Today I came across these three commands:
\@nil
\@cdr
\@car
Some are mentioned here and here, but I didn't get the idea of it. It seems to have to do with the macro character.
I haven't seen any description about it on google and TeX.SX.
asked Feb 13, 2017 at 21:54
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I retagged from tex-core to latex-base as these are latex macros– David CarlisleCommented Feb 13, 2017 at 22:01
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3This question explains them quite well: Lisp relics in LaTeXe logo– Alan MunnCommented Feb 13, 2017 at 22:02
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@AlanMunn ooh it does but this one has a better title– David CarlisleCommented Feb 13, 2017 at 22:03
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That's probably the reason I haven't found it– MaestroGlanzCommented Feb 13, 2017 at 22:04
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1 Answer
the names come from lisp.
In lisp, car returns the head of a list, cdr returns the tail of a list and nil is an empty list.
in latex
\def\@car#1#2\@nil{#1}
\def\@cdr#1#2\@nil{#2}
(\@nil is not defined at all)
so
\@car abc\@nil
expands to a
and
\@cdr abc\@nil
expands to bc
answered Feb 13, 2017 at 21:57
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But how does this work, if
\@nilis undefined. Why is there no undefined error? Commented Feb 13, 2017 at 22:02 -
6@MaestroGlanz tokens only make an undefined error if you try to expand them. tokens used to delimit a macro argument are not expanded, try
\def\foo#1\wibblethingy{hello #1}...\foo Maestro\wibblethingyCommented Feb 13, 2017 at 22:06 -
2@egreg we should put in a change request
\tl_car:nandtl_cdr:nto honour the cultural heritage Commented Feb 13, 2017 at 22:08 -