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Re: More semantic comments (XML Schema value spaces)

From: Dan Connolly <connolly@w3.org>
Date: 13 Nov 2002 09:16:43 -0600
To: Pat Hayes <phayes@ai.uwf.edu>
Cc: Jeremy Carroll <jjc@hplb.hpl.hp.com>, w3c-rdfcore-wg@w3.org, "Henry S. Thompson" <ht@cogsci.ed.ac.uk>
Message-Id: <1037200604.22866.21385.camel@dirk>

[+cc Henry in case he can clear this up quickly.
If not, Henry, don't feel obliged to reply; I'll
eventually report my problem with this
to www-xml-schema-comments]

On Wed, 2002-11-13 at 08:13, pat hayes wrote:
> >On Mon, 2002-11-11 at 17:13, pat hayes wrote:
> >[...]
> >>  >3.4
> >>  >
> >>  >I found the following unintelligible, possibly because I don't have
> >>  >access to the XSD example in your (or mayber Peter's) head.
> >>  >
> >>  >[[Users shoudl take care to distinghuish the value space ...
> >>  >identical when viewed as class members.]]
> >>
> >>  Yes, it is rather odd. The plain fact is that the XML schema specs
> >>  are logically contradictory when one thinks of a value space as a
> >>  set. Enquires have determined that XMLSchema value spaces are not
> >>  sets,
> >
> >?!?!?
> >
> >yes, they are:
> >
> >"A value space is the set of values for a given datatype."
> >  -- http://www.w3.org/TR/2001/REC-xmlschema-2-20010502/#value-space
> 
> There is little point in quoting one assertion from a text that has 
> internal contradictions.

I wasn't aware there were internal contradictions.
I'd be interested in help finding the text that
contradicts the above.

> But I spoke too hastily. What I should have said is that they are not 
> sets *of the values which the spec declares them as containing*. The 
> spec (plus published or just-to-be-published errata) insists that no 
> two primitive value spaces have the same members, by definition.

!?!?!

I was a member of the WG, and I remember no such decision.

I guess I'll have to watch the errata.

I don't see any relevant ones so far.
  http://www.w3.org/2001/05/xmlschema-errata
  $Id: xmlschema-errata.html,v 1.16 2002/11/04 15:05:09 ht Exp $

> It 
> also says that base64Binary and hexcodedBinary both have value spaces 
> containing the same members.

yes, that's clear enough:

"The ˇvalue spaceˇ of hexBinary is the set of finite-length sequences of
binary octets."

"The ˇvalue spaceˇ of base64Binary is the set of finite-length sequences
of binary octets."


> On being queried about this, Henry 
> Thompson gave the following replies: (1) primitive value spaces can 
> contain the same members (2) members of distinct primitive value 
> spaces are considered to be distinct, by definition.  This is 
> impossible in set theory, no matter what the spec says. The spec is 
> evidently confused.

Something or somebody is confused; I'm not convinced it's
the text of the spec.

> >  > but something like categories or algebras. The same thing, seen
> >>  as a member of two different value spaces, is considered to be two
> >>  different things.
> 
> That is the impression I formed from trying to make sense of what 
> Henry said and re-reading the spec in that light. But it is only an 
> impression. Another interpretation is that the spec uses a 
> nonstandard notion of 'value' in which there are two layers of 
> interpretation and the xsd-"value' is the thing in the first layer, 
> while the RDF-mathematical-"value" is in the second layer, with a 
> many-to-one mapping between them.

Blech.

> >Inquiries may *suggest* otherwise, but only excerpts from
> >the text of the spec can *determine* what XML Schema
> >value spaces are.
> >
> >I'm interested in what leads you to think they're not
> >sets.
> 
> Extended series of emails from Henry Thompson in reply to a direct 
> query on the topic from Brian, followed by searching enquiries from 
> me. I can forward them if you like. Henry's view is that the value 
> spaces are best seen as sets of pairs <value, value space>. These are 
> not the same sets as the class extensions of the datatype domains in 
> RDF.

Eek! Train wreck approaching!

We're supposed to model XML Schema datatypes.


> >In particular, I'm interested in seeing anything from
> >the spec that suggests they're not sets (i.e. is
> >inconsistent with the text excerpted above) reported as a bug
> >to www-xml-schema-comments@w3.org .
> 
> I thought of sending in such a bug report, but decided that it wasn't 
> a bug, exactly, and in any case it wasn't my business to help XML 
> Schema clean up its own mess.

Well, it is our business. You're certainly doing your fair share;
you're not the only one who can send the bug report...
but we are chartered to deal with XML Schema datatypes...

"RDF Schema must use and build upon XML Schema datatypes to the fullest
extent that is practical and appropriate. Specifically, the RDF Core
Working Group is not chartered to develop a separate data typing
language that duplicates facilities provided by XML Schema data types."
 -- http://www.w3.org/2001/sw/RDFCoreWGCharter

and if they're a mess, we have an interest in cleaning them up.


> Pat
> 
> 
> >
> >>  This is simply impossible in RDFS if we interpret
> >>  value spaces as classes. So I was trying to give the reader a warning
> >>  about this without being so coarse as to imply anything derogatory
> >>  about XML schema.

-- 
Dan Connolly, W3C http://www.w3.org/People/Connolly/
Received on Wednesday, 13 November 2002 10:18:16 EST

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