[Versa] More on literal queries

[Chimezie Ogbuji]

Okay, I have a suggestion regarding the need to query for a resource
by name and a literal by value.  This came from an exchange Uche and I
had on IRC.  If we leave the current Versa data model as is, we have
URI references (not 'resources' as we currently call them, which is
misleading)  - which may or may not refer to rdf resources  in the
underlying model - and literal values which only exist in the query
space and may not have any corresponding literal value in the
underlying model.

There is a need to query or match resources by URI and match literals
by value w/out having to resort to convoluted queries such as:

filter(all(),type(rdfs:Literal),eq("3.0"))
filter(all(),eq('<mailto:foo at yahoo.com>'))

We could define a function that takes a set of Versa data types
(resource or literal) and 'grounds' them to the underlying model.

For each data type leave it in the set if there is a corresponding RDF
resource in the graph (by identity operation).  Otherwise, remove it

So:

filter(all(),type(rdfs:Literal),eq("3.0"))

becomes

ground("3.0")    => empty set or set("3.0")  - depending on if there
is a "3.0" in the graph

filter(all(),eq('<mailto:foo at yahoo.com>'))

becomes

ground(<mailto:foo at yahoo.com>) => empty set or set(<mailto:foo at yahoo.com>)

I'm suggesting the term 'ground' because it is used to describe the
process of assigning unused URIs to blank nodes in a graph, which is
(somewhat) similar in some ways to what is being done here, resolving
a URI reference or literal in the query space to actual
resources/literals in the underlying graph.

We probably would also need to make sure our definition of eq is clear
about the semantics of comparing a resource in the underlying model
with a URI ref.  Do we mention that the model has the right to
determine how it calculates the identity operation, since this
apparently may involve more than just a string comparison of the
URI's?

Chimezie