Hi,

I've the following xml file:

<?xml version='1.0' standalone='yes?>

<content>
 <object mid="a">
  <text>a paragraph of text</text>
  <value>40.2</value>
 </object>
 <object mid="b">
  <text>another paragraph of text</text>
  <value>10.7</value>
 </object>
 <object mid="c">
  <text>still another paragraph!</text>
  <value>20.5</value>
 </object>
</content>

And want to calculate the total value of all the objects:

like so:

\startxmlsetups xml:example:base
  \xmlsetsetup{#1}{content|object}{xml:example:*}
\stopxmlsetups

\xmlregisterdocumentsetup{example}{xml:example:base}

\startxmlsetups xml:example:content
totalvalue: \xmlfunction{#1}{totalvalue}
\xmlflush{#1} \endgraf
\stopxmlsetups

\startxmlsetups xml:example:object
attribute : \xmlatt{#1}{mid} \par
\stopxmlsetups

% LUA

\startluacode
function xml.functions.totalvalue(root)
local score = 0
for e in xml.collected(root,"/object") do
tex.write('Okay!')
--score = score + xml.filter(e,"xml:///value/number()") or 0
end
tex.write(score)
end
\stopluacode

\starttext

\xmlprocessfile{example}{content.xml}{}

\stoptext

When I run this example I get:

totalvalue: Okay!Okay!Okay!0 attribute : a
attribute : b
attribute : c

That looks allright!

But when I incorporate the line:

score = score + xml.filter(e,"xml:///value/number()") or 0

the following Lua error is thrown:

! LuaTeX error [string "\directlua "]:5: attempt to perform arithmetic on a nil value

Has anybody any idea what I'm doing wrong here? The manual xml-mkiv.pdf doesn't give any clue about this!