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Re: [xml-dev] Formulas in XML values


This is an interesting point, and a subtle one (and that it comes up in XBRL surprises me not in the least). If I understand it correctly, what you're asking is, if you have an XML expression of the form:

<account-total value="29 + 12 * 1.25"/>

how you can effectively evaluate this statement via XPath. 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. This gets more complicated of course because there is nothing in the current data model for dealing with a function as a data type (the only way you could schematically define the above @value attribute is as xs:string, even though, as a numeric function, it should properly be defined as of type xs:double).

In practice, of course, you can always create an eval type function, though given XSLTForm's architecture, that could prove to be exciting (I'm guessing you'd have to run the expression through an XSLT again in order to properly evaluate it, assuming that you're just dealing with constants).

Kurt Cagle
Managing Editor

On Thu, Jun 25, 2009 at 10:35 AM, COUTHURES Alain <alain.couthures@agencexml.com> wrote:
Formulas in XML values are a persistent concern for me for some days and xml-dev appears to be the best place to discuss this...

From mathematical point of view, 35+7 and 42 are equals but human beings can interpret them differently. When a value has to be put in a field by an user, sometimes it's, in fact, the result of a basic computation and, when someone else has to verify the field value, the initial formula might help to understand. Excel users do that every day !

XML is a text notation so it's not disturbing to write 35+7 where 42 can be. But, schema languages should evaluate values before testing if they are valid or not. XPath should, at least, do the same or, better, be able to say that 15+6 multiplied by 2 is (15+6)*2. Students calculators can do that already, don't they ?

There are interesting possibilities with such a notation improvement : numbers such as 1/3, pi, sqrt(2) would be written as what they are and not just in a limited amount of digits. Of course, a bunch of mathematical functions and constants should be named...

I will probably implement some basic approach for this in my XForms implementation (XSLTForms : http://www.agencexml.com/xsltforms) because I already need it for setting values in XBRL documents.

XSLTForms has its own XPath implementation and XML Schema validation, both written in Javascript, and some eval() calls at the right place might be all what is needed. Formulas will then be submitted and stored as ordinary values would be. If needed, I will design an XSLT stylesheet to resolve formulas into values (XSLTForms already has an XPath 1.0 expression parser written in XSLT 1.0 ...).

Thank you for your remarks and suggestions.

Best regards,

Alain Couthures
Bordeaux, France
Twitter account: AlainCouthures


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