Fwd: Awkward TriG compatibility with N3

[re-sending, mostly for testing.   I'll send the official response 
shortly.  I originally sent this on the 17th.]

On this one, I propose replying along these lines:

The space of compatibility with n3, SPARQL, and existing TriG parsers is 
over-constrained. For now, N3 syntax compatibility was the one we 
prioritized lowest, since the WG had already rejected the N3 model of 
how to handle multiple graphs (since it was too different from SPARQL 
deployed usage).

Looking forward, we are encouraging everyone to move to SPARQL style 
TriG, without curly braces around default triples and with the GRAPH 
keyword. That style is compatible with N3, so perhaps in time a future 
language can include more N3 features and remove those old-style TriG 
constructs.

- Sandro

------------------------------------------------------------------------
*From:* Richard Smith <richard@ex-parrot.com>
*Sent:* Sun Nov 17 14:09:44 EST 2013
*To:* public-rdf-comments Comments <public-rdf-comments@w3.org>
*Subject:* Awkward TriG compatibility with N3

Consider the following TriG document:

{ ex:Jack a foaf:Person }
ex:foo { ex:Jill a foaf:Person }

Assuming the prefixes are bound, this is valid TriG
document.  The first line puts a statement that Jack is a
person into the default graph, and the second line puts a
similar statement about Jill into named graph ex:foo.

But it's very nearly a valid N3 document meaning something
quite different.  N3's formulae are conceptually much the
same as TriG's graphs, but N3's formulae can be subject or
object of statements.  So in N3, the following document
makes some sort of statement (with predicate ex:foo) about
the two formulae or graphs.

{ ex:Jack a foaf:Person } ex:foo { ex:Jill a foaf:Person } .

The only thing that stops that from being valid TriG is
terminating '.'.  To express this in TriG we would write:

_:a { ex:Jack a foaf:Person }
_:b { ex:Jill a foaf:Person }
_:a ex:foo _b

Now I can entirely understand TriG not wanting to support
this feature of N3.  But at the same time I would be pleased
if some future TriG-like language, maybe TriG 2.0, would
include them because they're very useful in technologies
like the W3C's PROV language where it's common to want to
write:

{ ex:Jack a foaf:Person ;
bio:birth [ dc:date "1905-01-11"^^xsd:date ] }
prov:wasDerivedFrom [ a foaf:Document,
dc:title "Jack's birth certificate" ] .

So it would be nice if TriG remained compatible with N3 by
allowing for the future possibility of such a feature.

Even though N3 is not a W3C recommendation, it (or a subset
of it) is in use in several current products, such as cwm.
Languages like N-Triples and Turtle are both compatible with
N3 insofar as no valid N-Triples or Turtle document that is
also a valid N3 document has different meanings in the
different languages.  (They're not quite subsets of N3, but
it would be easy enough to update N3 so that this were
true.)  An application wishing to support N-Triples, Turtle
and N3 may well do so with a single parser.

Strictly speaking it is also the case that TriG and N3
because the final '.' disambiguates it, but then makes the
grammar a long way from being LL(1) or LALR(1), as arbitrary
look-ahead is required.  Allowing a single parser to handle
both N3 and TriG easily is another reason why this syntax is
problematic.


If fixing this is considered desirable, requiring a '.'
after the closing '}' of the native (i.e. non-SPARQL) syntax
would do this.  (This would require trival ammendments to
grammar productions [2g] block and [3g] triplesOrGraph.)
But it does so at the cost of breaking compatibility with
the current (and earlier) drafts of TriG.  This has a
pleasing symmetry with the Turtle syntax for anonymous blank
nodes or collections:

( ex:a ex:b ) .
[ a ex:class ] .
{ ex:foo ex:pred ex:bar } .

which generate the following (slightly abbreviated) N-Quads:

<

    _:1 rdf:type rdf:List . <_:1 rdf:first ex:a . <_:1 rdf:rest _:2 .
    <_:2 rdf:first ex:b . <_:2 rdf:rest rdf:nil . <_:3 rdf:type ex:class . 

_:4  ex:foo  ex:pred    ex:bar   .


Richard



-- 
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