Xparse macro with optional parameter breaks value for PGF foreach

Question

I write a document class and want to execute something a certain number of times, depending on a user-defined quantity. So I thought I'd use a foreach loop from PGF together with accessor macros:

\documentclass{scrartcl}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}

\usepackage{xparse,etoolbox,pgffor}

\makeatletter
\csdef{my@test}{3}

\DeclareDocumentCommand{\val}{m g}{%
  \csname #1@#2\endcsname%
}
\makeatother

\begin{document}
  Testing direct: \csname my@test\endcsname

  Testing macro: \val{my}{test}

  \foreach \i in {1,...,\csname my@test\endcsname}{a}

  \foreach \i in {1,...,\val{my}{test}}{a}
\end{document}

Unfortunately, using val breaks foreach:

with error message Illegal unit of measure (pt inserted) (several times).

Funny: if I use parameter specification {m m} instead of {m g}, the problem goes away.

It is feasible to not use the macro inside the class definition -- I do need the optional parameter, though -- but what is going on here?

Background

Users can create a list of Xs by using a macro of the form

\addX{group}{name}{key1=val1,key2=val2,..} 

This macro is implemented with xparse and keyval. It stores the information in macros of the form \X@group@name@key. In order to hide this from users, I provide accessor macros like val above. There are also macros and environments that utilise the created list directly, one of which would contain a loop as defined above, repeating stuff (for every X) depending on user-provided values.

The last parameter of val is to be optional so that a default value can be returned if the user provides no key (which would be the value I suppose will be requested most often).

Answer 1

This seems to be an expandability issue. It seems that xparse's manner of defining functions with optional arguments is preventing \foreach from seeing the final value in its domain. In particular, \foreach cannot fully expand the value of your macro because xparse method for creating g type arguments prevents full expansion.

Even if the optional argument precedes the mandatory argument, you run into a similar error:

\documentclass{scrartcl}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}

\usepackage{xparse,etoolbox,pgffor}

\makeatletter
\csdef{my@test}{3}

\DeclareDocumentCommand{\val}{o m}{%
  \csname #1@#2\endcsname%
}
\makeatother

\begin{document}

  Testing direct: \csname my@test\endcsname

  Testing macro: \val[my]{test}

  \foreach \i in {1,...,\csname my@test\endcsname}{a}

  \foreach \i in {1,...,\val[my]{test} }{a}

\end{document}

In the documentation for xparse you can create fully expandable functions with optional arguments, but various restrictions apply:

• The optional argument may not be of type g

• The last argument may not be an optional argument.

You can write the following

\documentclass{scrartcl}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}

\usepackage{xparse,etoolbox,pgffor}

\makeatletter
\csdef{my@test}{3}

\DeclareExpandableDocumentCommand{\val}{o m}{%
  \csname #1@#2\endcsname%
}
\makeatother

\begin{document}

  Testing direct: \csname my@test\endcsname

  Testing macro: \val[my]{test}

  \foreach \i in {1,...,\csname my@test\endcsname}{a}

  \foreach \i in {1,...,\val[my]{test} }{a}

\end{document}

Doing a bit of gimmickry you can get a partial glance of what's happening expansion-wise

\documentclass{scrartcl}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}

\usepackage{xparse,etoolbox,pgffor}

\makeatletter
\csdef{my@test}{3}

%% an expandable version of your macro
\DeclareExpandableDocumentCommand{\eval}{o m}{%
  \csname #1@#2\endcsname%
}

%% a non-expandable version of your macro
\DeclareDocumentCommand{\val}{o m}{%
  \csname #1@#2\endcsname%
}
\makeatother
\setlength{\parskip}{2ex}
\begin{document}


 \makebox[1in][r]{Full expanded}: 
    \begin{minipage}[t]{3in}\ttfamily
      \detokenize\expandafter{\romannumeral-`G\eval[my]{test}}
    \end{minipage}

  \makebox[1in][r]{Expanded once}: 
    \begin{minipage}[t]{3in}\ttfamily
      \detokenize\expandafter{\eval[my]{test}}
    \end{minipage}

  \makebox[1in][r]{Not fully expandable}:
    \begin{minipage}[t]{3in}\ttfamily
      \detokenize\expandafter{\romannumeral-`G\val[my]{test}}
    \end{minipage}
\end{document}

Perhaps if you clarify how this macro is to behave, we can come up with a solution that better meets your needs.

UPDATE

You could make both your arguments mandatory, but test whether the second argument has been passed an empty argument.

Here's the code:

\csdef{my@default}{5}

\DeclareDocumentCommand{\eval}{ mm }{%%
  \expandafter\ifx\expandafter\relax\detokenize{#2}\relax
    \csname #1@default\endcsname
  \else
    \csname #1@#2\endcsname
  \fi
}

Then you can write something like

  \foreach \i in {1,...,\eval{my}{}}{D}

  \foreach \i in {1,...,\eval{my}{test}}{c}  

I should point out, though, that \eval{my}{ } will not test true for an empty argument.