus only on your problem.

Answer 2

The error you get (at least from the last example) is not “Undefined control sequence”, but “Use of \SubentryB doesn't match its definition”. Indeed you do

\def\SubentryB#1#2##1{\SubentryS { #1 #2 \href{http://a.beautiful.url/?q=\percentifier{##1}}{test-link} } \futurelet\next\SubentryA}

which defines \SubentryB to require being followed by whatever the first two arguments to the current mainentry environment are. You probably mean something like

\def\SubentryB##1##2{\SubentryS { ##1 ##2 \href{http://a.beautiful.url/?q=\percentifier{##1}}{test-link} } \futurelet\next\SubentryA}

but it's difficult to guess based on your example. In your case, you are doing

\def\SubentryB AB#1{\SubentryS { A B \href{http://a.beautiful.url/?q=\percentifier{#1}}{test-link} } \futurelet\next\SubentryA}

because TeX performs replacement of parameter with arguments and then turns ## into # when doing the definition. So you see why \SubentryB has to be followed by AB, but none of its calls is.