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Re: Syntax error in algebra example

From: Seaborne, Andy <andy.seaborne@hp.com>
Date: Thu, 27 Sep 2007 17:00:32 +0100
Message-ID: <46FBD3A0.7020201@hp.com>
To: Richard Newman <rnewman@franz.com>
CC: public-rdf-dawg-comments@w3.org



Richard Newman wrote:
> DAWG,
> 
>    12.2.2 has a minor error.
> ----
> 
> Example: group consisting of a basic graph pattern, a filter and an  
> optional graph pattern:
> 
> { ?s :p1 ?v1} FILTER (?v1 < 3 ) OPTIONAL {?s :p2 ?v2} }
> Filter( ?v1 < 3 ,
>    LeftJoin( BGP(?s :p1 ?v1), BGP(?s :p2 ?v2), true) ,
>    )
> ----

Fixed, thank you.

> 
>    Count the number of {s and }s.
> 
>    While I'm on the topic of 12.2.x, the algebra mapping algorithm in  
> 12.2.1 is somewhat unclear, and contains at least one error:
> 
> ----
> for each algebra sub-expression SA:
>     If SA is an OPTIONAL,
>        If SA is of the form OPTIONAL(Filter(F, A))
>             G := LeftJoin(G, A, F)
>        else
>             G := LeftJoin(G, A, true)
>     Otherwise for expression SA, G := Join(G, SA)
> ----
> 
>    in the 'else' clause, 'A' is out of scope, so this doesn't make  
> any sense.

Yes - changed to  "G := LeftJoin(G, SA, true)"

> 
> 
>    In a more general sense: it would probably aid the clarity of the  
> document if the DAWG tried to write an AST -> Algebra converter using  
> only the document as a reference, rather than writing the document  
> with the experience of having written the code. I'm writing such a  
> thing for the first time, and the document is not as clear as I would  
> like.

I can say that the doc was written before I implemented.

> 
>    It would also be helpful in 12.2.2 to provide concrete (syntax,  
> AST, algebra) triples, rather than only (syntax, algebra) pairs. At  
> present I am having to basically guess the AST that the DAWG had in  
> mind when writing this section.

The grammar rules give the abstract syntax and do define a parse tree. 
Inventing another notation to capture that would be confusing as to its status 
with respect to the SPARQL concrete syntax.

Eric has provided the yacker tool

   http://www.w3.org/2005/01/yacker/uploads/SPARQL

that prints out parse information which uses (I think) yapp (which is an LALR 
Perl-based parser generator).


	Thanks
	Andy

> 
>    Regards,
> 
> -R
> 

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Received on Thursday, 27 September 2007 16:00:53 GMT

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