What is the meaning of \meaning?

Question

In the code of endnotes.sty there is a command \meaning that is used in the following definition

\long\def\@endnotetext#1{%
     \if@enotesopen \else \@openenotes \fi
     \begingroup
        \def\next{#1}%
        \newlinechar='40
        \immediate\write\@enotes{\meaning\next}%
     \endgroup}

If I remove it, it doesn't work anymore (I get the funny error Undefined control sequence. \GenericError). What is the use of \meaning ?

Answer 1

~ $ tex
This is TeX, Version 3.14159265 (TeX Live 2016) (preloaded format=tex)
**\relax

*\def\test#1{This is a test #1 macro}

*\show\test
> \test=macro:
#1->This is a test #1 macro.
<*> \show\test

\show gives the meaning (definition) of a macro (or a primitive) in the log file and the terminal. \meaning shows the same but puts it in the document instead. In the example above it is written to the notes file.

Answer 2

If you process the document

\documentclass{article}
\usepackage{endnotes}

\begin{document}

x\endnote{Recall that $\sqrt{2}$ is irrational}

\end{document}

you'll get a file with extension .ent containing

\@doanenote {1}
macro:->Recall
that
$\sqrt
{2}$
is
irrational
\@endanenote 

This is a consequence of the definition

\long\def\@endnotetext#1{%
     \if@enotesopen \else \@openenotes \fi
     \immediate\write\@enotes{\@doanenote{\@theenmark}}%
     \begingroup
        \def\next{#1}%
        \newlinechar='40
        \immediate\write\@enotes{\meaning\next}%
     \endgroup
     \immediate\write\@enotes{\@endanenote}}

and of

\let\@doanenote=0
\let\@endanenote=0

which make the two control sequences into unexpandable tokens.

The \meaning primitive takes as argument a single token and expands to a string representation of the token: all characters will have category code 12, except for space tokens that are normalized to character code 32 (space) with category code 10. If the token is a macro (that is, it has been defined with \def, \edef, \gdef or \xdef), the representation will be of the form

macro:〈parameter text〉->〈replacement text〉

Thus, in the endnotes case above, \meaning\next will expand to

macro:->Recall that $\sqrt {2}$ is irrational

because the parameter text is empty. By rule, each control word in the string representation will be followed by a space.

The macro \@endnotetext does three \write operations. In the first, \@doanote {1} is written out, because \@doanote is not expandable so it is written out as is (with a trailing space), whereas \@theenmark will be expanded. Also { and } are unexpandable, so we get the first line in the example.

Then the expansion of \meaning\next is written, under the setting \newlinechar='40 which means that spaces (character code decimal 32, octal '40) will be transformed into newline characters. I guess this is done for avoiding overlong lines, in case of long endnotes.

Finally \@endanenote is written (on a new line).

When the .ent file is read in by the \theendnotes command, \@doanenote is given a new meaning; basically it reads until > is found, emits \begingroup, sets the mark to the tokens in braces and starts typesetting the note text; \@endanenote is redefined to issue \par\endgroup.

---

What does \meaning return in other cases? Here are a few examples; I use plain TeX, but it would be the same in all flavors of TeX.

\let\foo=0 % like in endnote.sty
\def\next{Recall that $\sqrt{2}$ is irrational}
\def\macro#1{Something with #1}

\tt % typewriter type

\meaning\relax

\meaning a

\meaning 0

\meaning\alpha

\meaning\foo

\meaning\next

\meaning\macro

\meaning ~

\meaning\&

\meaning\pageno % plain tex

\bye

A control sequence \let to a character will display the same as if the character was used. A control sequence defined with \chardef (like \&) gives \char" followed by the hexadecimal number of the character; similarly for \mathchardef (like \alpha).

The macros \next and \macro show the difference when there is a parameter text; finally, primitives represent themselves (like \relax). There are a few other cases, I showed \pageno that's defined with \countdef.