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Review section 4.1 of OWL AS&S

From: Graham Klyne <gk@ninebynine.org>
Date: Fri, 09 May 2003 18:44:33 +0100
Message-Id: <5.1.0.14.2.20030509175340.063636e8@127.0.0.1>
To: public-webont-comments@w3.org
These are some personal comments that arise out of a review I was asked to 
do for the RDFcore working group.   I must emphasize that they are offered 
as personal comments, as they do not cover any official RDFcore WG concerns.

-----

I looked briefly at the abstract syntax section [1] (it actually looks like 
a quite concrete serialization to me), which does appear to be very well 
presented, then at section 4.1 [2] of the OWL abstract syntax and semantics 
document, with a particular view to seeing if I could understand how to do 
the mapping.

I've spent about half a day looking at the "abstract syntax" (it actually 
looks like a quite concrete serialization to me), and have started to 
sketch a Haskell [3] implementation of the mapping [see attachment].  I 
come to the following conclusions:

1.  I do believe that I could implement a mapping to RDF based on this 
description, but with some guesswork.

2.  I think the presentation of the mapping leaves something to be 
desired.  I could not have started to grasp what was going on without 
actually trying to think about the details of an implementation.

There was some discussion about the purpose of these transformation 
rules.  From the description given, I think it is to translate OWL AS to an 
RDF graph, but others have
also suggested that they might be used for defining inverse transformation.
I don't think the mapping table is well-suited to that purpose.

More detailed comments are below.

[1]  http://www.w3.org/TR/2003/WD-owl-semantics-20030331/syntax.html

[2]  http://www.w3.org/TR/2003/WD-owl-semantics-20030331/mapping.html#4.1

[3]  http://www.haskell.org/

-----

Concerning section 4: 
http://www.w3.org/TR/2003/WD-owl-semantics-20030331/mapping.html

[[
This section defines a many-to-many relationship between
abstract syntax ontologies and RDF graphs. This is done using a
set of nondeterministic mapping rules.
]]

"Non-deterministic"?  Scary, if the goal is well-defined
semantics.

[[
The mapping is designed so that any of the RDF/XML graphs that
correspond to a particular abstarct ontology have the same
meaning, as do any of the abstract ontologies that correspond
to a particular RDF/XML graph.
]]

Good.  How confident can we be that this is so?

Why complicate the transformation with options that are not needed?
I'd suggest simply deleting the optional elements of the
transformation rules, and picking just one of the possible
alternative transformations.  If there's anything else useful
to be said, indicate it separately as additional information,
with appropriate commentary.

----

Concerning: 
http://www.w3.org/TR/2003/WD-owl-semantics-20030331/mapping.html#4.1

Table row 3:

[[
Annotation(ontologyPropertyID URIreference)
]]

Doesn't obviously match an abstract syntax production.
I think the URIreference should be ontologyId.

(I recognize these resolve to the same thing, but if one is going
to go to the trouble of defining a heap of equivalent symbols, they
might at least be used consistently.)

----

In reading the transformation rules, it is not immediately obvious from
the table as presented what parts of the abstract syntax expression
are variables that are carried into the transformed expression, but when
getting down to the level of trying to code an implementation it seems
clear enough what goes where.  Some more explicit convention for naming
"variables" in the transformation rules might help.

----

I *think*, though it's not exactly clear, that the third column
indicates what part of the transformed expression to use for the
node when T(expr) appears as a node in a triple for a transformed
piece of syntax.

I think this piece of text:
[[
The left column of the table gives a piece of syntax (S), the center column
gives its transformation (T(S)), and the right column gives an identifier
for the main node of the transformation (M(T(S))), but only for syntactic
constructs that can occur as pieces of directives.
]]
could usefully be expanded.  I think something like this is intended:

The table has three columns:

1.
   gives a piece of abstract syntax corresponding to an abstract syntax
   production rule (with further nonterminal symbols in an italic font)

2.
   gives a transformation of the abstract syntax into RDF triples, where
   the triples are presented as N-triples (with qnames for URIs).  Within
   these, further transformations, expressed as T(...), may be used to
   represent RDF triples, and also to represent individual nodes within
   such triples.

3.
   gives a node identifier to be used when a transformation of the
   corresponding abstract syntax is used as a node within a triple
   in some other (middle column) transformation expression.

The resulting RDF graph contains all of the triples generated by
the transformation of a given piece of abstract syntax.

(This may not be the best possible description, but I think something at
this level of detail would make the intent very much clearer.)

----

AFAICT, the mapping table is presented in the same order as the abstract
syntax production rules, with a 1:1 correspondence.  This is an important
clue, which I think would be better if made explicit by (a) labelling the
abstract syntax productions, and (b) using those labels to identify the
corresponding mapping rules.

----

I think the handling of the ontology node in the first two rows is not
entirely consistent with the treatment later given to other constructs
that result in the generation of new blank nodes.

(In my implementation sketch, I pass the explicit-or-generated
Ontology node as a parameter to the subsequent transformations.)

-----

Concerning: 
http://www.w3.org/TR/2003/WD-owl-semantics-20030331/mapping.html#4.2

This gives some rules to determine whether some RDF graph is an Owl-lite or 
Owl-DL ontology.  Presumably anything that doesn't satisfy either of these 
criteria is Owl-full.  But the rules are stated in terms of an AS->Graph 
transformation, so I'm not sure they constitute an effective procedure for 
making such a determination.

There's a point that isn't entirely clear to me:
   "the ontologies in O taken together provide a type for every individual ID"
What about mixtures of arbitrary RDF and OWL ontology statements?
Does this mean that every individual must have an explicit type statement?
Exactly what counts as an individual for the purposes of this assertion?

It has been my assumption that the various flavours of OWL can be used as a 
"logical layer" to perform some reasoning about things described by 
common-or-garden RDF.  I've since been told this may not be so.  If it is 
true that Owl DL and Owl-lite are not suitable for doing any reasoning 
about things described by ordinary "in the wild" RDF, I think this needs to 
be clearly stated.

-----

For interest, the attachment to this message is the start of a sketch for 
implementing the (section 4.1) transformation rules in Haskell.  The first 
part of the code is an encoding of the abstract syntax using Haskell data 
types.  The second part are some clauses of a Haskell function "transform" 
that implements the transformation T described by the mapping table, 
corresponding to the first 6 rows of the table.

The code is far from complete, and hasn't been near a Haskell 
compiler.  But I now think I see enough to complete the implementation.

#g


-------------------
Graham Klyne
<GK@NineByNine.org>
PGP: 0FAA 69FF C083 000B A2E9  A131 01B9 1C7A DBCA CB5E


Received on Friday, 9 May 2003 13:53:14 GMT

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