Using longtable inside xparse \NewDocumentEnvironment

Question

I am trying to create a wrapper for the longtable environment and it works fine with the standard \newenvironment command. Now I want to use \NewDocumentEnvironment from the xparse package to allow more optional arguments. But the result looks different, there appears an additional cell below the table. Maybe it's caused by a linebreak from the \NewDocumentEnvironment command because the error doesn't appear when I move the \end{longtable} from the end-code-block to the start-code-block of the environment definition (like I did in the example in myxparsetable2 environment).

I created a minimal example. The error appears with myxparsetable1.

\documentclass{scrartcl}
\usepackage{xparse}
\usepackage{longtable}

\NewDocumentEnvironment{myxparsetable1}{m}
{\begin{longtable}{#1}}
{\end{longtable}}

\NewDocumentEnvironment{myxparsetable2}{}
{%
  \begin{longtable}{|c|c|c|} \hline%
    col1 & col2 & col3 \\ \hline%
  \end{longtable}%
}
{}

\newenvironment{mytable}[1]
{\begin{longtable}{#1}}
{\end{longtable}}

\begin{document}

  Normal longtable:
  \begin{longtable}{|c|c|c|} \hline
    col1 & col2 & col3 \\ \hline
  \end{longtable}

  Custom mytable
  \begin{mytable}{|c|c|c|} \hline
      col1 & col2 & col3 \\ \hline
  \end{mytable}

  Custom myxparsetable1
  \begin{myxparsetable1}{|c|c|c|} \hline
      col1 & col2 & col3 \\ \hline
  \end{myxparsetable1}

  Custom myxparsetable2
  \begin{myxparsetable2}
  \end{myxparsetable2}

\end{document}

And here is the output:

Answer 1

It's a known problem: with \NewDocumentEnvironment{foo}, the macro \endfoo, that's internally used by LaTeX when \end{foo} is processed is “protected” and, because of this, LaTeX can't see that the alignment is to be ended until it has already started a new row.

The situation is mostly similar to having \multicolumn not as the first item in a cell.

What can I offer, if you want to scan multiple arguments for \begin{foo}, a kludge:

\newenvironment{myxparsetable}{}{\end{longtable}}
\RenewDocumentCommand{\myxparsetable}{mO{}o}{%
  ...something with #2 and #3...
  \begin{longtable}{#1}
}