- From: David Booth <david@dbooth.org>
- Date: Sun, 30 Jun 2013 16:45:19 -0400
- To: Pat Hayes <phayes@ihmc.us>
- CC: Markus Lanthaler <markus.lanthaler@gmx.net>, www-archive <www-archive@w3.org>, Andy Seaborne <andy@apache.org>, Manu Sporny <msporny@digitalbazaar.com>, "Hawke, Sandro" <sandro@w3.org>, "Wood, David" <david@3roundstones.com>, David Longley <dlongley@digitalbazaar.com>, Gregg Kellogg <gregg@greggkellogg.com>
[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 >> >> > > ------------------------------------------------------------ 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 > > > > > > > >
Received on Sunday, 30 June 2013 20:45:47 UTC