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Re: A possible structure of the datatype system for OWL 2 (related to ISSUE-126)

From: Michael Smith <msmith@clarkparsia.com>
Date: Thu, 10 Jul 2008 09:28:06 -0400
To: Rob Shearer <rob.shearer@comlab.ox.ac.uk>
Cc: public-owl-wg@w3.org
Message-Id: <1215696486.23047.173.camel@msmith-laptop-wired.int.clarkparsia.com>

On Thu, 2008-07-10 at 11:24 +0100, Rob Shearer wrote:
> > Further, I have concern that because the lexical space allows  
> > arbitrary precision and due to alternative, inconsistent rounding  
> > implementations (permitted by XSD 1.1 [2], see 3.3.5.2) the lexical to value-space
> > mapping could differ between implementations. Thus, two reasoners with
> > xsd conformant lexical to value mapping algorithms could correctly
> > produce different entailments.  (I don't have access to IEEE-754 to  
> > look into the details here, I'm basing my statements on [2]).
> 
> I've just been looking through IEEE-754. Although it does allow for  
> some differences in rounding implementations, it places (quite strict)  
> limits on the possible error introduced. My understanding is that this  
> means it is possible to encode any floating-point number in decimal to  
> sufficient precision that any IEEE-754-compliant translation back to  
> floating point results in exactly the same value.

> Assuming that my understanding is correct, this means that it is  
> perfectly safe to serialize floats as `xsd:float` within an OWL file  
> without loss of precision: although there do exist `xsd:float` strings  
> whose exact values are not precisely defined, users always have  
> unambiguous representations available to them.

Yes, I believe if we start with an IEEE-754 float value, it will go
lexical to value space and back without loss.

I was concerned about the large number of possible constants (e.g.,
"0.01") for which there is not a directly corresponding value in the
float value space.  [2] indicates to me that for some those, there are
alternative lexical to value space mappings, which could cause problems.


> So I recommend sticking with the semantics for lexical representations  
> defined in the XSchema spec.

[2] says, "since IEEE allows some variation in rounding of values,
processors conforming to this specification may exhibit some variation
in their ·lexical mappings·. "  It seems that unless we were to
constrain the set of permissible float constants to only those which are
mapped into the value space losslessly, then we have a problem.

-- 
Mike Smith

Clark & Parsia

[2] http://www.w3.org/TR/xmlschema11-2/#float
Received on Thursday, 10 July 2008 13:28:53 UTC

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