Hello,

There is currently a module in the expl3 bundle for dealing with lists
of tokens separated by commas. While there is a small application of
these things in areas such as XeTeX's font feature selection, this is
generally designed to offer convenient ways for users to input lists
of things.

My question for LaTeX3 is, "are commas enough?".

The current l3doc macros define the {function} environment that uses
vertical bar separated lists of function names:
\begin{function}{ \seq_push:Nn | \seq_push:NC }

I've written a LaTeX2e package called mlist that deals with "math
lists" that I define as two-d lists of items that eventually turn out
to be functions or matrices or vectors or whatever:
\matr{ a , b ;
c , d }

Whether we're dealing with commas, semicolons, or vertical bars, the
basic functions for manipulating the data would be largely identical.

There are two points of view here. One is that we should avoid making
things more complex than they already are and highly suggest that if
lists like this are going to be defined, then it's comma-lists or it's
nothing.

The other point of view is that we should allow some flexibility in
how users may end up needing to input list-like data, and providing a
range of delimited list types will be more convenient for (a) multi-
dimensional lists such as used by mlist, and (b) not having to escape
the delimiting characters as much; say if the list is likely to
contain commas then you can delimit it with vertical bars instead.

I lean towards the second philosophy. My tentative proposal is to
rename the clist module to, er, something else, and for it to
generalise the functions in clist such that functions for new
"delimiter"-lists can be instantiated with a single command.

I.e., the hypothetical command
\def_delimiter_list:nn {clist} {,}
would generate the suite of functions that are currently defined in
l3clist;
\def_delimiter_list:nn {slist} {;}
might then define equivalent commands for dealing with semi-colon
delimited lists.

* * *

Alternatively, maybe I'm taking this way too far and all we need is a
generalised mapping function that can loop through a token list using
an arbitrary substring as a delimiter. That would probably cover the
majority of the cases that \def_delimiter_list would be useful for.

* * *