[Adding Gregg Kellogg and David Longley, whom I neglected to copy
originally. (Sorry!)]
On 06/30/2013 10:25 AM, Pat Hayes wrote:
On Jun 27, 2013, at 10:19 PM, David Booth wrote:
[Copying public archive www-archive.w3.org for lack of a better
option]
PROBLEM SUMMARY
GOAL: Any two JSON-LD-compliant parsers should produce the exact
same RDF triples when parsing the same JSON-LD document, except for
blank node labels and (possibly) datatype conversions.
CURRENT PROBLEM: JSON-LD is intended to be a concrete RDF syntax,
but the JSON-LD data model has some extensions to the RDF data
model, and this causes some non-determinism and/or important
information loss when interpreting JSON-LD as RDF.
Wait. There are two issues getting muddled here. Yes, there can be
information loss in JSON-LD ==> RDF. No, it does not follow that the
mapping is nondeterministic or ambiguous. So information loss does
not compromise the GOAL as stated.
True. I thought it would be obvious that information loss is
undesirable (since otherwise we could just map to the empty graph), but
to clarify: the goal is to have a deterministic mapping *with* minimum
information loss.
David
Pat .
At present, the results of JSON-LD-compliant parsing of a JSON-LD
document to produce a set of RDF triples is non-deterministic
because JSON-LD allows blank node predicates and RDF does not.
That is a nonsequiteur. There is a perfectly deterministic algorithm
to map JSON-LD into RDF, with information loss. Option (a) below, for
example.
The JSON-LD specification currently suggests three potential
solutions but does not mandate one of them: (a) discard triples
that contain blank node predicates; (b) retain triples that contain
blank node predicates; or (c) skolemize blank nodes that are used
in the predicate position.
RANGE OF POTENTIAL SOLUTIONS
1. Change JSON-LD to prohibit JSON-LD blank nodes in positions
where the RDF interpretation of JSON-LD would cause them to be
mapped to illegal RDF blank nodes.
Pros: Easy enough spec change.
Cons: Loss of JSON-LD functionality? (Is there an important use
case for having blank nodes in predicate positions in JSON-LD?)
My comments: This seems to me like the best available option.
2. Change RDF to permit blank nodes as predicates.
Pros: Avoids information loss.
Cons: Not possible in the current RDF working group, because it is
specifically specified in the charter as being out of scope:
http://www.w3.org/2011/01/rdf-wg-charter "Some features are
explicitly out of scope for the Working Group . . . Removing
current restrictions in the RDF model (e.g., . . . blank nodes as
predicates"
My comments: To my mind, this would have been a second-best option
if it were available.
3. Change the JSON-LD-to-RDF-model mapping to specify that illegal
triples are discarded.
Pros: Easy change to the JSON-LD spec.
Cons: Significant information loss when interpreting JSON-LD as
RDF.
My comments: Not acceptable, due to the information loss.
4. Require skolemization of bnodes that appear in the predicate
positiont. (Note that if skolemization of a bnode is performed,
it must be performed uniformly on all instance of that bnode that
arise from that JSON-LD document.) RDF-standards-based
round-trippable skolemization would permit round-tripping of the
skolemized bnodes back to the original JSON-LD even if the return
trip is performed by a different party.
Pros: Avoids information loss.
Cons: (a) More complex than other options; (b) To avoid possible
URI clashes, the skolemizer would need a user-specific URI prefix
as a parameter, such as
http://example.com/.well-known/genid/alice/
My comments: Complex, but acceptable.
Are there other options or pros/cons that I did not list? Which
options would be preferable, acceptable or not acceptable to you?
I suggest adopting #1, but also adding a note to the JSON-LD spec
that recommends that parsers offer an *option* (disabled by
default) to retain triples with a blank node predicate.
Thanks, David
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