Can XPath concatenate two nodeset values? (for use in XForms)

I am wanting to concatenate two nodeset values using XPath in XForms.

I know that XPath has a concat(string, string) function, but how would I go about concatenating two nodeset values?

BEGIN EDIT: I tried concat function.. I tried this.. and variations of it to make it work, but it doesn't

<xf:value ref="concat(instance('param_choices')/choice/root, .)"/>

END EDIT

Below is a simplified code example of what I am trying to achieve.

XForms model:

<xf:instance id="param_choices" xmlns="">
    <choices>
        <root label="Param Choices">/param</root>
        <choice label="Name">/@AAA</choice>
        <choice label="Value">/@BBB</choice>
    </choices>
</xf:instance>

XForms ui code that I currently have:

<xf:select ref="instance('criteria_data')/criteria/criterion" appearance="full">
    <xf:label>Param choices:</xf:label>    
    <br/>
    <xf:itemset nodeset="instance('param_choices')/choice">
        <xf:label ref="@label"></xf:label>
        <xf:value ref="."></xf:value>
    </xf:itemset>
</xf:select>    

(if user selects "Name" checkbox..) the XML output is:

<criterion>/@BBB</criterion>

However! I want to combine the root nodeset value with the current choice nodeset value.

Essentially:

<xf:value ref="concat(instance('param_choices')/choice/root, .)"/>

or

<xf:value ref="(instance('definition_choices')/choice/root) + ."/>

to achieve the following XML output:

<criterion>/param/@BBB</criterion>

Any suggestions on how to do this? (I am fairly new to XPath and XForms)

p.s. what I am asking makes sense to me when I typed it out, but if you have trouble figuring out what I'm asking, just let me know..

thanks!

The evaluation of an XPath expression can not modify the structure/contents of an XML document -- it only selects (a set of) existing nodes.

In order to produce a node that doesn't exist in the XML document, one must use other means in conjunction with XPath (such as XSLT).

The problem is that you are using the ref attribute instead of the value attribute on your value element. The ref attribute is for binding elements to a single node, whereas the value attribute is for using the value from a resolved XPath expression.

There is also a small mistake in your XPath expression itself, so if you change your value element to this:

<xf:value value="concat(instance('param_choices')/root, .)" />

...you should find that it works correctly.

Dynamic XPath expression construction is not yet supported in XForms. There is no such function as eval() for Javascript.

As a very limited workaround you could use an expression such as

/*[name() = substring-after(instance('param_choices')/root,'/')]/@*[name() = substring-after(.,'/@')]