Literals: lexical spaces and value spaces

At 12:32 PM 11/5/01 +0200, Patrick.Stickler@nokia.com wrote:
> > FWIW, when I reviewed XML schema data types some time ago, I was deeply
> > uncomfortable that the primitive value spaces seemed to be defined (and
> > constrained) in terms of their lexical representations.  This seemed maybe
>
> > justified because XML schema was, by design, all about values that were
> > represented in a text-based format (i.e. XML).  But I'm not convinced that
>
> > the considerations apply equivalently to RDF.
>
>But exactly how is an RDF literal not a lexical representation?!

Ah yes, an RDF/XML or N-triples literal string *is* a lexical representation.

But my view is that the heart of RDF that really matters is the underlying 
abstraction.  I tend to think that there will be some things that RDF 
expresses (using whatever syntax) without reference to literal values, and 
that the data typing of such things should not be constrained by lexical 
(character string) representation.

I would be more comfortable with a scheme that defined value spaces 
independently of lexical representation, and then provided mappings for 
lexical representations.

My particular beef with XML schema datatypes concerns non-integral 
numbers.  I think the starting point for this value space should be the set 
of all rational numbers, since any number represented in and manipulated 
directly by a computer must necessarily be rational (**).  All fixed- and 
floating-point forms represent rational values.  But (as I recall) XML 
schema datatypes define non-integral number value spaces based on either 
fixed-point decimal character representations or IEEE floating point 
reprersentations.  This means that some simple, everyday values cannot be 
represented directly using primitive XSD numeric datatypes (e.g. 1/3, or 
the exact conversion factor for millimetres->inches: 10/254).  This isn't a 
purely academic argument:  we found rational numbers to be useful in CONNEG 
work, and they have been defined for CC/PP.

[(**) OK, you could devise exotic schemes where this isn't the case, but 
for practical purposes I still claim that rational numbers underpin just 
about all use of numbers in computers.]

>Insofar as associating a data type with a literal, both the value
>space and lexical space are relevant.

Yes, of course.

>Insofar as making logical inferences about the values themselves
>and their relation to other values, compliance with range constraints,
>etc. etc. then the lexical space is not relevant.

Quite.

#g


------------------------------------------------------------
Graham Klyne                    MIMEsweeper Group
Strategic Research              <http://www.mimesweeper.com>
<Graham.Klyne@MIMEsweeper.com>
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Received on Monday, 5 November 2001 07:44:25 UTC