- From: Pat Hayes <phayes@ihmc.us>
- Date: Fri, 7 Nov 2008 18:19:23 -0600
- To: "Boris Motik" <boris.motik@comlab.ox.ac.uk>
- Cc: "'Bijan Parsia'" <bparsia@cs.man.ac.uk>, <public-owl-wg@w3.org>
- Message-Id: <C04BA4BE-AD43-4E52-BC71-FA4C823E9F41@ihmc.us>
On Nov 7, 2008, at 5:28 PM, Boris Motik wrote:
> Hello,
>
>> -----Original Message-----
>> From: public-owl-wg-request@w3.org [mailto:public-owl-wg-request@w3.org
>> ] On Behalf Of Pat Hayes
>> Sent: 07 November 2008 22:03
>> To: Boris Motik
>> Cc: 'Bijan Parsia'; public-owl-wg@w3.org
>> Subject: Re: typo in OWL 2 structural specification, not semantics.
>>
>>
>>
>> On Nov 7, 2008, at 1:15 PM, Boris Motik wrote:
>>
>>> Yes, you're right. I'm sorry - I got confused.
>>
>> Yes, I agree, Bijan is right, contrary to my original note.
>>
>> Still, this is VERY confusing. I would suggest that the use of
>> "positive zero" and the notation "+0", while I am sure well-meant, is
>> responsible for much of the difficulty. The whole idea of a "positive
>> zero" is incoherent in any case, and the positive/negative contrast
>> and the symmetry of the notation both strongly suggest a symmetry in
>> the underlying structure, which in fact is quite asymmetric.
>>
>> If this convention is already in widespread use, I guess y'all have
>> to
>> go with it; but if not, I'd strongly suggest getting rid of
>> 'positive
>> zero' and '+0', and just calling it 'zero' and writing '0'. You know,
>> like people have for about a millennium now.
>>
>
> The reason why we have positive and negative zeros is because this
> is how things work in XML Schema Datatypes 1.1; these are in turn
> based on IEEE 754 for the representation of floating-point numbers.
IEEE 754 has -0, OK. But you don't need to confuse things by re-naming
plain old zero as 'positive zero', was my point.
> Our goal was to be compatible with these specs, so we included
> them into OWL. If this were not the case, OWL ontologies could not
> capture all floating point numbers that can occur in practice,
> which some members of the WG considered problematic.
>
> The solution was to "promote" the standard zero to +0, and to
> introduce a new -0 which is not a real number.
My point can be made by agreeing with the need to introduce -0, but
suggesting that you avoid "promotion". Zero doesn't need to be
promoted: just leave it alone, then it continues to have the
properties it always had (without anything needing to be said), and
you don't need to talk about it, and it doesn't seem to be a
symmetrical version of -0.
But I will drop the subject, having made the suggestion.
>
>
>> Another thing that is highly confusing in the text is the use of
>> 'equal' having a distinct meaning from 'identical'. If these are not
>> synonyms, then 'equal' does not mean =. Boris, I think your first
>> response was confused on exactly this issue.
>>
>> I think what y'all mean by
>>
>> "+0 is equal to -0 but not identical to it"
>>
>> is in fact
>>
>> "+0 isn't equal to -0 but they are equivalent as far as arithmetic is
>> concerned"
>>
>
> The XML Schema Datatypes 1.1 spec explicitly distinguishes identity
> from equality.
I know it does. But you are writing an OWL 2 spec here, and "equality"
is a word with a very precise meaning in logic. Its not up for re-
definition along the XML Schema lines unless you are VERY clear in the
text that you are using "equality" in this section of your spec in a
logically non-standard way.
BTW, I was, having already walked into this particular minefield, able
to guess your intended meaning at this point, but I am still not able
to tell if you use "equal" in this idiosyncratic way throughout the
OWL 2 specification, all seven volumes of it. I would suggest that
internal coherence of the OWL spec on such a basic topic is more
important than an adherence to XML Schema usage.
> In particular, +0 and -0 are not identical (i.e.,
> they are different things in the interpretation domain), but they
> are equal.
No, they are not. XML Schema notwithstanding, equal means identical.
You are using "equal" here in an alien way, not the way it is used
throughout logic (as in "FOL with equality", "Equational reasoning",
etc.) and indeed throughout most of mathematics. You are here using it
to mean "Equivalent under the XML Schema real-number datatype
equivalence". If you were intending OWL 2 to be a sub-specification
of XML Schema, maybe this usage would be acceptable: but I don't think
you do intend this, in fact.
> Equality is used in OWL mainly for the definition of
> facets.
No, equality in OWL is owl:sameAs. Check the semantics if you don't
believe me :-)
> For example, the following data range is interpreted as a set of two
> elements containing +0 and -0:
>
> DatatypeResctriction( owl:realPlus minInclusive "0"^^xsd:integer
> maxInclusive "0"^^xsd:integer )
>
> The reason for this is that both +0 and -0 are equal, but they are
> not identical.
I know what you mean, but this way of saying it is nonsensical. To say
they are equal is to say that the relation indicated by the
mathematical sign "=" holds of them; and this is possible only if
they are one and the same thing, i.e. identical.
> In contrast, the following data range is a singleton and contains
> only +0:
>
> DatatypeResctriction( owl:real minInclusive "0"^^xsd:integer
> maxInclusive "0"^^xsd:integer )
>
>
> I'm afraid there is not much we can do if we want to remain
> compatible with XML Schema Datatypes 1.1 and IEEE 754.
I am only talking about the wording, not the substance. Of course you
can retain such compatibility, but you are not obliged to re-use the
XML Schema's idiosyncratic (and, strictly, mistaken) usage of the word
"equal". If you feel that you must re-use it, at the very least you
should include some explanatory text to clarify what you actually
mean, for the benefit of readers who speak English rather than
XMLSchema-ese.
Best wishes
Pat
>
>
> Regards,
>
> Boris
>
>> Pat
>>
>> PS. I presume the ordering intended is
>>
>> -INF < any negative real < either zero < any positive real < +INF
>>
>> but without any order assigned to {zero, -zero}. Is this right?
>>
>>> I've now reverted the spec into the original condition. While doing
>>> this, I've just
>>> noticed that we haven't defined the relationship between -0 and +INF
>>> and -INF. Therefore, I've added the following condition:
>>>
>>> * -INF < -0 < +INF
>>>
>>> Again, sorry for the confusion.
>>>
>>> Regards,
>>>
>>> Boris
>>>
>>>> -----Original Message-----
>>>> From: Bijan Parsia [mailto:bparsia@cs.man.ac.uk]
>>>> Sent: 07 November 2008 19:06
>>>> To: Boris Motik
>>>> Cc: 'Pat Hayes'; public-owl-wg@w3.org
>>>> Subject: Re: typo in OWL 2 structural specification, not semantics.
>>>>
>>>> On 7 Nov 2008, at 18:55, Boris Motik wrote:
>>>>
>>>>> Hello,
>>>>>
>>>>> Strictly speaking, this wasn't a typo because -0 is equal +0. I
>>>>> can
>>>>> see, however, that this can confuse people, so I've therefore
>>>>> changed the document as you've suggested.
>>>>>
>>>>> Thanks for this comment!
>>>>
>>>> Actually, I think this is a bit misleading and may even change the
>>>> meaning.
>>>>
>>>> +0 is a real, thus covered by the first clause (we should switch
>>>> these to an ordered list).
>>>>
>>>> -0 is *not* a real (though it is equal to a real), so, just like -
>>>> INF
>>>> and +INF needs an additional clause.
>>>>
>>>> (At the moment, I don't see a hook up between = and <. As far as I
>>>> can tell, it's possible for -0 = +0 *and* -0 < +0. Indeed, if you
>>>> change the definition to +0, then I would say that -0 is not < than
>>>> any other element in realPlus and vice versa.)
>>>>
>>>> For example, with this change, I believe your example no longer
>>>> holds. I.e., from that definition -1 and 1 only have +0 between
>>>> them.
>>>>
>>>> Cheers,
>>>> Bijan.
>>>
>>>
>>>
>>>
>>
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>>
>>
>>
>
>
>
>
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Received on Saturday, 8 November 2008 00:20:42 UTC