Loading an array of fontfaces with fontspec and pgffor

Question

I'm trying to produce a document to compare a bunch of fontfaces. I'd like to parse an array to load and try them all.

This is what I have done:

\documentclass{article}

\usepackage{lipsum}
\usepackage{pgffor}
\usepackage{fontspec}

\def\LoadFont#1{\expandafter\newfontface\csname#1\endcsname{#1}}
\def\UseFont#1{\csname#1\endcsname}

\def\FontList{Charter,Lato,Linux Libertine O}

% these work just fine
%\LoadFont{Charter}
%\LoadFont{Lato}
%\LoadFont{Linux Libertine O}

% THIS DOES NOT WORK
\foreach \FontName in \FontList {\LoadFont\FontName}

\begin{document}
\foreach \FontName in \FontList {\UseFont\FontName\lipsum[1]}
\end{document}

While the cycle with \UseFont works fine, the one with \LoadFont does not. On its own, \LoadFont does what's expected.

I think this could be an expansion problem, but I'm not sure where it stands exactly; blind attempts led nowhere.

What's wrong with the loading cycle?

Answer 1

As already explained, the behaviour you observed comes from the established feature of \foreach to do its iterations inside scope-limiting groups.

Here is with \xintFor. I switched to another font, as I don't have Lato installed.

\documentclass{article}

\usepackage{lipsum}
\usepackage{xinttools}
\usepackage{fontspec}

\def\LoadFont#1{\expandafter\newfontface\csname#1\endcsname{#1}}
\def\UseFont#1{\csname#1\endcsname}

% original
% \def\FontList{Charter,Lato,Linux Libertine O}
% as I don't have Lato:

\def\FontList {Charter, TeX Gyre Heros, Linux Libertine O}

\xintFor #1 in \FontList \do {\LoadFont {#1}}

\begin{document}

\xintFor #1 in \FontList \do {\UseFont {#1}\lipsum[1]}

\end{document}

% Local Variables:
% TeX-engine: xetex
% End:

Answer 2

When you do

\foreach \FontName in \FontList {\LoadFont\FontName}

the \newfontface instruction is performed inside a group, so the definition of the font face is lost as soon as the group ends. This is a feature of \foreach.

Here's a set of macros in expl3 that can give you some ideas for extending them.

\documentclass{article}

\usepackage{fontspec}
\usepackage{lipsum}

\ExplSyntaxOn
\NewDocumentCommand{\LoadFont}{m}
 {
  \exp_args:Nc \newfontface { #1 } { #1 }
 }
\NewDocumentCommand{\LoadFonts}{m}
 {
  \clist_map_inline:nn { #1 }
   {
    \LoadFont{##1}
   }
 }
\NewDocumentCommand{\UseFont}{m}
 {
  \use:c { #1 }
 }
\NewDocumentCommand{\UseFonts}{m +m}
 {
  \clist_map_inline:nn { #1 }
   {
    \group_begin:
    \UseFont{##1} ##1:~#2
    \group_end:
   }
 }
\ExplSyntaxOff

\LoadFonts{Charter,Lato,Linux Libertine O}

\begin{document}

\UseFonts{Charter,Lato,Linux Libertine O}{\lipsum[2]}

\end{document}

Note that \exp_args:Nc \newfontface { #1 } { #1 } is essentially the same as

\expandafter\newfontface\csname #1\endcsname{#1}

If you want to use a symbolic name for a list, define a *-variant:

\documentclass{article}

\usepackage{fontspec}
\usepackage{lipsum}

\ExplSyntaxOn
\NewDocumentCommand{\LoadFont}{m}
 {
  \exp_args:Nc \newfontface { #1 } { #1 }
 }
\NewDocumentCommand{\LoadFonts}{sm}
 {
  \IfBooleanTF{#1}
   { \clist_map_inline:Nn #2 }
   { \clist_map_inline:nn { #2 } }
   {
    \LoadFont{##1}
   }
 }
\NewDocumentCommand{\UseFont}{m}
 {
  \use:c { #1 }
 }
\NewDocumentCommand{\UseFonts}{s m +m}
 {
  \IfBooleanTF{#1}
   { \clist_map_inline:Nn #2 }
   { \clist_map_inline:nn { #2 } }
   {
    \group_begin:
    \UseFont{##1} ##1:~#3
    \group_end:
   }
 }
\ExplSyntaxOff

%\LoadFonts{Charter,Lato,Linux Libertine O}

\newcommand\FontList{Charter,Lato,Linux Libertine O}
\LoadFonts*{\FontList}

\begin{document}

%\UseFonts{Charter,Lato,Linux Libertine O}{\lipsum[2]}

\UseFonts*{\FontList}{\lipsum[2]}

\end{document}