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Syntax: Literals and defined datatypes

From: Markus Krötzsch <markus.kroetzsch@comlab.ox.ac.uk>
Date: Tue, 08 Mar 2011 08:51:20 +0000
Message-ID: <4D75EE08.2010902@comlab.ox.ac.uk>
To: public-owl-comments@w3.org
Re-reading the OWL 2 specification, I stumbled upon some syntactic 
issues that I want to record here. Relevant statements:

(1) OWL 2 allows ontologies to define datatypes DT as "synonyms" for 
dataranges DR such that "in any expression in the ontology containing 
such an axiom, DT can be replaced with DR without affecting the meaning 
of the ontology" [1].

(2) Dataranges can have a number of forms, using features like union, 
intersection, and facet restrictions on datatypes [2].

(3) A literal "consists of a lexical form, which is a string, and a 
datatype" where the "lexical form of each literal occurring in an OWL 2 
DL ontology MUST belong to the lexical space of the literal's datatype." [3]


I suppose that it was indented to not allow defined datatypes in 
literals, and that the hint "the datatypes supported in OWL 2 are 
described in more detail in Section 4" [3] might suggest this (Section 4 
defines built-in types, not defined ones). But the normative grammar 
uses the same non-terminal "Datatype" both in literals and in defined 
types, and I could not find another statement that disallows defined 
types in literals explicitly.

Using defined datatypes in literals would not be an option for OWL DL 
since there does not seem to be any place where the *lexical space* of 
defined datatypes is specified; so if it is unclear how to check the 
requirements of (3). In any case, the direct semantics does not cover 
this possibility, and it would also not be desirable to evaluate 
datatype definitions for checking an ontology's syntactic/structural 
validity.

Some editorial clarification might be useful here (if the intended 
editorial update after XML 1.1 completion ever happens).

Cheers,

Markus


[1] http://www.w3.org/TR/owl2-syntax/#Datatype_Definitions
[2] http://www.w3.org/TR/owl2-syntax/#Data_Ranges
[3] http://www.w3.org/TR/owl2-syntax/#Literals
[4] 
http://www.w3.org/TR/owl2-syntax/#Appendix:_Complete_Grammar_.28Normative.29

-- 
Dr. Markus Krötzsch
Oxford  University  Computing  Laboratory
Room 306, Parks Road, Oxford, OX1 3QD, UK
+44 (0)1865 283529    http://korrekt.org/
Received on Tuesday, 8 March 2011 08:51:50 GMT

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