DAML and reification

From: Mike Dean (mdean@bbn.com)
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

Comments?  Can/should we include a (simplified) mechanism
for tagging statements in DAML?


[1] http://www.w3.org/TR/1999/REC-rdf-syntax-19990222

[2] http://www.w3.org/2000/01/rdf-schema

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