निरंजन
I was experimenting with these two commands and I realized that there are cases where they aren't identical.
```
\documentclass{article}
\NewDocumentCommand{ \foocmd }{ }{%
\NewDocumentCommand{ \barcmd }{ >{\SplitList{,}} m }{%
\ProcessList{##1}{\textbf}%
}%
}
\NewDocumentEnvironment{ fooenv }{ }{%
\NewDocumentEnvironment{ barenv }{ >{\SplitList{,}} m }{%
\ProcessList{##1}{\textbf}%
}%
}
\begin{document}
\foocmd{Hello world\barcmd{a,b,c,d}}
\begin{fooenv}%
Hello world%
\begin{barenv}{%
a,b,c,d%
}%
\end{barenv}%
\end{fooenv}
\end{document}
```
I expect the output of both the commands to be same which it isn't.
This is rather strange. Is this the expected behavior or a bug?
Top Answer
Skillmon
The issue is that `\NewDocumentEnvironment` has an argument more than `\NewDocumentCommand`, that argument defines what should be done in the `\end` part of said environment. So the syntax is:
```
\NewDocumentEnvironment {<name>} {<args>}
{<begin>}
{<end>}
```
(this is analogue to `\newenvironment`, that also allows you to define the action in `\begin` and `\end`).
Now when you call your environment `fooenv` it'll do the `\NewDocumentEnvironment{barenv}` definition, and that one will grab an argument more than you provide (in your example that argument is `H`). Now the environments do what you tell them to, and on `\end{barenv}` the `H` is reinserted (as that's the definition for the `\end`-part now).
Please note that I'd say using `\NewDocumentEnvironment` to define a nested environment seems wrong. What I'd do is define it globally, but also define a toggle that'll throw an error if you don't use it nested in `fooenv`.
Consider the following example:
```
\documentclass{article}
\makeatletter
\NewDocumentEnvironment{ fooenv }{ }{%
\let\barenv@\@empty
}{}
\newcommand*\barenv@
{\GenericError{}{niranjan}{Misplaced environment barenv}{}}
\NewDocumentEnvironment{ barenv }{ >{\SplitList{,}} m }{%
\barenv@
\ProcessList{#1}{\textbf}%
}{}
\makeatother
\begin{document}
\begin{fooenv}%
Hello world%
\begin{barenv}{%
a,b,c,d%
}%
\end{barenv}%
\end{fooenv}
\end{document}
```