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RE: [xml-dev] Formulas in XML values

 The expression in the value attribute is essentially an anonymous function -

<account-total value="fn(){29 + 12 * 1.25}"/>

In the current implementation of XPath, there is no mechanism to enable the evaluation of anonymous functions, because that's a second order operation. There is some discussion in the XQuery 1.1 spec about working with higher functions, I'd have to dig into it a bit in order to tell you precisely where, however.  
You won't find it in the current public documents. A detailed proposal for higher-order functions in XQuery has been accepted by the Working Group (it adds "function" to the data model as a third kind of item, alongside nodes and atomic values), and it is implemented in Saxon 9.2 which is now in a limited beta release, but the spec has not yet found its way into a public draft. There are still decisions to be made about whether and how to expose the functionality in XSLT/XPath.
Of course dynamic XPath evaluation has been available in XSLT processors, as a vendor extension, for years.


Michael Kay

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