Extract first character of string stored in macro using Expl3

Question

I want to extract a set of n characters stored in a macro like this:

\def\foo{bar}
\extract{\foo}{1}

prints: b using expl3.

I have the following code:

\DeclareExpandableDocumentCommand{\extract}{mm}{%
  \tl_item:nn { #1 } { #2 }%
}

which works on a literal string: \extract{foo}{2} prints fo, but it does not work when using a macro. More specifically, a string stored in pgfkeys.

The final result should be fully expanded and not something that will most likely give the desired character (see: xstring package).

Full MWE:

\documentclass{article}

\usepackage{xparse}
\usepackage{tikz}

\ExplSyntaxOn
\DeclareExpandableDocumentCommand{\extract}{mm}{%
  Extracted: \tl_item:nn { #1 } { #2 }%
}
\ExplSyntaxOff

\begin{document}

\extract{foo}{2}     % output: fo as expected

\def\foo{bar}

\extract{\foo}{2}    % output: nothing

\pgfkeys{/foo/.is family,
     /foo/.unknown/.style = %
     {\pgfkeyscurrentpath/\pgfkeyscurrentname/%
     .initial = #1}}
\pgfkeys{/foo/bar = some text here}

\pgfkeysvalueof{/foo/bar}

\extract{\pgfkeysvalueof{/foo/bar}}{2} % output: /foo/bar

\end{document}

Answer 1

\ExplSyntaxOn

\cs_generate_variant:Nn \tl_item:nn { f }

\DeclareExpandableDocumentCommand \extract { m m }
 {
  Extracted: ~ \tl_item:fn { #1 } { #2 }
 }

\ExplSyntaxOff