How to treat lists like regular text (regarding vertical spacing)?

Question

Context: I want to treat lists (itemize, enumerate, and description) just like regular text for what regards vertical spacing. Namely:

1. If the list starts a paragraph, insert a vertical \parskip beforehand — no vertical spacing otherwise;
2. No vertical spacing between items of the list;
3. If the list ends a paragraph, insert a vertical \parskip afterwards — no vertical spacing otherwise.

I have notably used information found in \topsep, \itemsep, \partopsep and \parsep - what does each of them mean.

Requirement #1 is solved with \setlist{topsep=-\parskip,partopsep=\parskip}.

Requirement #2 is solved with \setlist{noitemsep}.

Problem: There remain two issues with Requirement #3:

• A vertical space is added after the list if the latter starts a paragraph, even if this list is immediately followed by text. (I.e. there is no such thing as an idependent parbottomsep length).
• If a new paragraph starts after the list, this paragraph is not preceded with a \parskip.

Question: How to comply with Requirement #3?
(I currently use manual patches — see MWE below — but it is of course not satisfactory.)

---

MWE

\documentclass[parskip=half]{scrartcl}
    \usepackage{enumitem}
    \setlist{%
        topsep=-\parskip,
        partopsep=\parskip,
        noitemsep,
    }
\begin{document}
    This sentence is a paragraph on its own; there is thus a vertical parskip prior next paragraph.

    Following list is \emph{within} a paragraph, with preceding and appended text.
    \begin{itemize}
        \item One,
        \item Two,
        \begin{itemize}
            \item Two and a half;
            \item Almost three.
        \end{itemize}
        \item Three.
    \end{itemize}
    This text is appended to the previous list.
    However, following list starts a new paragraph on its own.

    \begin{enumerate}
        \item Did you notice the vertical spacing preceding this list?
        \item Two,
        \begin{enumerate}
            \item Two and a half;
            \item Almost three.
        \end{enumerate}
        \item Three.
    \end{enumerate}
%   \vspace{-\parskip}                                  %quick and dirty solution
    \textbf{There shouldn't be a vertical spacing here.}
    This text is appended to the previous list too.

    And finally, a list with preceding text only.
    \begin{itemize}
        \item One,
        \item Two,
        \begin{itemize}
            \item Two and a half;
            \item Almost three.
        \end{itemize}
        \item Three.
    \end{itemize}

%   \null\par                                            %quick and dirty solution
    \textbf{There should be a vertical spacing here.}
    This is a new paragraph.
    It should thus be preceded with parskip.
\end{document}

Answer 1

This is hardly less dirty, but it uses the tools provided by enumitem, playing with the after key:

\documentclass[parskip=half]{scrartcl}
\usepackage{enumitem}
\setlist{%
    topsep=-\parskip,
    partopsep=\parskip,
    noitemsep,
}

\begin{document}

This sentence is a paragraph on its own; there is thus a vertical parskip prior next paragraph.

Following list is \emph{within} a paragraph, with preceding and appended text.
\begin{itemize}
    \item One,
    \item Two,
    \begin{itemize}
        \item Two and a half;
        \item Almost three.
    \end{itemize}
    \item Three.
\end{itemize}
This text is appended to the previous list.
However, following list starts a new paragraph on its own.

\begin{enumerate}[after =\vspace*{-\partopsep}]
    \item Did you notice the vertical spacing preceding this list?
    \item Two,
    \begin{enumerate}
        \item Two and a half;
        \item Almost three.
    \end{enumerate}
    \item Three.
\end{enumerate}
\textbf{There shouldn't be a vertical spacing here.}
This text is appended to the previous list too.

And finally, a list with preceding text only.
\begin{itemize}[after = \vspace*{\partopsep}]
    \item One,
    \item Two,
    \begin{itemize}
        \item Two and a half;
        \item Almost three.
    \end{itemize}
    \item Three.
\end{itemize}


\textbf{There should be a vertical spacing here.}
This is a new paragraph.
It should thus be preceded with parskip.

\end{document} 

Answer 2

Here is a possible solution using lua to get the next line in the source and check its emptiness. Of course, it only works with lualatex. It may need a bit of refining for quasi-empty lines (only spaces ?).

\documentclass{article}
\usepackage{luacode}
\begin{luacode}
function manageNextLine(s)
  luatexbase.remove_from_callback("process_input_buffer", "manageNextLine")
  if s == "" then
    return "\leavevmode\par"
  else
    return s
  end
end
\end{luacode}
\parskip=10ex  % to see it better
\usepackage{enumitem}
\setlist{noitemsep,
         topsep=-\parskip,
         partopsep=\parskip,
         after=\luadirect{luatexbase.add_to_callback("process_input_buffer", manageNextLine , "manageNextLine")}
  }
\begin{document}
Para
Para2
\begin{itemize}
\item 1
\item 2
\end{itemize}
%
Continuing the same para.
Para3
\begin{itemize}
\item A.
\item B.
\end{itemize}
Suite with vertical spacing.
\end{document}