Re: [dotNetRDF-Develop] TriXWriter/Reader and Empty Graphs

[Rob V. <rv...@do...>]

Peter

Question 1

It is not the query processor but the ISparqlDataset that is used to wrap
the ITripleStore instance for SPARQL execution which adds the empty default
graph.  The SPARQL specification says queries and updates operate over a
dataset which always has at least an empty default graph and possibly some
named graphs so it makes logic easier elsewhere to always ensure an empty
default graph is present

Question 2

Yes though if it is the default graph then that could probably be omitted.

In general many RDF/SPARQL systems won't necessarily support empty graphs
explicitly (particularly quad oriented systems) so it would not be
unreasonable to omit empty graphs entirely.

Question 3

Yes it should, please submit a fix if you have time

Question 4

Not necessarily, going back to Question 2 not all systems support empty
graphs explicitly.  Longer term I intend for dotNetRDF to become one of them
so I would just ignore empty graphs in the TriX input, the DOM based parsing
mode used on non-portable platforms likely already does just this.

If everything is working on more platforms than we currently support it
would be great to have a pull request that expands our supported .Net
builds/portable profile.  Personally I don't have the time/energy to keep
track of all the multitude of profiles that MS keep introducing (which IMO
is the single worst thing they ever did to .Net) so if you have some time to
do so that'd be much appreciated.

Rob

From:  Peter Kahle <pk...@ka...>
Reply-To:  dotNetRDF Developer Discussion and Feature Request
<dot...@li...>
Date:  Wednesday, 10 December 2014 03:32
To:  "dot...@li..."
<dot...@li...>
Subject:  [dotNetRDF-Develop] TriXWriter/Reader and Empty Graphs

> Hi Folks,
> 
> In playing around testing against WinRT/WP 8.1 (CORE-429 ), I’ve found what I
> think is a bug somewhere in the portable version of the TriXWriter/Parser
> stack (or possibly in the SPARQL engine), but I don’t really know where to go
> poking to fix it.
> 
> I’m querying against a TripleStore with two named graphs. This works fine, but
> when calling “new LeviathanQueryProcessor(myTripleStore)”, a new default graph
> (with a null Uri) is added to the store.
> 
> Then, when I go to save the store using TriXWriter, it writes  “<graph />” as
> the third graph element. This seems to be correct behavior to me, or at least
> potentially correct.
> 
> Unfortunately, when I go to load the file, I see:
> 
> VDS.RDF.Parsing.RdfParseException was unhandled by user code
>   HResult=-2146233088
>   Message=[Source XML]
> 
> 
> Unexpected </TriX> encountered, either <triple> elements or a </graph> was
> expected
>   Source=dotNetRDF
>   EndLine=-1
>   EndPosition=-1
>   HasPositionInformation=false
>   StartLine=-1
>   StartPosition=-1
>   StackTrace:
>        at VDS.RDF.Parsing.TriXParser.TryParseGraph(XmlReader reader,
> IRdfHandler handler)
>        at VDS.RDF.Parsing.TriXParser.TryParseGraphset(XmlReader reader,
> IRdfHandler handler)
>        at VDS.RDF.Parsing.TriXParser.Load(IRdfHandler handler, TextReader
> input)
>        at VDS.RDF.Parsing.TriXParser.Load(ITripleStore store, TextReader
> input)
>        at TripleNote.Store.<LoadStore>d__14.MoveNext()
>     --- End of stack trace from previous location where exception was thrown
> ---
>        at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task
> task)
>        at 
> System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotific
> ation(Task task)
>        at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
>        at TripleNote.Store.<test>d__0.MoveNext()
>   InnerException:
> 
> I’m guessing this is where the problem lies, and that the correct behavior is
> to check if we’ve got a blank element in the TriXParser.TryParseGraph.
> 
> The questions are:
> 1. Should LeviathanQueryProcessor add a default graph to the TripleStore?
> 2. 
> 3. Should an empty graph be written by the TriXWriter?
> 4. 
> 5. Should an empty graph be accepted by the TriXParser? I think yes.
> 6. 
> 7. If so, should it result in an empty graph in the TripleStore? This would
> require a change to IRdfHandler to handle this case.
> 
> Incidentally, while I’m not testing it in any organized manner, all of this is
> working in WinRT and  WP8.1 WinRT apps (a universal app, actually), including
> reading, writing, querying, and asserting triples.
> 
> Peter
> Sent from Windows Mail
> 
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