@nameuse as conditional

Question

Please consider the following, non-working, example:

\documentclass{standalone}

\makeatletter
  \parindent\z@
  \@namedef{blafasel}{foo}
  \def\foo{foo}
  \def\bar{bar}
\makeatother

\begin{document}
1:
\expandafter\ifx\@nameuse{blafasel}\foo
  \message{true}
\else
  \message{false}
\fi    

2: 
\expandafter\ifx\@nameuse{blafasel}\bar
  \message{true}
\else
  \message{false}
\fi

\end{document}

How must I alter this code so that the output is "correct" (1: true\n2: false)? So far, I tried up to 9 \expandafter before \ifx, but none gave me the desired output. Why is that?

EDIT

Okay, my mistake was that @ wasn't a letter anymore... Using three \expandafters did the trick. Nonetheless i'd like to know what happens here. How does LaTeX expand @namedef and @nameuse resp.? Extending the example by the following code also gives weired results:

3: 
\expandafter\ifx\csname blafasel\endcsname\bar
  true
\else
  false
\fi

4: 
\expandafter\ifx\csname blafasel\endcsname\bar
  true
\else
  false
\fi

Answer 1

You're almost there. Two problems only.

First, \@nameuse contains an @ in the name, so you need to \makeatletter before using it.

Second, if you \show\@nameuse (after a \makeatletter, of course) you see:

> \@nameuse=macro:
#1->\csname #1\endcsname 

so one expansion of \@nameuse{foo} yields \csname blafasel\endcsname. The \csname...\endcsname requires another expansion to make \blafasel, which is what you want. So now you know you need two expansions of \@nameuse.

With

\expandafter\ifx\@nameuse{blafasel}\foo

you expand it once. To expand twice you need one more \expandafter before each token that precedes \@nameuse. That is, one \expandafter for the "old" \expandafter, and another for the \ifx:

\expandafter\expandafter\expandafter\ifx\@nameuse{blafasel}\foo

Now the code:

\documentclass{standalone}

\makeatletter
  \parindent\z@
  \@namedef{blafasel}{foo}
  \def\foo{foo}
  \def\bar{bar}
\makeatother

\begin{document}
1:
\makeatletter
\expandafter\expandafter\expandafter\ifx\@nameuse{blafasel}\foo
  \message{true}
\else
  \message{false}
\fi    

2:
\expandafter\expandafter\expandafter\ifx\@nameuse{blafasel}\bar
  \message{true}
\else
  \message{false}
\fi

\end{document}

prints true false somewhere in the terminal.

Answer 2

You need to expand two levels, so \expandafter\expandafter\expandafter, besides making @ a letter in order to avoid TeX compare the first two tokens in the expansion of \@ (precisely, \spacefactor and \@m).

A single \expandafter is sufficient with \expandafter\ifx\csname blafasel\endcsname\foo.

There are simpler ways to compare macros expanding to strings of characters, the simplest one being

\ifnum\pdfstrcmp{\@nameuse{blafadel}}{foo}=0
  true
\else
  false
\fi

that doesn't need to define \foo. Since \pdfstrcmp is available with different names in other engines, it's best to do

\usepackage{pdftexcmds}

and use \pdf@strcmp.

Much more powerful string comparison functions are available with expl3.