[texhax] Some puzzling TeX (\@sptoken)

[Uwe Lueck]

Continuing puzzles about latex.ltx's \@sptoken,

    http://tug.org/mailman/htdig/texhax/2011-February/016809.html

-- guess what happens with

    \makeatletter \let\atest\@sptoken \show\atest \stop

and why ...

Cheers,

    Uwe.


-----Ursprüngliche Nachricht-----
Von: "Stephen Hicks" <sdh33 at cornell.edu>
Gesendet: 17.02.2011 01:50:44
An: "Rod Harries" <rod at harries.net.nz>
Betreff: Re: [texhax] Some puzzling TeX

>On Wed, Feb 16, 2011 at 3:44 PM, Rod Harries <rod at harries.net.nz> wrote:
>> I am trying to understand why one would write
>>
>> \def\:{\let\@sptoken= } \: %
>>
>> instead of just \let\@sptoken= %
>> or \def\@sptoken{ }
>>
>> Explanations (preferably designed for the simple-minded) would be appreciated.
>
>Have you tried either of those two options? It turns out they don't
>behave the same at all -- see the following session:
>
>====
>$ latex \\makeatletter
>This is pdfTeX, Version 3.1415926-1.40.10 (TeX Live 2009/Debian)
> restricted \write18 enabled.
>entering extended mode
>LaTeX2e <2009/09/24>
>Babel <v3.8l> and hyphenation patterns for english, usenglishmax, dumylang, noh
>yphenation, loaded.
>
>*\let\@sptoken= %
>
>*\relax
>
>*\show\@sptoken
>> \@sptoken=\relax.
><*> \show\@sptoken
>
>?
>
>*\def\@sptoken{ }
>
>*\show\@sptoken
>> \@sptoken=macro:
>-> .
><*> \show\@sptoken
>
>?
>
>*\def\:{\let\@sptoken= } \: %
>
>*\show\@sptoken
>> \@sptoken=blank space .
><*> \show\@sptoken
>
>?
>
>*
>! Emergency stop.
><*> \end
>
>No pages of output.
>Transcript written on texput.log.
>====
>
>What's happening here? It turns out that spaces are particularly
>tricky in TeX. I don't have my TeXBook on me right now, but the way
>the \let primitive is defined, it allows an optional "=" in between
>the token it's assigning to and the token it's assigning from. This
>optional "=" may be surrounded by any number of spaces. So when you
>simply say "\let\@sptoken= %" it's still waiting for the token to
>assign to \@sptoken, and the next non-space it encounters will be it
>(hence the \relax on the next line). In the case of "\def\@sptoken{
>}" that's completely different. This is defining a macro, which has a
>completely different category code than a space. The point here is
>that the author presumably wants \@sptoken to really be in every way a
>space, probably so he can use \ifcat to check if something else has
>the same catcode. But the \def'ed \@sptoken (and all normal control
>sequences) has catcode 16, rather than 9.
>
>So what is this actually doing? It defines the macro to actually
>contain the space. Moreover, when control characters (i.e. \: rather
>than \x) are expanded, they don't eat any following spaces. So then
>"\: " expands to \let\@sptoken=<space><space>, which normally is
>impossible to achieve since TeX eats adjacent spaces when it first
>tokenizes the file.
>
>Cheers,
>steve
>
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