How do you signal an error from within an expandable function?

Question

I would like to do some error checking from within an expandable function. But my attempts at signalling an error are not processed correctly, but rather returned to the input stream---contrary to my expectations.

Here's the code:

\documentclass{article}
\usepackage{pstricks,pst-eucl}
\psset{unit=0.25cm}
\usepackage{xparse}
\ExplSyntaxOn
\msg_new:nnnn {testing}{testing}{illegal~property}{}
%% property list
\prop_new:N \g__ace_all_properties_plist
%% setter
\cs_new_protected:Npn \__ace_set_property:nn #1#2
    {
        \prop_gput:Nnn \g__ace_all_properties_plist { #1 } { #2 }
    }
%% getter
\cs_new:Npn \__ace_get_property:n #1
    {
        \prop_get:Nn \g__ace_all_properties_plist { #1 }
    }
%%--- User interface for settr and gettr
\DeclareDocumentCommand{\settr}{ m m }
    {
        \__ace_set_property:nn { #1 }{ #2 }
    }
\DeclareExpandableDocumentCommand{\gettr}{ m }
    {
        \prop_if_in:NnTF \g__ace_all_properties_plist { #1 }
            {
                \__ace_get_property:n { #1 }
            }
            {
                %% this next line does not get executed
                %% even when there #1 is not in the list
                %<var 1>%\msg_info_text:n { #1 ~ is ~ not ~ defined }
                \msg_fatal_text:n { testing}
            }

    }        
\ExplSyntaxOff
\pagestyle{empty}
\begin{document}
\settr{displacement}{10}%

This picture gets processed correctly\par
\begin{pspicture}(0,0)(10,10)
    \pstGeonode[PointName=default,PointSymbol=default,PosAngle=0](0,0){A}(0,1){B}
    \pstTranslation[DistCoef=\gettr{displacement}]{A}{B}{A}[C]
\end{pspicture}

\vspace{1cm}

No error signaled here $\rightarrow$
\gettr{unknownproperty}

\vspace{1cm}

This picture should signal an error when commented line uncommented\par
\begin{pspicture}(0,0)(10,10)
    \pstGeonode[PointName=default,PointSymbol=default,PosAngle=0](0,0){A}(0,1){B}
    %\pstTranslation[DistCoef=\gettr{unknown-property}]{A}{B}{A}[C]
\end{pspicture}

\end{document}

Initially, I thought this was an issue of calling a protect function \msg_term:x from within an expandable function. But, my errors are not properly triggered even when using using \mgs_fatal_text:n. This later approach is not really what I want. But signalling an error in any way would be nice.

EDIT

Following @DavidCarlisle's suggestion, I tried replacing

\msg_fatal_text:n{ testing }

with

\ERROR

So, the compilation stops, but since the wrong type of error message is signaled, I can imagine wasting a lot of time looking for the wrong error. So, instead of \ERROR I tried

\use:c { Attempting ~ to ~ call ~ undefined ~ property -> {#1} }

I figured at least with this, I'd get an error that made it clearer what the real issue was. But this doesn't signal an error at all---contrary to my expectations.

Answer 1

If you actually want to generate the error in an expansion only context such as a \write or a PS \special the usual way is just to use an undefined command such as \ERROR You can not trigger any other error from there (and LaTeX3 can't really change that).

! Undefined control sequence.
l.1 ...ptingToCallUndefinedPropertyunknownproperty



\documentclass{article}
\usepackage{pstricks,pst-eucl}
\psset{unit=0.25cm}
\usepackage{xparse}
\ExplSyntaxOn
\msg_new:nnnn {testing}{testing}{illegal~property}{}
%% property list
\prop_new:N \g__ace_all_properties_plist
%% setter
\cs_new_protected:Npn \__ace_set_property:nn #1#2
    {
        \prop_gput:Nnn \g__ace_all_properties_plist { #1 } { #2 }
    }
%% getter
\cs_new:Npn \__ace_get_property:n #1
    {
        \prop_get:Nn \g__ace_all_properties_plist { #1 }
    }
%%--- User interface for settr and gettr
\DeclareDocumentCommand{\settr}{ m m }
    {
        \__ace_set_property:nn { #1 }{ #2 }
    }
\DeclareExpandableDocumentCommand{\gettr}{ m }
    {
        \prop_if_in:NnTF \g__ace_all_properties_plist { #1 }
            {
                \__ace_get_property:n { #1 }
            }
            {
                %% this next line does not get executed
                %% even when there #1 is not in the list
                %<var 1>%\msg_info_text:n { #1 ~ is ~ not ~ defined }
\foo{ Attempting  To  Call  Undefined  Property  #1 }
            }

    }

\ExplSyntaxOff

{\catcode`\@=0
@catcode`\\=12
@gdef@foo#1{@scantokens{\#1}}}

\pagestyle{empty}
\begin{document}
\settr{displacement}{10}%

This picture gets processed correctly\par
\begin{pspicture}(0,0)(10,10)
    \pstGeonode[PointName=default,PointSymbol=default,PosAngle=0](0,0){A}(0,1){B}
    \pstTranslation[DistCoef=\gettr{displacement}]{A}{B}{A}[C]
\end{pspicture}

\vspace{1cm}

No error signaled here $\rightarrow$
\gettr{unknownproperty}

\vspace{1cm}

This picture should signal an error when commented line uncommented\par
\begin{pspicture}(0,0)(10,10)
    \pstGeonode[PointName=default,PointSymbol=default,PosAngle=0](0,0){A}(0,1){B}
    %\pstTranslation[DistCoef=\gettr{unknown-property}]{A}{B}{A}[C]
\end{pspicture}

\end{document}

(AM) David's trick is interesting:

{
  \catcode`\|=0
  |catcode`\\=12
  |gdef|throwundefined#1{|scantokens{\#1}}
}
\throwundefined{x}

Answer 2

Translaton of @DavidCarlisle's clever slight of hand:

\cs_new:Npn \foo #1 
    {
        \tl_rescan:nn
            {  
                \char_set_catcode_escape:N \@
            }{
                @#1 
            }
    }