\@chapapp doesn't expand properly in memoir header

Question

I'm converting a book from FrameMaker to the memoir class using XeLaTeX and am having to design my own chapter and page styles to suit the publisher's requirements. Mostly it works fine, except that my attempt to pull 'Chapter' or 'Appendix' as the case may be by placing \@chapapp into the left mark doesn't seem to work: instead of giving me the correct literal I just get 'chapapp'.

MWE follows:

% !TEX TS-program = xelatexmk
 \documentclass[%
    a4paper,            % minus margins = A5
    11pt,               % reduce to 10 if required
    twoside,            % two-sided printing
    one column,         % single column of text on a page
    openright,          % chapters/lessons begin on odd pages only; review later
%   draft               % mark overfull lines and provide change bars; change later
    ]{memoir}

% CHAPTER STYLE
\makechapterstyle{olms}{%
    \setlength{\beforechapskip}{50pt}
    \setlength{\midchapskip}{20pt}% unused
    \setlength{\afterchapskip}{40pt}
    \renewcommand{\chapnamefont}{\sffamily\LARGE\bfseries}
    \renewcommand{\chapnumfont}{\sffamily\LARGE\bfseries}
    \renewcommand{\afterchapternum}{:\space}
    \renewcommand{\chaptitlefont}{\sffamily\LARGE\bfseries}
}
\chapterstyle{olms}

% PAGE STYLE
\makepagestyle{olms}
\makeevenhead{olms}{\thepage}{\leftmark}{}
\makeoddhead{olms}{}{\rightmark}{\thepage}
\makepsmarks{olms}{%
    \createmark{chapter}{both}{shownumber}{\@chapapp\space}{:\space}
    \createmark{section}{right}{shownumber}{}{\space}
\createplainmark{toc}{both}{\contentsname}
\createplainmark{lof}{both}{\listfigurename}
\createplainmark{lot}{both}{\listtablename}
\createplainmark{bib}{both}{\bibname}
\createplainmark{index}{both}{\indexname}
\createplainmark{glossary}{both}{\glossaryname}
}

\pagestyle{olms}
\nouppercaseheads

\usepackage{lipsum}

\begin{document}
\frontmatter
\setcounter{page}{5}
\tableofcontents

\mainmatter
\chapter{First Chapter}
\lipsum[1]
\section{First section}\lipsum[3-5]

\appendix
\chapter{First Appendix}
\lipsum[1]
\section{First section}\lipsum[3-5]

\end{document}

The header on page 2 looks like this:

I'm stumped, so any help would be appreciated.

Answer 1

You need

\makeatletter
\makepsmarks{olms}{%
  \createmark{chapter}{both}{shownumber}{\@chapapp\space}{:\space}
  \createmark{section}{right}{shownumber}{}{\space}
\makeatother

because you're using \@chapapp - a macro with an @ in it (outside of a .style file/class). See What do \makeatletter and \makeatother do? that explains what happens within this pair.

If you're not using an appendix, you can get away with using \chaptername instead of \@chapapp:

\makepsmarks{olms}{%
  \createmark{chapter}{both}{shownumber}{\chaptername\space}{:\space}
  \createmark{section}{right}{shownumber}{}{\space}