current() function in XForms XPath expressions?

In some forms, it is necessary to perform a simple table lookup based on user input. For example, suppose a form contains an element conversionTable that provides conversion rates for various currencies to the user's local currency. Based on the conversion selected by the user, the appropriate exchange rate would be used in a calculation.

XPath provides the ability to perform this type of search using predicates. To continue the above example, suppose each child of conversionTable is a rate element with the exchange rate as content and a currency attribute. Further, let a separate instance element called currency provide the storage for the currency selected by the user, and let amount contain the amount to convert. The converted amount could then be obtained using the following XPath: /root/amount * /root/conversionTable/rate[@currency=/root/currency].

The problem with this approach is that it uses absolute referencing to obtain the currency value selected by the user. If the converted amount, amount, and currency elements were moved to a deeper point in the instance data, then the XPath would have to be rewritten.

Requirement: Allow an XPath expression to perform a table search using a key whose value is referenced relative to the initial context node of the XPath expression.

Hi All,

Is there currently any support for a function similar to XSLT's current() in OPS?

The nature of the problem is decribed in the current XForms 1.1 requirements (http://www.w3.org/TR/xforms-11-req/), section 2.5:

Quoting from the above referenced document:

In some forms, it is necessary to perform a simple table lookup based on user input. For example, suppose a form contains an element conversionTable that provides conversion rates for various currencies to the user's local currency. Based on the conversion selected by the user, the appropriate exchange rate would be used in a calculation.

XPath provides the ability to perform this type of search using predicates. To continue the above example, suppose each child of conversionTable is a rate element with the exchange rate as content and a currency attribute. Further, let a separate instance element called currency provide the storage for the currency selected by the user, and let amount contain the amount to convert. The converted amount could then be obtained using the following XPath: /root/amount * /root/conversionTable/rate[@currency=/root/currency].

The problem with this approach is that it uses absolute referencing to obtain the currency value selected by the user. If the converted amount, amount, and currency elements were moved to a deeper point in the instance data, then the XPath would have to be rewritten.

Requirement: Allow an XPath expression to perform a table search using a key whose value is referenced relative to the initial context node of the XPath expression.

--end quote

Based on the example given in the snippet above, I'd like to be able do something like:
<xforms:bind nodeset="convertedAmount" calculate=
"../amount * /root/conversionTable/rate[@currency=current()/../currency]"/>
Note the use of current() like in XSLT to refer to the current context node outside of the predicate.
I hope that's clear enough.

So is this possible right now with OPS?   If not, I'd like to request it be implemented since it is a very common use case.  Without something like current(), it is impossible to perform a calculation on fields in a repeat element using values from a separate lookup table, since you cannot reference specific elements inside a repeat using an absolute path.

Larry T. Chen
Software Engineer
Intelenet Communications, Inc.

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