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Re: test for DATATYPE on plain literals, please

From: Seaborne, Andy <andy.seaborne@hp.com>
Date: Mon, 14 Nov 2005 14:08:34 +0000
Message-ID: <43789A62.9040808@hp.com>
To: Dan Connolly <connolly@w3.org>
CC: RDF Data Access Working Group <public-rdf-dawg@w3.org>



Dan Connolly wrote:
> "It returns <xsd:string> if the argument is an untyped literal."
>  -- http://www.w3.org/2001/sw/DataAccess/rq23/#func-datatype
> 
> 
> SPARQLer doesn't seem to agree; I tried this query:
> 
> SELECT ?book ?title WHERE { ?book dc:title ?title FILTER
> ( DATATYPE(?title)) }
> 
> http://sparql.org/books?query=PREFIX+books%3A+++%3Chttp%3A%2F%
> 2Fexample.org%2Fbook%2F%3E%0D%0APREFIX+dc%3A++++++%3Chttp%3A%2F%
> 2Fpurl.org%2Fdc%2Felements%2F1.1%2F%3E%0D%0ASELECT+%3Fbook+%3Ftitle
> +WHERE+%7B+%3Fbook+dc%3Atitle+%3Ftitle+FILTER+%28+DATATYPE%28%3Ftitle%
> 29%29+%7D%0D%0A&stylesheet=xml-to-html.xsl
> 
> and got no results.

ARQ 1.0 is broken in this area - I've rewritten the whole area.  But your 
query still returns nothing :-(

> 
> I suspect SPARQLer is not quite caught up there. To make sure
> developers learn about this detail, let's make sure there's a test,
> please.
> 
> Steve, would you please add a test or get somebody to do it?
> Or if one is already there, point me to it?
> 
> I looked, and the closest I see is something with doubles.
> http://www.w3.org/2001/sw/DataAccess/tests/#datatype-1
> 

There are several things going on here as well:

1/ Whether DATATYPE(?title) should xsd:string

2/ The EBV rules

3/ Whether EBV applies at all.

- - - - - -

1/ Whether DATATYPE(?title) => xsd:string

It's a D-entailment:
http://www.w3.org/TR/rdf-mt/#DtypeRules

xsd 1a  uuu aaa "sss".  		=> uuu aaa "sss"^^xsd:string .
xsd 1b 	uuu aaa "sss"^^xsd:string . 	=> uuu aaa "sss".

that equates plain literals and xsd:strings.  It seems reasonable to treat 
this like any other XSD vocabulary interpretation but I see datatype() as 
acting on the syntax to extract the datatytpe of the literal, not acting on 
entailments in the graph.  Concretely, in a graph with both untyped literals 
and literals with xsd:strings, how can the application sort them out for, say, 
consistency checking?  Raising an error on an a plain literal seems more 
consistent than returning xsd:string

- - - - - -

2/ EBV rules

As Steve pointed out, the boolean effective value rules are "default true".

I have implemented Boolean Effective Value to test for the enumerated cases in 
rq23 but on unknown types and things that aren't literals at all it raises an 
error.  As written, the rules say somesort of coercion is always possible - 
what if there is an error raised?

ARQ (in CVS) returns xsd:string on plain literals so still rejects all 
solutions. Maybe it should return false (xsd:string is known not to be 
xsd:boolean value true) - ARQ behind SPARQL is the 1.0 release and is buggy in 
this area.

I would prefer the BEV rules to say something like:

-----------------------------

11.2.2 Effective Boolean Value

When an operand is coerced to xsd:boolean through invoking a function that 
takes a xsd:boolean argument, the following rules apply:

If the operand is not a literal, a type error is generated.

If the literal is known to be the value true the result is true.

If the literal is of unknown result is false.

If the literal is an XSD datatype then the result is TRUE unless any of the 
following are true:

     * The operand is unbound.
     * The operand is an xsd:boolean with a FALSE value.
     * The operand is a 0-length untyped RDF literal or xsd:string.
     * The operand is any numeric type with a value of 0.
     * The operand is an xsd:double or xsd:float with a value of NaN
-----------------------------

- - - - - -

3/ Whether EBV applies at all?

rq23 says "When an operand is coerced to xsd:boolean through invoking a 
function"  FILTER is not a function.  The definition of Value Constraint 
applies only on boolena valued expressions so somewhere between the two, we 
need to fix the text.

	Andy
Received on Monday, 14 November 2005 14:10:45 GMT

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