How to expand pgfkeys value when the key path contains macros?

Question

This question represents a second, more simplified, attempt at a problem I asked about some time ago but for which I received no answer.

In short, I need to put the fully expanded result of a call to \pgfkeys in a macro for later use, and the key path itself contains macros. The purpose is that I have parameters which change depending on the type and part of the document one is in, and I want to have a single call to \pgfkeys (which will appear in the definitions of other macros/environments) automatically look up the right parameter for the currently active settings.

I tried using .estore in, but got the error TeX capacity exceeded, sorry [input stack size=5000].

Using .store in lets me set the key, but since the macro contains the unexpanded \pgfkeys function, I can't use it in a context like the argument to another macro/environment, and I haven't been able to figure out how to expand it before calling another macro. (In my various experiments with \edef, I usually wind up with unending compilation.) In the MWE, opening the multicols environment with the macro defined this way causes the error !Missing number, treated as zero.

So, how can get a fully expanded value into a macro set by \pgfkeys when the right side of the assignment is itself a call to \pgfkeys?

My preference is a solution that still lets me use \pgfkeys rather than \pgfkeysvalueof or some other key-value package, because I make heavy use of the .search also handler, but at this point I'm willing to consider other approaches, including l3keys, even if it requires a massive rewrite elsewhere.

MWE:

\documentclass{article}
\usepackage{pgfkeys}
\usepackage{multicol}

\newcommand{\ActiveFormat}{generic}
\newcommand{\ActiveDomain}{default}
\newcommand{\colnum}{1}

\pgfkeys{/stdt/.cd,
    num-columns/.store in=\colnum,
%   num-columns/.estore in=\colnum,
    generic/default/num-columns/.initial=2,
}

\begin{document}

\textbackslash colnum = \colnum

\pgfkeys{/stdt/num-columns=\pgfkeys{/stdt/\ActiveFormat/\ActiveDomain/num-columns}}

% This is fine with the .sore in version
\textbackslash colnum = \colnum

% There will be an error here
\begin{multicols}{\colnum}

Some text.

Some more text.

\end{multicols}

\end{document}

Answer 1

If I understand correctly, I think you should use the /stdt/generic/default keys the same as how Tikz uses its styles, to set other keys in the base directory (in this case /stdt instead of /tikz).

The following example results in:

\documentclass{article}
\usepackage{pgfkeys}
\usepackage{multicol}

\newcommand\ActiveFormat{generic}
\newcommand\ActiveDomain{default}
\newcommand{\colnum}{1}

\pgfkeys{/stdt/.cd,
    num-columns/.store in=\colnum,
    generic/default/num-columns/.initial=2,
    generic/default/.code=\pgfkeys{/stdt/num-columns=2},
    generic/non-default/.code=\pgfkeys{/stdt/num-columns=4},
}

\begin{document}

\textbackslash colnum = \colnum

%\pgfkeys{/stdt/num-columns=\pgfkeys{/stdt/\ActiveFormat/\ActiveDomain/num-columns}}
\pgfkeys{/stdt/\ActiveFormat/\ActiveDomain}

% This is fine with the .store in version
\textbackslash colnum = \colnum

% No errors
\begin{multicols}{\colnum}

Some text.

Some more text.

\end{multicols}

\def\ActiveDomain{non-default}

\pgfkeys{/stdt/\ActiveFormat/\ActiveDomain}

\textbackslash colnum = \colnum

% No errors
\begin{multicols}{\colnum}

Some text.

Some more.

Some more.

Final column.

\end{multicols}

\end{document}