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Re: new model theory for DAML+OIL

From: Pat Hayes <phayes@ai.uwf.edu>
Date: Wed, 10 Oct 2001 22:42:18 -0500
Message-Id: <p05101050b7ea7eb7993b@[205.160.76.193]>
To: "Peter F. Patel-Schneider" <pfps@research.bell-labs.com>
Cc: joint-committee@daml.org, w3c-rdfcore-wg@w3.org
I am cross-posting this to both the RDFcore and DAML groups. - Pat

>  > See my recent message; could you change this to XLS: LL -> LV, where
>>  LL is the set of all *occurrences* of literal labels (or, maybe, of
>>  nodes which have literals as labels, given that we do not require
>>  tidiness on such nodes) ?  This would allow a node labelled with 05
>>  to be mapped into either 5 or "5", but not both. (I think this might
>>  also simplify the later treatment of DTs, since the extra conditions
>>  they introduce would indeed be semantic conditions in the usual
>>  sense, ie restrictions on the class of interpretations.)
>
>I think that the next step is for me to try to come up with some (larger)
>set of examples to see if that approach will work.

OK, after due consideration and *really* reading your model theory, 
let me propose a compromise along the above lines, which preserves 
the basic RDF/S MT virtually intact, keeps my 
datayping-as-basically-syntactic aesthetic happy, and also provides a 
smooth interface to your data-typing machinery. Also it seems less 
complicated, though it is a bit more trouble to set up.

First, the changes to the model theory. We remove the (unnecessary) 
condition that the XL mapping be 'global', and just refer to it as 'a 
mapping'. I will also put in some smoothing remarks that data typing 
is complicated and this is just a placeholder, and refer ahead to a 
treatment of datatyping. Finally, we will also emphasize that 
*exactly what counts as a literal label* in an RDF graph is left 
unspecified.  Everything else then goes as before.

Once that is done, data-typing is introduced as follows. (From now on 
this is all a notational/terminological variation on your version....)
First, we point out that literals, taken literally, are peculiar if 
we have datatyping, since then the very same Ntriples literal has to 
be able to mean different things in different triples (the '05' 
example). So we cannot assign a single value to a Ntriples literal; 
so (when there is datatyping), we are forced into re-thinking what 
counts as a literal for semantic purposes. So will understand the 
"literal label" to *include the graph node itself*. That is, for 
literals, in the presence of data typing, the 'literal node label' is 
actually a *pair* consisting of the node and the literal <n,l>  ( 
<n,LN(n)> in your notation. Alternatively, we can say that data 
typing applies to *occurrences* of literal labels on nodes, ie to 
labelled nodes rather than to the labels alone; that would be more in 
line with your treatment).

Then we introduce a dataype scheme (not a theory yet) as a set DT and 
mappings DTS and DTC, as you did:
DTS: DT -> (L ->LV)
{DTS(x)(y) : y in L} is a subset of DTC(x)
also I would add an exclusion clause:
if x =/= y then DTC(x) is disjoint from DTC(y)  (Is this kosher in 
XML? It seems reasonable, and makes things neater, but we could do 
without it.)

(Notice, nothing said yet about relationship to IR or a graph, so 
this is interpretation-independent so far.)

Now we introduce a new kind of type mapping, this time from *nodes* 
to DT (so this is rather different from an interpretation mapping of 
a vocabulary) and we say that a typed interpretation of an RDF graph 
is a pair of a type mapping D (on the graph nodes; only really needed 
on nodes labelled with literals, but what the hell) and an 
interpretation mapping I (on the vocabulary) which satisfies the 
following condition:

LV(<n,l>) = DTS(D(n))(l)

(This is the first and only time anything has been said about LV; 
until now it was just an unspecified mapping between two unspecified 
sets. Its also the only place that the mapping D is used.)

and if I is an rdfs-interpretation then also:

ICEXT(d) is a subset of DTC(d) for any d in (DT intersect IR)

and for any typename URIs nnn in the reserved vocabulary, then also:

I(nnn) is in DT.

I think that is all we need. Now, if you assert something that 
entails that a typed literal is in a class, then what you said would 
be false if that literal node's datatype doesn't contain that class. 
So for example, if you assert

foo baz lit .
baz rdfs:range ttt .

where ttt is a reserved datatype URI, then it follows from the rdfs 
semantic conditions that the range of IEXT(I(baz)) is a subset of 
ICEXT(I(ttt)); so from the second and third conditions  that it is a 
subset of DTC(I(ttt)). It follows therefore from the rdfs conditions 
that I(<n,lit>)=LV(<n,lit>) is in DTC(I(typename)), where n is the 
node identifying that occurrence of 'lit' in the graph, which by the 
first condition and the standard truth-conditions means that 
DTS(D(n))(l) is in DTC(I(typename)); and by the exclusion condition 
on DTC, this means that l is here being understood in the required 
way. (Actually maybe we don't need the exclusion conditions, since if 
two type classes overlap then as long as you hit the intersection you 
aren't really wrong, right?)

This assumes that the datatype has a name that can be used in the 
second triple, of course. That is the DT intersect IR case, and its 
the only one we need to include in the semantics. This keeps I and D 
'separate', but imposes a kind of semantic connection between them in 
the interaction between the three conditions, which force them to 
cooperate in order to make rdfs:range assertions true when applied to 
literals.

BTW, stating the conditions in terms of ICEXT in this way would also 
work smoothly if the coreWG ever were to decide to remove the 
prohibition on literals as subjects (or even as properties, for that 
matter.)

In the absence of any distinguishing marks, as it were, there could 
be a default(?) 'simple-string' datatyping with DTS(x)(y)=y for all x 
and y. (That would require LV to contain all the literals, but that 
seems reasonable under the circumstances.) In this case DT can be any 
nonempty set and need not intersect IR. This particular scheme has 
the singular feature that it can be composed with any other one 
without changing it, in the following sense: the literal value of any 
literal can be treated as a literal in the other datatype scheme, and 
you will get the same final value as you would have gotten by 
starting with the literal in the first place. (Obvious, but might be 
worth pointing out as it is kind of cute.) In fact this is really the 
only 'safe' datatyping that allows literals to be treated as simple 
labels, ie so that every occurrence of a literal has the same meaning.

I think this might enable us both to hold our heads up high. Whaddaya 
think? If you can go along with this, I promise to recant my last 
message to RDF-comment, howzat?

Pat
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Received on Wednesday, 10 October 2001 23:42:27 EDT

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