Command inside tabular with automatic \hline placement

Question

I am trying to define a command that simplifies building tables. I want the last entry to be automatically preceded by a horizontal line. I tried to do a minimal working example, and it worked (surprisingly), but it does not work in my more elaborate code.

\documentclass{article}
\usepackage{xparse}

%%% Works

\newcommand{\foo}[2]{#1 & #2\\}

\newcommand{\foolast}[2]{\hline
    #1 & #2}

\newcounter{premn}
\setcounter{premn}{0}

%%% Does not work if \hline is present

\newcommand{\premform}{\arabic}

\NewDocumentCommand{\preml}{O{\premform{premn}} m o}{%
    \IfNoValueTF{#3}%
    {%
        \refstepcounter{premn}%
        (#1) & #2 & \\
    }
    {%
        \refstepcounter{premn}%
        (#1) & #2 & #3\\
    }
}

\NewDocumentCommand{\conl}{O{\premform{premn}} m o}{%
    \IfNoValueTF{#3}%
    {%\hline
        \refstepcounter{premn}%
        (#1) & #2 & %
        \setcounter{premn}{0}%
        \ignorespaces%
    }
    {%\hline
        \refstepcounter{premn}%
        (#1) & #2 & #3 %
        \setcounter{premn}{0}%
        \ignorespaces%
    }
}

%%% MWE

\begin{document}

\begin{tabular}{ll}
\foo{a}{b}
\foolast{a}{b}
\end{tabular}

\begin{tabular}{lll}
\preml{a}
\conl{b}
\end{tabular}
    
\end{document}

I am sure there is an answer to this, but it is beyond my knowledge.

Answer 1

With the help of the tabularray package you can access the row number of each table row and also make sure that a horizontal lines is automatically drawn above the last row:

\documentclass{article}
\usepackage{tabularray}

\begin{document}

\begin{tblr}{colspec={>{(\arabic{rownum})\enspace}ll}, hline{Y}}
a & b \\
a & b \\
a & b \\
a & b \\
a & b \\
a & b \\
\end{tblr}
    
\end{document}

Answer 2

You can do that with {NiceTabular} of nicematrix and Tikz.

\documentclass{article}
\usepackage{nicematrix,tikz}

\begin{document}

\begin{NiceTabular}
  [code-after = { \tikz \draw (\arabic{iRow}-|1) -- (\arabic{iRow}-|last) ; } ]
  {>{(\arabic{iRow})\enspace}ll}
a & b \\
a & b \\
a & b \\
a & b \\
a & b \\
a & b \\
\end{NiceTabular}
    
\end{document}

You need several compilations (because nicematrix uses PGF/Tikz nodes under the hood).