How does one fully expand a macro between \csname and \encsname?

Question

The best way to ask this question is to present two MWEs, one of which works and one of which does not. Naturally, I would like to get this into the second form, but I am at a loss as to what is happening. I believe this is an order-of-expansion issue. I tried this originally with the TeX primitives \csname and \endcsname and, failing that, went with the etoolbox utility instead thinking that it would know what I was doing wrong and have built-in safeguards. But I am getting the same errors. Without further ado, why does

\documentclass[inline]{svmono}

\usepackage{xstring,etoolbox}

\usepackage{fontspec}
\setmainfont[Mapping=tex-text]{TeX Gyre Termes}
\setsansfont[Scale=0.9,Mapping=tex-text]{TeX Gyre Heros}
\setmonofont[Scale=0.9]{Consolas}

\DeclareDocumentCommand{\extract}{m}{%
    \edef\c{\detokenize{#1}}%
    \StrCount{\c}{\string /}[\lastslash]%
    \StrCount{\c}{\string .}[\lastdot]%
    \StrBetween[\lastslash,\lastdot]{\c}{\string /}{\string .}[\temp]%
    \csedef{\temp}{true}
}

\DeclareDocumentCommand{\mark}{m}{%
    \csedef{\extract{#1}}{true}
}

\begin{document}
    \extract{\input{../../Lyx/MacroMasterXeTeX.tex}}
    \MacroMasterXeTeX
\end{document}

work, but (showing only changes)

\DeclareDocumentCommand{\extract}{m}{%
    \edef\c{\detokenize{#1}}%
    \StrCount{\c}{\string /}[\lastslash]%
    \StrCount{\c}{\string .}[\lastdot]%
    \StrBetween[\lastslash,\lastdot]{\c}{\string /}{\string .}[\temp]%
    \temp
}

\DeclareDocumentCommand{\mark}{m}{%
    \csedef{\extract{#1}}{true}
}

\begin{document}
    \mark{\input{../../Lyx/MacroMasterXeTeX.tex}}
%   \MacroMasterXeTeX
\end{document}

not work? (I commented out the last line in the document because that macro is never getting defined.) I would like to reuse the extraction code and would prefer to separate it from the "marking" process.

Answer 1

First off, redefining \c and \mark is not a good idea: the former is an accent command, the latter is nothing less than a TeX primitive.

I see no reason for passing \input to the \MARK command; if you actually want to input the file, you can just do it in the code of the macro.

With this simplification, we can use built-in functions:

\documentclass{article}

\ExplSyntaxOn

\NewDocumentCommand{\MARK}{m}
 {
  \tl_set:cn { \exp_last_unbraced:Ne \use_ii:nnn { \file_parse_full_name:n { #1 } } } { true }
  %\file_input:n { #1 }
 }

\ExplSyntaxOff

\begin{document}

\MARK{../../Lyx/MacroMasterXeTeX.tex}

\MacroMasterXeTeX

\end{document}

Explanation:

1. \file_parse_full_name:n { ../../Lyx/MacroMasterXeTeX.tex } will leave {../../Lyx}{MacroMasterXeTeX}{tex} on the input stream;

2. with \exp_last_unbraced:Ne, the \file_parse_full_name:n function is performed before passing control to \use_ii:nnn;

3. as a result, the input stream will have \use_ii:nn {../../Lyx}{MacroMasterXeTeX}{tex} and this just expands to MacroMasterXeTeX;

4. finally, a tl variable with that name is set to true.