[Versa] Issue 100A proposal, extension functions

[Chimezie Ogbuji]

On 8/29/05, Michael Olson <Mike.Olson at fourthought.com> wrote:
> 
> 
> I would like to propose adding a section 7.7 to the specification.
> This section would discuss function names.  We would also change the
> "identifier" production in the EBNF for a function-call to match a xml
> qname production.  function names without a prefix would be assumed to
> be in the "versa core function" namespace.  All other namespace must
> have been registered with the implementation yada yada yada.

I plan on finishing up this section with the following requirements
for resolving function references:

The default namespace for function name resolution is:
http://rdfinference.org/versa/0/2/

(should this change since we no longer own that domain?)

It is automatically bound to the 'versa' prefix.  The core functions
are defined in this namespace.  If a function is referenced with a
namespace outside of the query processor is required to do the
following:

First check if the namespace is bound in the current context (defined
in a prefix declaration).  If it isn't, raise an error complaining
about the unbound namespace prefix reference.  If it is, check if any
function with the matching namespace and name are registered with the
query processor.  If so, invoke the function with the appropriate
arguments.  If there is no matching function, convert each argument to
a resource (following the conversion rules - a little trickier if the
argument is a literal) and return the objects of all statements where
the subject is any of the arguments (converted to resources) and the
predicate is identified by the URI of the function name.  I.e.,:


ns:functionName(res)  :=   resource - ns:functionName -> *

(where ns:functionName is not registered with the implementation)

If there are no arguments, raise an exception stating that the
referenced function is not registered with the implementation.  I
think this is more useful than simply complaining if the function
isn't registered, and allows the use of functions as First Order
Predicate mappings.  These can be overridden by registered extension
functions.  In this way, you can allow certain  'high-level' semantics
to be provided by extension functions that override certain predicates
(in the same way that CWM uses predicates in the log namespace --
http://www.w3.org/2000/10/swap/log.n3 -- to perform high level RDF
management).

Beyond this, it is completely up to the implementation to determine
how extension functions are registered.

Chimezie