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response to issue pfps-10

From: pat hayes <phayes@ai.uwf.edu>
Date: Tue, 4 Feb 2003 18:23:23 -0600
Message-Id: <p05111b2eba64b7d84292@[10.0.100.86]>
To: w3c-rdfcore-wg@w3.org
Cc: pfps@research.bell-labs.com


I believe that untyped literals do not work correctly in the RDF model
theory. The semantic constraint on rdfs:Literal is, from Section 3.3

     ICEXT(I(rdfs:Literal)) is a subset of LV

However, the denotations of untyped literals are only required to be
in LV.   Therefore the denotation of "a" is not necessarily in
ICEXT(I(rdfs:Literal)).

This situation has observable consequences.  For example
         ex:foo ex:rel "a" .
does not RDFS-entail
         ex:foo ex:rel _:x .
         _:x rdf:type rdfs:Literal .

The situation is made even stranger in datatyped interpretations.  If
there is a datatype whose value spaces includes the string "a", then
entailment over this datatype has the above entailment.
-----

This comment is substantially correct. The design was deliberate, 
partly to keep rdfs-entailment simpler,  but I now tend to agree with 
Peter that it would be preferable to make the untyped case more 
aligned with the typed case, and Patrick has indicated that this was 
his understanding of what should have been done in any case.

I therefore propose to make changes to the model theory, as follows. 
This is the only proposed change which materially affects normative 
parts of the semantics document. Of these, 1. is simply a matter of 
mathematical style; 2 is the substantial change which requires the 
new rules (detailed below); the rest are clarifications of existing 
intentions which can now be more simply expressed in the light of 2. 
4. has the consequence that all aspects of the built-in datatype that 
can be expressed using the RDF namespace are now incorporated into 
RDF entailment, which seems appropriate.


1. LV will be considered part of an interpretation, like IP and IC. 
The semantic conditions on literals are then conventional semantic 
constraints on interpretations. This also deals with an issue raised 
by Graham. In many ways this makes the model theory more 
'conventional'.
2. ICEXT(I(rdfs:Literal)) = LV (not a subset of LV) in all RDFS 
interpretations.
3. All ill-formed typed literals denote non-literal values (ie not in LV)
4. Similar conditions for XML literals will be added to the RDF semantics.
5. In datatyped interpretations, If x is in D, then ICEXT(x) is a subset of LV

In the rules this has the following corresponding changes:

add the following rdf inference rule:
-----
xxx aaa lll .
where lll is a well-formed XML typed literal .
==>
xxx aaa _:n .
_:n rdf:type rdf:XMLLiteral .
where _:n is a new bNode.
----
and the following rdfs inference rule:
-----
xxx aaa "sss"[@tag] .
==>
xxx aaa _:n .
_:n rdf:type rdfs:Literal .

where _:n is a new bNode.
------

and the axiomatic triple

rdf:XMLLiteral rdfs:subClassOf rdfs:Literal .

--------
There will also be some corresponding changes to some of the proofs 
in the appendix; I will not try to summarize these here.

The first of these rules corresponds exactly to rule rdfD 1 for typed 
literals, and the second also corresponds if we interpret untyped 
literals as having a 'trivial' type where L2V is the identity 
mapping.  This is therefore a completely uniform treatment for all 
literals.


I hope to have an editors draft incorporating these changes ready by 
tomorrow (Wednesday).


Pat



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Received on Tuesday, 4 February 2003 19:21:41 EST

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