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Re: stumbled upon ACTION-200 CONSTRUCT WHERE shortcut...

From: Andy Seaborne <andy.seaborne@epimorphics.com>
Date: Mon, 13 Dec 2010 21:38:07 +0000
Message-ID: <4D06923F.1020806@epimorphics.com>
To: Axel Polleres <axel.polleres@deri.org>
CC: SPARQL Working Group <public-rdf-dawg@w3.org>


On 13/12/10 20:12, Axel Polleres wrote:
> Hi all,
>
> I found this old action lying around, haven't found any notes on it, so I thought I just draft something:
>
> If we want that still, my suggestion would be to
>
> 1) change
>
> [9]  ConstructQuery  ::=  'CONSTRUCT' ConstructTemplate DatasetClause* WhereClause SolutionModifier
>
> to
>
> [9]  ConstructQuery  ::=  ( 'CONSTRUCT' ConstructTemplate DatasetClause* WhereClause SolutionModifier ) |
>                            ( 'CONSTRUCT' DatasetClause* 'WHERE' ConstructTemplate  SolutionModifier )
>
> would that work with the grammar?

(I haven't time to check it programmatically but looking at it visually...)

Sort of.

"yes" in the sense that if the WHERE is mandatory, CONSTRUCT WHERE is a 
unique rule head so LL(1) works unambiguously.

LALR(1) will, I think, have conflicts and needs checking because the 
ConstructTemplate looks like a graph pattern but is a different rule.

"no" for some practical considerations:

1/ WHERE is optional in the WhereClause form so there is cause for 
confusion (minor - that's a value judgement we would have to make)

2/ ConstructTemplate isn't a full graph pattern and parsers may be 
treating it differently internally because the evaluation later is 
different.

It's hard be certain of the effect of (2) on existing systems. It may 
turn out to be a significant change to existing implementations in an 
area they weren't expecting to work on and the value is small.

It would be change in ARQ but I don't care and am quite prepared to 
rework the CONSTRUCT form. (I came across a similar effect with DELETE 
WHERE but that was all new code anyway.)

> 2) add a subsection
>
>
> <div3 id="CONSTRUCTWHERE">
> <head>CONSTRUCT WHERE</head>
> <p>In case the construct template is identical to the WHERE clause, i.e. the triples matched by the WHERE clause
> form the result graph, the CONSTRUCT Query form can be used in an abbreviated version, omitting the template.</p>

It is not identical.  The treatment of blank nodes is different.  We 
need to be precise.

> <p>For example, for the same data graph as before, the following query copies all foaf:name triples to the result graph.</p>
>
> <pre class=query
>    PREFIX foaf:<http://xmlns.com/foaf/0.1/>
>    PREFIX site:<http://example.org/stats#>
>
>    CONSTRUCT WHERE { ?person foaf:name ?name }
> </pre>
> </div3>

	Andy
Received on Monday, 13 December 2010 21:38:46 GMT

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