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Re: XML Schema datatypes

From: <ewallace@cme.nist.gov>
Date: Thu, 15 Nov 2007 17:47:36 -0500 (EST)
Message-Id: <200711152247.RAA29402@clue.mel.nist.gov>
To: public-owl-wg@w3.org


Carsten Lutz wrote:

>On Tue, 13 Nov 2007, Jeremy Carroll wrote:
>>
>> 7) rounding errors behave very differently from in traditional numeric
>> applications - hence a solved problem (rounding) becomes an unsolved problem
>>
>> (I will also add another area of concern which is a mismatch between the 
>> real numbers and the XSD datatypes)
>>
>> ===
>
>This is an interesting subject. I don't think it is really about
>"rounding", though, and neither is it really about n-ary datatypes.  I
>think what your example shows is that using bounded or fixed-precision
>datatypes such as float or decimal in the semantics of an ontology
>language is a bad idea. This means that already the unary datatypes in
>OWL 1.0 are in some sense broken, and the XML Schema datatypes are
>probably *not* the right thing to use in OWL. Sorry if this sounds like
>blasphemy.

A minor nit:

The built-in XML Schema datatype Decimal is neither bounded nor fixed
precision (as defined).  The type is basically the Reals subsetted to those 
values that can be expressed with finite length decimal numerals.  So
in the terminology that Jeremy used in an earlier email, it is sparse
in comparison to Real or even Rational numbers.  I believe that this was
a type that was meant to support a wider range of uses, where the 
precision and range would be bounded according to the needs of the
application, not the representation.  It's closer to what we need then
standard computational types like Float, but still not good enough.

I agree with Carsten though that the issue here isn't really rounding.  
There were three types of concerns in the Turner - Carroll paper
that were mixed: 
1) computational differences/limitations in types,
2) representational differences/limitations in types and
3) naive expectations of users.

I would like to see us explore defining a Rational datatype and 
corresponding computation requirements for reasoners to address limitation
concerns 1 and 2 for use cases such as exact value coefficients for unit 
conversion.  This may turn out to be out of scope if further research is
really needed, or a majority of implementers balk at adding it.

Concern 3 is still legitimate, though not a problem unique to datatype 
reasoning with OWL.  We should at least make a start on a user guide for 
this as well, though that's the kind of document that would have belonged
in the Best Practices group (and "XML Datatypes in RDF and OWL" did
touch on some of these issues).

-Evan
Received on Thursday, 15 November 2007 22:47:50 GMT

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