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Issue 80: terminology

From: Dave Reynolds <der@hplb.hpl.hp.com>
Date: Wed, 25 Mar 2009 18:47:56 +0000
Message-ID: <49CA7C5C.7030507@hplb.hpl.hp.com>
To: RIF WG <public-rif-wg@w3.org>
We had a useful but not fully conclusive discussion on this at the last 
telecon and tried to clarify our terminology. I felt afterwards we 
didn't quite have it right so here's an attempt to articulate that.

There are actually three notions of "equality" buried in here.

(1) Identity. Two literals are identical if they correspond to the same 
value in the value space.

(2) Equality. XSD defines algorithms for equality which mean that two 
non-identical points in a given value space can be equal. However, for 
XSD this equality is restricted to be within a datatype[1] and so 
doesn't include the "1"^^xsd:int = "1.0"^^xsd:float case.

(3) Equivalence[3]. XPath F&0 defines equality operators (a different 
notion from the mathematical equality functions which characterize the 
XSD value spaces). These can span multiple datatypes, specifically 
numeric-equal does.  This is the one that does type promotion and 
permits "1"^^xsd:int = "1.0"^^xsd:float.

Now, in XML Schema 1.0, which is the version we current reference in 
DTB, equality and identity are the same[2]. So the issues we were 
worrying about in the telecon don't apply.

In XML Schema 1.1 some datatypes have a non-identity equality. 
Specifically xsd:dateTime and float/double. Though with float/double the 
exceptions (-0 = +0, Nan != NaN) are perhaps not so interesting.

For OWL then they define xsd:dateTimeStamp to have a value space 
corresponding to a timeline instead of following the XML Schema 1.1 
structural model and so for OWL's definition identity and equality 
coincide again.

So my contention is that the way literal-not-equal is defined in DTB at 
present (non-identity) corresponds to the equality notion in OWL for all 
the relevant datatypes and I keep my preference for options 2 or 1 [4].

This contention is trivially true if we stick to XML Schema 1.0 because 
there is no distinction between equality and identity there. If we move 
to XML Schema 1.1 (is there a proposal to do this? I assume so) there 
are differences but the main problematic datatype is not included in OWL[5].


[1] http://www.w3.org/TR/xmlschema11-2/#value-space
"For purposes of this specification, the value spaces of primitive 
datatypes are disjoint, even in cases where the abstractions they 
represent might be thought of as having values in common."

[2] http://www.w3.org/TR/xmlschema11-2/#value-space
"Note: In the prior version of this specification (1.0), equality was 
always identity.  This has been changed to permit the datatypes defined 
herein to more closely match the "real world" datatypes for which they 
are intended to be used as transmission formats."

[3] I don't know what a good name for this is so I'm using "equivalence" 
for now. You could call it "programmatic equality" or "pragmatic 
equality" or something.

[4] http://lists.w3.org/Archives/Public/public-rif-wg/2009Mar/0076.html

[5] OK, that does leave the corner cases of float/double (-0 = +0, Nan 
!= NaN) I haven't checked how OWL handle these. If those are the only 
differences then I personally don't care either way.
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Received on Wednesday, 25 March 2009 18:48:50 UTC

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