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Re: [RDF-CONCEPTS] Skolemization

From: Pat Hayes <phayes@ihmc.us>
Date: Wed, 12 Jun 2013 11:20:32 -0500
Cc: Ivan Herman <ivan@w3.org>, public-rdf-comments <public-rdf-comments@w3.org>
Message-Id: <6641751E-CE7B-4548-9DCD-8B559AC43A19@ihmc.us>
To: David Booth <david@dbooth.org>

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. But it would not change the semantics, 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. 

>  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. 

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 16:20:59 UTC

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