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Thoughts on abstract syntax

From: Chris Welty <cawelty@frontiernet.net>
Date: Wed, 29 Nov 2006 08:33:45 -0500
Message-ID: <456D8C39.2080609@frontiernet.net>
To: "Public-Rif-Wg (E-mail)" <public-rif-wg@w3.org>

<chair hat off>
Trying to put some order to my thoughts on the subject:

We decided at a recent telecon to continue developing an abstract syntax 
side-by-side with the current BNF.  

I think it would be productive to agree as a group with Paul's 
suggestion that we will produce an abstract syntax in some form (asn06, 
OWL, RDFS, ...), a meta-model (in UML), a human readable syntax (in BNF) 
and an XML syntax.  We can agree do to this and then later we can decide 
which of them are normative.  The charter indicates the XML syntax 
shoudl be normative, but doesn't preclude us from making any others 
normative, too. Also, we may find we want the meta-model to be the 
abstract syntax or vice-versa.

I think most people agreed that these various forms are isomorphic, 
modulo a technical limitation here and there (to be explored).

As anyone who has ever done any kind of modeling knows, when you try to 
maintain more than one artifact that is supposed to describe *the same 
thing* you always end up with a maintenance problem keeping them in 
synch.  Thus it seems it would be productive to agree as a group that 
one of these will be the "primary" object of maintenance in our work 
(note that this is not the same as being normative) and the others 
either generated automatically through some tools (ideal) or by human 

Choosing the abstract syntax as the primary (though, again, perhaps 
non-normative) syntax appeals to me because it allows us to focus on 
certain aspects of RIF while postponing others.  For example, we need to 
consider in Phase 1 how to integrate RDF into RIF (to the extent it is 
compatible with the CORE semantics).  How will this be done?  RDF only 
allows binary predicates, and treats unary predication as a special kind 
of binary predicate, e.g. the atom P(A) in a rule language would be 
expressed as type(A,P) in RDF.  Do we define a mapping from RDF triples 
into RIF syntax, or does RIF carry RDF triples as RDF?  We also need to 
consider how to transmit RDF data as ground atoms in RIF.  Are we going 
to invent our own special RIF syntax for RDF triples, or are we going to 
invent a special RDF syntax for higher-arity predicates, or are some of 
our ground atoms going to be in RDF and the rest in our own syntax, or 
something else?  Are rules going to look different than ground atoms? 
etc etc.  We have not really begun to discuss the alternatives here.

By working at the level of an abstract syntax, we can postpone that 
decision and work on the current issues.  It seems to me that we would 
need to directly address these questions if we are to have hope of a BNF 
syntax, and that the current BNF syntax already biases us away from some 
of the options for handling RDF (not to mention OWL) which we haven't 
discussed (it is biased because radical changes to the syntax would be 
needed if we went with some of the approaches).  In an abstract syntax, 
we say something like

  Predicate: term
  Args: list of term

  Args: NonVariableTerm

etc., without committing to a particular decision on how RDF ground 
atoms are to be treated specifically.  We all know rules will have atoms 
in them and that the atoms may contain variables, etc., and there is 
considerable work that can be done without dealing with the specific 
problem of rdf-ground-atoms now.

Note of course that there are already a slew of other issues like the 
RDF one I use here that could have the same impact on a concrete (e.g. 
BNF) syntax, but far less impact on an abstract syntax.

</chair hat off>


Dr. Christopher A. Welty                    IBM Watson Research Center
+1.914.784.7055                             19 Skyline Dr.
cawelty@frontiernet.net                     Hawthorne, NY 10532
Received on Wednesday, 29 November 2006 13:34:04 UTC

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