[Versa] Issue 1, better literal and datatype support

[Chimezie Ogbuji]

On 8/12/05, Michael Olson <Mike.Olson at fourthought.com> wrote:
> 
> I'd like to propose adding better literal and data type support to
> Versa.  I don't think we need to do too much to the spec itself to
> support this.  One thing we need is a literal data type.  This would be
> a representation of a plain (with or without language) or typed
> literal.  I think we leave hard issues like comparison and sorting
> between typed literals to the underlying model.  If the model support
> D-entailment then it can properly compare "01"^^xsd:int and
> "1"^^xsd:int.  If it doesn't....

I had exactly the same suggestion.  So your suggestion a literal
datatype as a *replacement* for strings and numbers?  If so, I'd
agree, since that would completely cover all literals (especially if
we include the capablity to associate xsd - or other - datatypes to
the underlying model) and would be more inline with RDF Literals.

> Besides the addition of the literal datatype, I think all that is
> needed is to determine how the conversions work (should be pretty
> straight forward) and some functions.  ie. it would be nice to filter
> on language and/or datatype in a traversal.

Luckily, I think there is alot of precedent on this (with XQuery's use
of datatypes and SPARQL's - the latter simply burrows from XQuery):

http://www.w3.org/TR/xpath-functions/#casting-from-primitive-to-primitive
 
> One last thing, we would need to allow these literals as "subjects" in
> traversals.  This does not mean much from the specification point of
> view, maybe just a mention, but implementors will need to understand
> this.

I agree, and I think this becomes neccessary once you allow datatyping
because of RDFs entailment rules
(http://www.w3.org/TR/rdf-mt/#simpleRules  - rdfs1) and RDF Literals.

Specifically:

uuu aaa lll (where lll is a plain literal (with or without a language tag)

implies

 _:nnn rdf:type rdfs:Literal .

where _:nnn identifies a blank node allocated to lll by rule rule lg.

So, the literals themselves are thought of as resources by the RDF
model (especially if they are typed)