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. 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 > > ------------------------------------------------------------ IHMC (850)434 8903 or (650)494 3973 40 South Alcaniz St. (850)202 4416 office Pensacola (850)202 4440 fax FL 32502 (850)291 0667 mobile phayesAT-SIGNihmc.us http://www.ihmc.us/users/phayes
