key values getting improperly passed through a property list getter function

Question

I'm creating rather complicated diagram and would like to put all the controls all in one easy to find location and not scattered throughout the document. To do this, I've created setter and getter functions for a LaTeX3 property list. Here's the code:

\documentclass{article}
\usepackage{pstricks,pst-node}
\usepackage{xparse}
\ExplSyntaxOn
\prop_new:N \g__sunset_plist
\NewDocumentCommand{\setattr}{ mm }
    {
        \prop_gput:Nnn \g__sunset_plist {#1} {#2}
    }
\DeclareExpandableDocumentCommand{\gettr}{ m }
    {
        \prop_get:Nn \g__sunset_plist { #1 }
    }
\ExplSyntaxOff
\setattr{my_linestyle}{linestyle=dashed,linecolor=blue}
\setattr{my_line}{\psline[linestyle=dashed,linecolor=blue](A)(B)}
\pagestyle{empty}
\begin{document}

\begin{pspicture}(10,10)

    \pnode(0,0){A}
    \pnode(10,10){B}
    \pnode(10,0){C}
    \pnode(0,10){D}

    \gettr{my_line}   %% <-- works as expected
    \psline[\gettr{my_linestyle}](C)(D)  %% <-- not working

\end{pspicture}

\end{document}

The line

    \gettr{my_line}   %% <-- works as expected

works but I don't want to approach the problem this way. I'd prefer to write

    \psline[\gettr{my_linestyle}](C)(D)  %% <-- not working

because this line shows me exactly what I'm doing: drawing a line from point C to point D. What's hidden are the attributes to that line. I'd prefer this approach. But what seems to be happening is that when \gettr{my_linestyle} is expanded, LaTeX sees:

    \psline[{linestyle=dashed,linecolor=blue}](C)(D)  %% <-- not working

How do I get my getter function to expand without the surrounding {...}?

EDIT

Here's the error message

! Package xkeyval Error: `linestyle=dashed,linecolor=blue' undefined in familie
s `,pstricks,pst-node'.

See the xkeyval package documentation for explanation.
Type  H <return>  for immediate help.

So it seems that some kind of expansion is happening before xkeyval is getting the keys.

Answer 1

You can define a "prefix" macro:

\documentclass{article}
\usepackage{pstricks,pst-node}
\usepackage{xparse}
\ExplSyntaxOn
\prop_new:N \g__ellett_sunset_plist
\NewDocumentCommand{\setattr}{ mm }
 {
  \prop_gput:Nnn \g__ellett_sunset_plist {#1} {#2}
 }
\NewDocumentCommand{\gettr}{ m }
 {
  \prop_get:Nn \g__ellett_sunset_plist { #1 }
 }
\NewDocumentCommand{\expandoptions}{mO{}}
 {
  \use:x { \exp_not:N #1 [ #2 ] }
 }
\ExplSyntaxOff

\setattr{my_linestyle}{linestyle=dashed,linecolor=blue}
\setattr{my_line}{\psline[linestyle=dashed,linecolor=blue](A)(B)}

\pagestyle{empty}

\begin{document}

\begin{pspicture}(10,10)
  \pnode(0,0){A}
  \pnode(10,10){B}
  \pnode(10,0){C}
  \pnode(0,10){D}

  \gettr{my_line}   %% <-- works as expected
  \expandoptions\psline[\gettr{my_linestyle}](C)(D)  %% <-- not working

\end{pspicture}

\end{document}

Using \use:x is a "last resort", but four expansions of \gettr still are far from the final result. Beware of other options containing dangerous tokens; text and lengths are safe.