Why do TeX list items use two backslashes?

Question

In TeX packages one finds definitions of lists as:

\def\l{\\\alpha\\\beta\\\gamma}

The one backslash is understandable, it represent the active character for the macro i.e, \gamma, but what do the other two do, why are they there?

Answer 1

The \\ is just an arbitrary (short) control sequence. It allows you to do something with the list just by defining it as a one-parameter macro:

\begingroup
\def\\#1{\typeout{\string#1}}
\l  % prints out the tokens in the list
\endgroup

However, I think \\ is a bad choice because it has a meaning in TeX/LaTeX. LaTeX generally uses \do for this purpose, and packages such as expl3 use a special hidden separator so that you can add \\ or \do as normal list elements.

Answer 2

If you want a list of control sequences, you don't need the extra \\ — you can use token lists and "pop" off the first item. Eijkhout's TeX by Topic gives some code that does that in his chapter on token lists.

If you want more general contents in the list, you need a separator. The \\ is what Knuth used as a separator in Appendix D of The TeXbook, so I guess that is the convention, but you could use an active character like ~ as a separator.