[texhax] defin'd?

[Reinhard Kotucha]

On 2015-12-23 at 13:30:15 +0000, Joseph Wright wrote:

 > On 23/12/2015 13:17, Uwe Lueck wrote:
 > > Discussing with Reinhard Kotucha, Philip Taylor argued
 > >  
 > >> P.S. I think the following demonstrates why \@undefined is
 > >> markedly inferior to \ifcsname :
 > > [etc.]
 > > 
 > > probably meaning inferiority of \LaTeX's \@ifundefined to the
 > > \varepsilon-\TeX primitive \ifcsname, in that \@ifundefined
 > > uses \csname and thus may assign a meaning to a named token
 > > that before didn't have a meaning, or simply may increase the
 > > hash table without increasing functionality. Both the two of
 > > them (the German comedian equivalent may be "die zwei beiden")
 > > also referred to a similar discussion on the list many years
 > > ago (Phil provided the URL).
 > > 
 > > I would like to remind readers of a third approach that uses
 > > \csname and though does not increase the hash table
 > > (permanently -- made in Germany): Bernd Raichle's german.sty
 > > has (had)
 > > 
 > >     \begingroup\expandafter\expandafter\expandafter\endgroup
 > >     \expandafter\ifx\csname ProvidesPackage\endcsname\relax
 > > 
 > > So definedness of \ProvidesPackage as \ifx-equivalent to 
 > > \relax is restricted to a local group, outside true 
 > > undefinedness is not affected.
 > > 
 > > There could be a variant \@IfUndefined of \@ifundefined with 
 > > an only local side effect as follows:
 > > 
 > >     \def\@IfUndefined#1{%
 > >         \begingroup\expandafter\expandafter\expandafter\endgroup
 > >         \expandafter\ifx\csname #1\endcsname\relax
 > >               \expandafter\@firstoftwo
 > >         \else \expandafter\@secondoftwo \fi}
 > 
 > Noting that this is not expandable, in contrast to LaTeX's \@ifundefined
 > and \ifcsname. (The latter is clearly the best approach when allowed.)

... and the macro above doesn't prevent anything from going into the
hash table.

Regards,
  Reinhard

 > > P.S.: I have ignored \varepsilon-\TeX\ until very recently ...
 > > sympathizing with minimalistic implementations of \TeX. On
 > > the other hand, I don't agree much when I read that the NTS
 > > project "failed" -- \varepsilon-\TeX\ has been a useful result.
 > 
 > Given that other than "tex", all widely-used binaries in use nowadays
 > feature the extensions, I think e-TeX is clearly a success. Whether it
 > achieved everything desirable is a different matter, of course.
 > 
 > Joseph
 > 
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