taple.tex

\documentclass{jarticle}

\makeatletter

% registers are ALWAYS allocated globally (darn!)
\newcount\@fuck@i
\newcount\@fuck@ii

\def\tuple#1{%
	\begingroup
		\@fuck@i = -1
		\@fuck@ii = 0
		\@for\mem:=#1\do{\advance\@fuck@i by 1\relax } %harmful space chopped
		\left(
		\@for\mem:=#1\do{%
			% no use fiddling around \@tempcnta etc...
			\ifnum \@fuck@ii<\@fuck@i\relax
				\mem,
				\advance\@fuck@ii by 1\relax
			\else
				\mem
			\fi
		}
		\right)
	\endgroup
}
\makeatother

\begin{document}
$ \tuple{ \tuple{a, b, 1, 2}, \tuple{d, i, a}, a, 1, 2, \tuple{1, 2}} $

\end{document}