Notation 3 support

[Gregory Williams]

On Jul 4, 2010, at 5:18 PM, Toby Inkster wrote:

> I woke up this morning with a clear idea of how to add support for
> property paths. Implemented it this evening in less than 20 lines of
> code. The attached patch should now be able to parse Notation 3 fully.

Nice work.

> Next steps are an N3 serialiser, and then to clean it all up and release
> it all as a separate distribution on CPAN. (I plan on calling it
> "RDF::TriN3".)

That's almost too clever for its own good :)

> In the mean time, here's a Turtle serialisation bugette I've discovered.
> Create an RDF graph along these lines:
> 
> 	_:foo <#a> <#b> .
> 	<#c> _:foo <#d> .
> 
> It will be serialised something like this:
> 
> 	[] <#a> <#b> .
> 	<#c> _:r1234567890 <#d> .
> 
> Putting aside the fact that the second triple shouldn't be serialised
> in Turtle at all, as it's not legal RDF, the serialiser has lost the
> connection between the two appearances of the blank node.
> 
> I imagine when it decides to use '[]' rather than '_:something' to
> serialise a blank node, it first checks whether the node is the object
> of any triples - it doesn't check whether it's the predicate.

Yes, this is exactly why it won't serialize properly.

> There's a slightly bigger option of what to do when a serialiser tries
> to serialise a triple that cannot legally be represented in the format
> its outputting. I'd suggest there should be a constructor option along
> the lines of:
> 
> 	$s = RDF::Trine::Serializer->new('turtle', illegals=>'die');
> 
> Where 'illegals' can be set to 'die', 'warn', 'skip' or 'force'. If
> 'die' the serializer just dies when serialising something illegal; if
> 'warn' it issues a warning to STDERR and then skips the triple; if
> 'skip' it silently skips the triple; if 'force' it attempts to
> serialise the triple even if it means that the output is not strictly
> valid.

OK. I'm open to discussing changes like this. Right now all my stuff generally throws an exception via L<Error>, but I know there's some resistance to that since Error has been deprecated. I still haven't found a suitable replacement for my (subjective) needs, but could probably add in some alternative behaviors like those you suggest. What does anyone else think about these?

.greg