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RE: [xml-dev] Formulas in XML values

Interesting idea.

This would be nice:

1. Xml Schema to have a built in simple expression language.
<foo expr=="1/3 + 2/3"

xs:attribute name="expr" type="xs:rationalExpr
xs:attribute name="expr" type="xs:float"

2. schema to have a plugin or type system extension mechanism.
<foo expr="1/3 + 2/3"
xs:attribute name="expr" type="js:rationalExpr"

Then for either 1 or 2 you need a corresponding evaluation engine. But you
can separate type checking from evaluation as separate concerns.

Not sure these ideas would ever happen as it requires a spec change and this
kind of change has lots of issues.

Simplest would be

xs:attribute name="expr" type="xs:string"

then write an XSLT or SAX processor a extension function the does the type
checking for that kind of expression.  Again, runtime evaluation can be a
separate concern.

As usual, there are some exact number representation and rounding issues at
play - part of a different discussion.


-----Original Message-----
From: COUTHURES Alain [mailto:alain.couthures@agencexml.com] 
Sent: Thursday, June 25, 2009 10:35 AM
To: xml-dev@lists.xml.org
Subject: [xml-dev] Formulas in XML values

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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