How to write \StrGobbleLeft output to an external file?

Question

I have not learnt how the TeX expansion works behind the scene. Shortly speaking, how to write \StrGobbleLeft output to an external file?

\documentclass{article}
\usepackage{filecontents}
\begin{filecontents*}{rawdata.txt}
beamerthemeAnnArbor.sty
beamerthemeAntibes.sty
beamerthemeBergen.sty
beamerthemeBerkeley.sty
beamerthemeBerlin.sty
beamerthemeBoadilla.sty
beamerthemeboxes.sty
beamerthemeCambridgeUS.sty
beamerthemeCopenhagen.sty
beamerthemeDarmstadt.sty
beamerthemedefault.sty
beamerthemeDresden.sty
beamerthemeEastLansing.sty
beamerthemeFrankfurt.sty
beamerthemeGoettingen.sty
beamerthemeHannover.sty
beamerthemeIlmenau.sty
beamerthemeJuanLesPins.sty
beamerthemeLuebeck.sty
beamerthemeMadrid.sty
beamerthemeMalmoe.sty
beamerthemeMarburg.sty
beamerthemeMontpellier.sty
beamerthemePaloAlto.sty
beamerthemePittsburgh.sty
beamerthemeRochester.sty
beamerthemeSingapore.sty
beamerthemeSzeged.sty
beamerthemeWarsaw.sty
beamerthemebars.sty
beamerthemeclassic.sty
beamerthemecompatibility.sty
beamerthemelined.sty
beamerthemeplain.sty
beamerthemeshadow.sty
beamerthemesidebar.sty
beamerthemesplit.sty
beamerthemetree.sty
\end{filecontents*}

\usepackage{xstring}

\begin{document}
% writer
\newwrite\writer
\immediate\openout\writer=list.txt\relax
% reader
\newread\reader
\openin\reader=rawdata.txt\relax
\makeatletter
\loop
    \read\reader to \x
    \unless\ifeof\reader
        \filename@parse{\x}
        \immediate\write\writer{\StrGobbleLeft{\filename@base}{11}}
\repeat
\makeatother
\closein\reader
\immediate\closeout\writer
\end{document}

Answer 1

\StrGobbleLeft is not expandable, therefore it will not work inside \write. But the package xstring is so kind to provide an optional argument at the end. If present, it takes a macro name that stores the result.

\StrGobbleLeft{\filename@base}{11}[\Result]
\immediate\write\writer{\Result}

Answer 2

You need an a macro that works via expansion if you want to place it in the write for example:

\documentclass{article}
\usepackage{filecontents}
\begin{filecontents*}{rawdata.txt}
beamerthemeAnnArbor.sty
beamerthemeAntibes.sty
beamerthemeBergen.sty
beamerthemeBerkeley.sty
beamerthemeBerlin.sty
beamerthemeBoadilla.sty
beamerthemeboxes.sty
beamerthemeCambridgeUS.sty
beamerthemeCopenhagen.sty
beamerthemeDarmstadt.sty
beamerthemedefault.sty
beamerthemeDresden.sty
beamerthemeEastLansing.sty
beamerthemeFrankfurt.sty
beamerthemeGoettingen.sty
beamerthemeHannover.sty
beamerthemeIlmenau.sty
beamerthemeJuanLesPins.sty
beamerthemeLuebeck.sty
beamerthemeMadrid.sty
beamerthemeMalmoe.sty
beamerthemeMarburg.sty
beamerthemeMontpellier.sty
beamerthemePaloAlto.sty
beamerthemePittsburgh.sty
beamerthemeRochester.sty
beamerthemeSingapore.sty
beamerthemeSzeged.sty
beamerthemeWarsaw.sty
beamerthemebars.sty
beamerthemeclassic.sty
beamerthemecompatibility.sty
beamerthemelined.sty
beamerthemeplain.sty
beamerthemeshadow.sty
beamerthemesidebar.sty
beamerthemesplit.sty
beamerthemetree.sty
\end{filecontents*}

\usepackage{xstring}

\begin{document}
% writer
\newwrite\writer
\immediate\openout\writer=list.txt\relax
% reader
\newread\reader
\openin\reader=rawdata.txt\relax
\makeatletter
\loop
    \read\reader to \x
    \unless\ifeof\reader
        \filename@parse{\x}%
        \def\zaptheme beamertheme{}%
        \immediate\write\writer{\expandafter\zaptheme \filename@base}
\repeat
\makeatother
\closein\reader
\immediate\closeout\writer
\end{document}