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Re: RDF* semantics, particularly malformed literals in embedded triples

From: <phayes@ihmc.us>
Date: Thu, 4 Feb 2021 18:53:18 -0600
Message-ID: <80AB5962-4339-4BBC-8975-1815DCD54ADD@ihmc.us>
CC: Pierre-Antoine Champin <pierre-antoine.champin@ercim.eu>, <public-rdf-star@w3.org>
To: "Peter F. Patel-Schneider" <pfpschneider@gmail.com>


> On Feb 4, 2021, at 1:56 PM, Peter F. Patel-Schneider <pfpschneider@gmail.com> wrote:
> 
> 
> On 2/4/21 1:00 PM, Pierre-Antoine Champin wrote:
>> 
>> On 04/02/2021 00:18, Peter F. Patel-Schneider wrote:
>>> On 2/3/21 4:09 PM, Pierre-Antoine Champin wrote:
>>>> On 28/01/2021 01:59, Peter F. Patel-Schneider wrote:
>>>>> On 1/25/21 4:21 AM, Pierre-Antoine Champin wrote:
>>>>> 
>>>>>> [...]
>>>>>> 
>>>>>> One example of "missed" entailement would be (I think) with the
>>>>>> "malformed-literal-bnode" test
>>>>>> (https://w3c.github.io/rdf-star/tests/semantics/manifest.html#malformed-literal-bnode).
>>>>>> 
>>>>>> 
>>>>>> But granted, that one is a corner-case, that maybe not everybody agrees on
>>>>>> anyway.
>>>>> I don't see why this entailment would be missed.
>>>> (this is based on your proposal here:
>>>> https://lists.w3.org/Archives/Public/public-rdf-star/2021Jan/0059.html)
>>>> 
>>>> * the input of the test has an rdf*:object arc, but no rdf:object arc
>>>> (because o is a malformed litteral)
>>>> 
>>>> * the expected entailed graph as a spurious rdf:object arc, and misses the
>>>> rdf*:object arc (because o is a blank node)
>>> The example you mention is, I beleive,
>>> 
>>> prefix :<http://example.com/ns#>
>>> prefix xsd:<http://www.w3.org/2001/XMLSchema#>
>>> << :a :b "c"^^xsd:integer >> :p1 :o1.
>>> 
>>> when xsd:integer is a recognized datatype MUST entail
>>> 
>>> prefix :<http://example.com/ns#>
>>> << :a :b _:x >> :p1 :o1.
>> Correct
>>> This stays, intuitively, that an embedded triple with a malformed literal
>>> entails one with a blank node.
>>> 
>>> I agree that my proposal does not have this entailment, contrary to my
>>> previous impression.  But I view this as a good thing.  Malformed literals do
>>> not denote in RDF so there should not be any thing for the blank node to
>>> match.  So don't view this as a desireable entailment.
>> 
>> I know this one is controversial. :)
>> 
>> My position was: in RDF, one can replace any term with a fresh blank node,
>> and the resulting graph is entailed by the original one. Following the
>> principle of least surprise, the same should hold in RDF*.
> 
> 
> Well, a surprise is that malformed literals in embedded triples do not cause a
> semantic inconsistency like they do elsewhere.   So given that there is a
> surprise already, further surprises are not indicative of further incorrect
> behaviour. 
> 
> The argument the other way is that malformed literals do not denote anything,
> so there is nothing for the blank node to denote, so the entailment should not
> follow.

Yes, and that argument is correct. This entailment:

x:a x:b 'notaninteger'^^xsd:integer .
entails
x:a x:b _:thing .

is valid in RDF (recognizing XML schema datatypes) not because it preserves truth, but because the antecedent is necessarily false in all interpretations. It is a trivial /ex falso quodlibet/ entailment. To emphasize the point, this is also a valid entailment, for exactly the same reason:

x:a x:b 'notaninteger'^^xsd:integer .
entails
https://dbpedia.org/resource/Pat_Hayes <https://dbpedia.org/resource/Pat_Hayes> owl:sameAs https://dbpedia.org/resource/Pablo_Picasso <https://dbpedia.org/resource/Pablo_Picasso> .

Pat Hayes



Received on Friday, 5 February 2021 00:53:38 UTC

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