Legitimate "local variable" (argument) command in a macro raising "Undefined control sequence"?

Question

Consider this MWE:

\documentclass[varwidth,tightpage,border=1bp]{standalone}

\usepackage{tikz}

\begin{document}
\begin{tikzpicture}

\makeatletter
\gdef\getMyInfoString{%
  \let\trfm\relax % ! Undefined control sequence.
  \pgfgettransform{\trfm}% with previous commented: ! Undefined control sequence. <argument> \trfm
  \edef\tout{%
    pgfgettransform: \trfm ; %
    pgf@pt@aa, pgf@pt@ba, pgf@pt@ab, pgf@pt@bb: \pgf@pt@aa, \pgf@pt@ba, \pgf@pt@ab, \pgf@pt@bb %
  }
  \tout % output tout
}
\makeatother

\pgfgettransform{\trfmX}%
\makeatletter
\typeout{Manually check: %
    pgfgettransform: \trfmX ; %
    pgf@pt@aa, pgf@pt@ba, pgf@pt@ab, pgf@pt@bb: \pgf@pt@aa, \pgf@pt@ba, \pgf@pt@ab, \pgf@pt@bb %
}
\makeatother

\typeout{This is via macro function: \getMyInfoString}%

\end{tikzpicture}
\end{document}

If you compile this, the first \typeout prints this to stdout:

Manually: pgfgettransform: {1.0}{0.0}{0.0}{1.0}{0.0pt}{0.0pt}; pgf@pt@aa, pgf@p
t@ba, pgf@pt@ab, pgf@pt@bb: 1.0, 0.0, 0.0, 1.0

... while the second \typeout raises an error:

! Undefined control sequence.
\getMyInfoString ->\let \trfm 
                              \relax \pgfgettransform {\trfm }\edef \tout {p...
l.30 \typeout{This is: \getMyInfoString}
                                        %
? 

Clearly, latex wants to execute the \trfm control sequence/command/token/macro - while I intend it to be a "local variable", which will be used as an "argument" to \pgfgettransform (which will then populate it with data).

Is there a way to "protect" this \trfm macro -- so the \getMyInfoString executes correctly, and returns a string (in an expandable way, I guess, since the intent is to use it in \typeout), when called?

Answer 1

If you want to get the current transform data in the log file, just execute the commands before the \typeout.

\documentclass{article}

\usepackage{tikz}

\makeatletter
\def\getMyInfoString{%
  \pgfgettransform{\trfm}%
  \edef\tout{%
    \space\space pgfgettransform: \trfm ;^^J%
    \space\space pgf@pt@aa: \pgf@pt@aa;^^J%
    \space\space pgf@pt@ba: \pgf@pt@ba;^^J%
    \space\space pgf@pt@ab: \pgf@pt@ab;^^J%
    \space\space pgf@pt@bb: \pgf@pt@bb
  }%
  \typeout{Here's the current transform:^^J\tout} % output tout
}
\makeatother

\begin{document}
\begin{tikzpicture}
\getMyInfoString
\end{tikzpicture}
\end{document}

The output in the log file is

Here's the current transform:
  pgfgettransform: {1.0}{0.0}{0.0}{1.0}{0.0pt}{0.0pt};
  pgf@pt@aa: 1.0;
  pgf@pt@ba: 0.0;
  pgf@pt@ab: 0.0;
  pgf@pt@bb: 1.0

You cannot perform assignments in a \typeout; a definition is an assignment.

Answer 2

How about:

\documentclass{scrartcl}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
\pgfgettransformentries{ \a }{\b }{ \c }{ \d }{ \sx }{ \sy }
\typeout{a=\a, b=\b, c=\c, d=\d, sx=\sx, sy=\sy}
\end{tikzpicture}
\end{document}

which produces a=1.0, b=0.0, c=0.0, d=1.0, sx=0.0pt, sy=0.0pt and avoids nasty plain TeX hacks?

Edit: Ok, after your comment I understand that you are in the usual hell of what command is expanded when. It seems like typeout is pretty nasty here. Unfortunately I can not give you some \expandafter magic which solves this case, but a simple workaround would be to split setting variables and producing output like

\documentclass{scrartcl}
\usepackage{tikz}
\newcommand\setVars{\pgfgettransformentries{\a}{\b}{\c}{\d}{\sx}{\sy}}
\newcommand\printVars{a=\a, b=\b, c=\c, d=\d, sx=\sx, sy=\sy}
\begin{document}
\setVars\typeout{\printVars}
\end{document}

Answer 3

I'll post this as an answer, as it does somewhat help - if the crashing "local option" macros are "predefined" beforehand with \gdef (just the relevant snippet here):

\makeatletter
\gdef\tout{}\gdef\trfm{} % ** this
\gdef\getMyInfoString{%
  \let\trfm\relax % ! Undefined control sequence.
  \pgfgettransform{\trfm}% with previous commented: ! Undefined control sequence. <argument> \trfm
  \edef\tout{%
    pgfgettransform: \trfm ; %
    pgf@pt@aa, pgf@pt@ba, pgf@pt@ab, pgf@pt@bb: \pgf@pt@aa, \pgf@pt@ba, \pgf@pt@ab, \pgf@pt@bb %
  }
  \tout % output tout
}
\makeatother

... then the MWE seems to pass without compilation errors; and it prints out:

Manually check: pgfgettransform: {1.0}{0.0}{0.0}{1.0}{0.0pt}{0.0pt}; pgf@pt@aa,
 pgf@pt@ba, pgf@pt@ab, pgf@pt@bb: 1.0, 0.0, 0.0, 1.0
This is via macro function: \let \relax \edef {{1.0}{0.0}{0.0}{1.0}{0.0pt}{0.0p
t}} \edef {pgfgettransform: ; pgf@pt@aa, pgf@pt@ba, pgf@pt@ab, pgf@pt@bb: 1.0, 
0.0, 0.0, 1.0} 

Obviously, the printout via the macro is not exactly as intended - which is why it would be nice to have a proper answer; but at least in this case, the required info leaks to stdout - and that is good enough for me for now :)