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Re: [RDF-CONCEPTS] Skolemization

From: David Booth <david@dbooth.org>
Date: Wed, 12 Jun 2013 13:41:43 -0400
Message-ID: <51B8B2D7.60400@dbooth.org>
To: Pat Hayes <phayes@ihmc.us>
CC: Ivan Herman <ivan@w3.org>, public-rdf-comments <public-rdf-comments@w3.org>
First off, sorry I didn't see Pat's response to Ivan before I replied. 
More . . .

On 06/12/2013 12:20 PM, Pat Hayes wrote:
>
> On Jun 12, 2013, at 10:53 AM, David Booth wrote:
>
>> On 06/12/2013 10:04 AM, Ivan Herman wrote:
>>>
>>>
>>> David Booth wrote:
>>>> I'd like to propose a small change in section on
>>>> Skolemization:
>>>> https://dvcs.w3.org/hg/rdf/raw-file/default/rdf-concepts/index.html#section-skolemization
>>>>
>>>>
>>>>
>>>>Regarding: "Systems wishing to do this SHOULD mint a new, globally 
unique IRI (a
>>>> Skolem IRI) for each blank node so replaced." it seems to me
>>>> that this conformance requirement should be a MUST -- not a
>>>> SHOULD -- because the system has already made the free choice
>>>> to skolemize.
>>>
>>> I do not follow this. Why should be a MUST?
>>
>> Because an IRI that is not globally unique would not be logically
>> equivalent to a bnode, and thus could significantly change the
>> semantics, and that would violate the intent of skolemization.
>
> It would not be skolemization, but that's just a matter of
> definition.

Right, that is my point: it would not conform to the RDF spec's 
definition of skolemization.  But if the conformance word were SHOULD, 
then it would conform.  That is why I am pointing out that the 
conformance word should be MUST.

> But it would not change the semantics,

WTF???  I don't know what you were thinking when your hands typed that!

> and even a
> skolemization is not *logically equivalent* to the bnode version. See
> https://dvcs.w3.org/hg/rdf/raw-file/default/rdf-mt/index.html#skolemization-1
> for the full monty on skolemization.

I meant logically equivalent in the sense explained in the Semantics:
https://dvcs.w3.org/hg/rdf/raw-file/default/rdf-mt/index.html#skolemization-1
[[
Nevertheless, they are in a strong sense almost interchangeable, as 
shown the next two properties. The third property means that even when 
conclusions are drawn from the skolemized graph which do contain the new 
vocabulary, these will exactly mirror what could have been derived from 
the original graph with the original blank nodes in place. The 
replacement of blank nodes by IRIs does not effectively alter what can 
be validly derived from the graph, other than by giving new names to 
what were formerly anonymous entities. The fourth property, which is a 
consequence of the third, clearly shows that in some sense a 
skolemization of G can "stand in for" G as far as entailments are 
concerned. Using sk(G) instead of G will not affect any entailments 
which do not involve the new skolem vocabulary.
]]

>
>> If it were a SHOULD then
>>
>> _:b :foo :bar .
>>
>> could be changed to
>>
>> :bar :foo :bar .
>>
>> If someone makes a change like that they should not be able to
>> claim that the change was conformant to the RDF spec.
>
> Sure they can. It *is* conformant with the spec, in fact. Its not a
> logically valid entailment, but users are not prohibited from making
> non-valid inferences in RDF. The user might happen to know, for
> out-of-band reasons, that the _:b is in fact this :bar guy.

You are really going to confuse people if you say things like that. 
Skolemizing

   _:b :foo :bar .

into

   skolem:b :foo :bar .

(where skolem:b is a skolem URI) is *completely* different from changing 
it into

   :bar :foo :bar .

The former "does not effectively alter what can be validly derived from 
the graph", whereas the latter obviously does.

David

>
> Pat
>
>
>>
>> Bear in mind that the decision to perform the skolemization is
>> still optional -- it's a MAY.  The MUST only kicks in after they
>> have made that choice: if they choose to do it they MUST do it
>> properly.
>>
>> David
>>
>>>
>>> Ivan
>>>
>>>
>>>>
>>>> Specific wording changes that I suggest:
>>>>
>>>> 1. Change:
>>>>
>>>> "Systems wishing to do this SHOULD mint a new, globally unique
>>>> IRI (a Skolem IRI) for each blank node so replaced."
>>>>
>>>> to:
>>>>
>>>> "Systems choosing to do this MUST mint a new, globally unique
>>>> IRI (a Skolem IRI) for each blank node so replaced. Each such
>>>> Skolem IRI SHOULD conform to the syntactic requirement for a
>>>> well-known IRI [WELL-KNOWN] with the registered name genid.
>>>> This is an IRI that uses the HTTP or HTTPS scheme, or another
>>>> scheme that has been specified to use well-known IRIs; and
>>>> whose path component starts with /.well-known/genid/."
>>>>
>>>> 2. Delete the paragraph: [[ Systems that want Skolem IRIs to be
>>>> recognizable outside of the system boundaries should use a
>>>> well-known IRI [WELL-KNOWN] with the registered name genid.
>>>> This is an IRI that uses the HTTP or HTTPS scheme, or another
>>>> scheme that has been specified to use well-known IRIs; and
>>>> whose path component starts with /.well-known/genid/. ]]
>>>>
>>>> Thanks, David
>>>>
>>>
>>
>>
>
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Received on Wednesday, 12 June 2013 17:42:11 UTC

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