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language tags in typed RDF literals

From: pat hayes <phayes@ai.uwf.edu>
Date: Tue, 11 Feb 2003 18:01:21 -0600
Message-Id: <p05111b42ba6f36b3787e@[]>
To: www-rdf-comments@w3.org
Cc: bwm@hplb.hpl.hp.com, Jan.Grant@bristol.ac.uk

The current design of RDF literals is needlessly complicated and kind 
of silly.  The syntax allows language tags to occur in typed 
literals, but in all cases other than rdf:XMLLiteral, these tags are 
required to have no meaning, so the semantics is obliged to provide a 
valid inference rule which allows any language tag in any such typed 
literal to be removed or replaced by any other.  This considerably 
complicates the statement of the semantics, adds a burden to any 
implementation, nullifies the implicit design principle that literals 
can be compared for identity using simple lexical matching (since an 
engine is required to strip out all such lang tags while performing 
inferences or checking for identity), and provides no useful 
expressive function.

A related point is that the requirement in the semantics that 
datatypes other than rdf:XMLLiteral *must* ignore language tags seems 
to restrict possible future datatyping proposals needlessly.

I suggest therefore that


(1) lang tags be forbidden by the RDF syntax from appearing in 
non-XML typed literals.


(2) the notion of the lexical space of a datatype be generalized to 
allow (not require)  lang tags to be taken into consideration by a 
datatype, so that the lexical space may be a set of strings or pairs 
of strings, i.e. a set of simple literals. This would have the effect 
that it would no longer be valid to make arbitrary changes to a lang 
tag in any literal, typed or not. It would also bring the treatment 
of all RDF datatypes into alignment so that rdf:XMLLiteral need not 
be considered a special case.

Either of these changes will simplify the semantics and make it more 
coherent, but in slightly different ways.

Either change will produce fewer inference rules and lead to less 
processing in a reasoning engine.

Pat Hayes
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Received on Tuesday, 11 February 2003 19:01:25 UTC

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