Re: [Dev-luatex] # or ##, that is the question

17 Oct 2007


      "Jonathan Sauer"  writes:
...
While writing unit tests for some of my macros, I stumbled on the
following weird behaviour when expanding \toks registers inside
\edef using \the (PlainTeX):
There is nothing strange here.
...
\catcode`\@=11
\def\log#1{\immediate\write\sixt@@n{#1}}
\toks@{|##|}
% Obvious result: Doubled
\edef\foo{\the\toks@}
\log{1: \meaning\foo}
% Obvious as well: Doubled
\edef\foo{\the\toks@\the\toks@}
\log{2: \meaning\foo}
% Now this is weird: First doubled, then not
\edef\foo{\the\expandafter\toks@\the\toks@}
\the\toks@ passes # unmolested into \edef.  In every other case (apart
from the equivalent use of \unexpanded), a doubled ## is required for
producing a single # in the result, because then # is a parameter
delimiting token for the \edef.  The \expandafter takes the ## out of
the token register and into the regular input stream, so the
#-preserving character of \the inside of \edef is no longer in action.
...
So when jumping out of the first expansion of the \toks register and
into the second expansion, TeX seems to forget that it is inside an
\edef and thus does not double the ##.
TeX _never_ doubles the # in \edef: you are confusing this with a
printing artifact.
...
I do not know what to make of this. I could not find any place in
the TeXbook where it is stated that # is doubled when expanding a
\toks register inside \edef, so this might not be a bug, but simply
some weird undocumented phenomenon.
It is not doubled, but simply left uninterpreted.
...
And it might be useful too, to build up the body of a complicated
macro inside a \toks register before finally creating the macro
using \edef:
What do you think?
You are about 20 years late.  Please check LaTeX's definition (and
usage) of \g@addto@macro:

\long\def\g@addto@macro#1#2{%
  \begingroup
    \toks@\expandafter{#1#2}%
    \xdef#1{\the\toks@}%
  \endgroup}
\def\AtEndOfPackage{%
  \expandafter\g@addto@macro\csname\@currname.\@currext-h@@k\endcsname}
\let\AtEndOfClass\AtEndOfPackage
\@onlypreamble\AtEndOfPackage
\@onlypreamble\AtEndOfClass
\def\AtBeginDocument{\g@addto@macro\@begindocumenthook}
\def\AtEndDocument{\g@addto@macro\@enddocumenthook}


-- 
David Kastrup