From: Mike Dean ([email protected])
Date: 03/05/01
There was some discussion about reification in the DAML Language breakout session at the DAML PI meeting (which I unfortunately missed), which apparently led many to conclude that reification may be too problematic for DAML (in particular, its formal semantics). I think a problem for RDF and DAML is that "quoting" (e.g. "Ralph says that Ora is the author of Ora's Home Page", with an implication that I may not choose to believe that Ora is the author or Ora's Home Page") has been used as the canonical example for reification. Instead, I think data tagging (e.g. "statement X was harvested from http://www..." or "statement X was made on 2001-02-27") should be considered the canonical example, with quoting at best secondary. Such tagging is incredibly useful, and (IMHO) one of the main benefits offered by RDF/DAML over XML and other representations. I've seen a number of military databases that add per-record or per-field timestamps and source information, significantly cluttering their data model. I've been using RDF API for such tagging of statements harvested by the DAML Crawler. The API is very convenient: a Statement is a Resource, and can be used as the subject of another Statement. The serialized form uses a URI containing a hash of the subject/predicate/object to identify the statement -- I'd like to have an optional mechanism to explicitly provide an ID for the statement. I looked back at the RDF Specification [1] and found that the specification wasn't quite as I remembered it. Section 5 shows a reified statement as being represented by 4 statements, involving the subject, predicate, object, and type properties. Section 4.1 states A new resource with the above four properties represents the original statement and can both be used as the object of other statements and have additional statements made about it. The resource with these four properties is not a replacement for the original statement, it is a model of the statement. A statement and its corresponding reified statement exist independently in an RDF graph and either may be present without the other. The RDF graph is said to contain the fact given in the statement if and only if the statement is present in the graph, irrespective of whether the corresponding reified statement is present. without specifying whether the original statement can also be used as the subject of other statements. However, [2] shows that a (reified) Statement is a subclass of Resource, so I believe the RDF API interpretation is reasonable. Comments? Can/should we include a (simplified) mechanism for tagging statements in DAML? Mike [1] http://www.w3.org/TR/1999/REC-rdf-syntax-19990222 [2] http://www.w3.org/2000/01/rdf-schema
This archive was generated by hypermail 2.1.4 : 04/02/02 EST