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Re: adding inline graphs to TriG

From: Gregg Kellogg <gregg@greggkellogg.net>
Date: Wed, 17 Jul 2013 11:36:07 -0700
Cc: W3C RDF WG <public-rdf-wg@w3.org>
Message-Id: <4217A504-C6C4-46F7-A5BB-7646AB4AB61D@greggkellogg.net>
To: Sandro Hawke <sandro@w3.org>
On Jul 16, 2013, at 5:53 PM, Sandro Hawke <sandro@w3.org> wrote:

> I know this is going to seem to some people like it's going too far, but it seems to me like such a good idea, in good conscience I have to at least seriously propose it.    If there aren't at least three +1's for this email, I wont mention it again.
> 
> Right now, in TriG, you can say things like:
> [ a :Patch;
>   :deletes _:g1;
>   :inserts _:g2 ].
> GRAPH _:g1 { ... }
> GRAPH _:g2 { ... }
> 
> I think it would make a whole lot of sense to allow a little syntactic sugar.  I'd like to allow this:
> [ a :Patch;
>    :delete { ... };
>    :inserts { ... } ].
> 
> The mechanism for this would be exactly like the mechanism for square-bracket [ ... ] expressions.  Just like [ ... ] is syntactic sugar for a blank node that is "used" once, inline graphs would be syntactic sugar for a named graph that is "used" once.    Any TriG document with inline graphs could be re-written to not have inline graphs by just replacing the inline graph with a new blank node label, then adding to the end of the document a name-graph pair of that label and that graph.     Nested inline graphs work fine by this rule, with no special handling.
> 
> For example:
> :alice :said { :bob :said { :charlie said { :spot a :Dog } } } }
> is syntactic sugar for:
> :alice :said _:u1.
> GRAPH _:u1 { :bob :said _:u2 }
> GRAPH _:u2 { :charlie said _:u3 }
> GRAPH _:u3 { :spot a :Dog }
> I'm unsure about whether to allow this in the subject position or just the object position.  It's probably hard to parse in the subject position; I'm not sure it can be done with an LL(1) grammar.   (Consider that "{s p o} s p o" is currently valid TriG, distinguishing that from "{s p o} p o" is tricky.)   I don't think it's hard in just the object position, and most of my use cases are fine with the object position.  If people like this idea in general, I'll investigate the grammar more.

I couldn't get a bare graph to work in the subject position, but it works well in the object position. The modified EBNF I used was the following:

[1g]   trigDoc               ::= (statement)* 
[2g]   statement             ::= directive | tripleOrBareGraph | graphword | wrappedDefault
[3g]   tripleOrBareGraph     ::= graphName (wrappedDefault | PropertyListNotEmpty '.')
                               | graphName1 (wrappedDefault | PropertyList '.')
                               | collection PropertyListNotEmpty '.'
[4g]   graphword             ::= GRAPH graphName wrappedDefault
[5g]   wrappedDefault        ::= '{' TriplesTemplate? '}'
[6g]   graphName             ::= iri | BlankNode
[7g]   graphName1            ::= BlankNodePropertyList

Plus, a modification to the "object" production:

[12]   object                ::= iri | BlankNode | collection | BlankNodePropertyList | literal | wrappedDefault

This should allow statements similar to what you suggested.

Minor changes to the parser logic are to remember the current graph name when beginning to process object, and assign a new blank node as the graph name, if the next production is a wrappedDefault, restoring it afterwards. The wrappedDefault returns the current graph name as the resulting resource, which allows it to be used like anything else in the object production.

Trying to allow wrappedDefault in the subject position creates conflicts.

Gregg

> People familiar with N3 will note this makes TriG even more like N3.  I'd say it brings TriG from being 85% of N3 to maybe 95% of N3.  This is probably a good thing.   (I'm measuring in terms of feature usage/importance, and excluding features that are really just RDF vocabulary.)  
> 
> So, anyone else think it's worth fully spec'ing this and adding it as a TriG "Feature At Risk" for last call?   
> 
> If not, I can just put it on my wishlist for the next turtle-like dataset syntax, someday.
> 
>       -- Sandro
Received on Wednesday, 17 July 2013 18:36:41 UTC

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