W3C home > Mailing lists > Public > w3c-rdfcore-wg@w3.org > June 2002

datatyping unstaked

From: patrick hayes <phayes@ai.uwf.edu>
Date: Thu, 13 Jun 2002 22:04:49 -0500
Message-Id: <p05111a01b92ee4914211@[65.217.30.123]>
To: w3c-rdfcore-wg@w3.org

Ive been thinking about the datatyping stuff again, and would like to 
put forward (again) an idea I once had, for possible discussion at 
the F2F, should the topic come up. If people feel that its out of 
scope now, or whatever, then OK, I just wanted to get it on the table 
for discussion should we decide to discuss it.

I think this proposal comes closer than any other to satisfying the 
most people at the least cost, and also is most likely to conform to 
existing usage. Still, there is no free lunch, and it does require 
making some changes to the current RDF docs, particularly the MT, and 
maybe some of the test cases. Details below.

I will use the following graph to illustrate the idea:

<ex:Jenny> <ex:age> "10" .
_:f <dc:title> "10" .
_:f <rdf:type> <ex:movie> .

which is supposed to have four nodes and three triples, i.e. it is tidy.

The first problem is how to arrange that this could be interpreted so 
that Jenny's age was ten (not '10') and _:f's title was '10', even 
though there is only one literal node.

The second problem is that we want it to be possible to add 
datatyping information to a graph like this without changing any 
meanings. On the face of it, that seems impossible, since we have to 
say that the "10" in the undatatyped graph denotes *something*, and 
in the absence of any datatyping information, and given that it's in 
several triples, it seems like the only sensible thing to say is that 
it denotes itself; and once we have done that, we are stuck.

The trick is to admit that indeed the literal alone does denote 
itself (just as now) but to provide some wriggle room in the meaning 
of a triple containing a literal object.

Right now, all triple meanings follow the same very simple rule: 
I(SPO)=true iff IEXT(I(P)) contains <I(S), I(O)> . But we could say 
that this is the meaning of triples with bnodes or urirefs as 
objects, but give a slightly different meaning to urirefs with 
literal objects: what *they*  mean is effectively what the rdfd:lex 
idiom means at present, ie

I(SPL)=true iff IEXT(I(P)) contains <I(S), D(I(O)) > for some 
literal-to-value mapping D.

Everything else works as before; graphs are tidy on literal nodes.

What follows? First, note that this allows the literal to 'indicate' 
a different value than it denotes (ie something other than itself), 
and a different one of those in each triple. So although it is indeed 
true that "10" denotes "10" in this graph, nevertheless the meanings 
of the the two triples containing that literal can refer to different 
things than "10". This means in turn that the entailment rules need 
to be modified slightly: it isn't valid to existentially generalize 
on literals. For example, the above graph does NOT entail

<ex:Jenny> <ex:age> _:x .
_:f <dc:title> _:x .
_:f <rdf:type> <ex:movie> .

It does, however, entail

<ex:Jenny> <ex:age> _:x .
_:f <dc:title> _:y .
_:f <rdf:type> <ex:movie> .

via the graph below.

This is the only significant change to RDF entailment which arises in 
this version, and it does require re-stating some of the lemmas in 
the MT. (This change which is needed to the 'pre-datatype' RDF is the 
chief cost of this proposal.)

Any literal triple is now equivalent to its rdfd:lex pair version, eg 
the graph both entails and is entailed by:

<ex:Jenny> <ex:age> _:x .
_:x <rdfd:lex> "10" .
_:f <dc:title> _:y .
_:y rdfd:lex "10" .
_:f <rdf:type> <ex:movie> .

(6 nodes, 5 triples) which is the best we can do by way of 
existential generalization. If the object of a triple is a uriref 
then we can simply generalize the node:

aaa bbb ccc .
ddd eee ccc .
--->
aaa bbb _:xxx .
ddd eee _:xxx .

but if it is a literal then we have to generalize one triple at a time:

aaa bbb LLL .
ddd eee LLL .
--->
aaa bbb _:xxx .
ddd eee LLL .
--->
aaa bbb _:xxx .
ddd eee _:yyy .

What this is all about, of course, is tidyness; we can't assume that 
a tidy literal indicates the same thing in each triple.

Now, given this change to the basic RDF MT, it's easy to make 
datatyping work smoothly. We just say that the datatype 'associated' 
with a triple (read on) provides the D mapping, ie that now instead 
of just being *some* D mapping, it has to be the L2V(I(ddd)) thingie 
of the associated datatype of that triple. Attaching a datatype to a 
property range says that any triple with that property has to have 
that datatype associated with it. A triple containing a datatype 
property is always associated with that datatype, obviously, and the 
rdfd:lex idiom is handled by the special rule that says that an 
rdfd:lex triple inherits its associated datatype from the datatype 
associated with any triple whose object is its subject. Then all the 
current idioms work out as they do now, except that the in-line idiom 
is datatype-sensitive, as Patrick wants.

Adding all this datatyping does not change either the denotation of 
the literal itself or contradict the meanings of the triples in the 
undatatyped graph.

This also means that if G' is a generalization from G  using rdfd:lex 
to keep the connections between the bnodes and the literal, then 
datatyping G and G' has the same effect. For example:

aaa <ex:age> "10" .
bbb <ex:age> "10" .
--->>>
aaa <ex:age> _:x .
bbb <ex:age> -:y .
_:x <rdfd:lex> "10" .
_:y <rdfd:lex> "10" .

are equivalent; and if you add (Ive changed the name again, just for fun)

<ex:age> <rdfd:rangedatatype> <xsd:integer> .

to either graph, they are still equivalent: they both then say that 
the ages of aaa and bbb are ten. The second one has nodes that denote 
ten, the first one doesn't, but they express the same information. 
(Of course, if you had missed out the rdfd:lex links, you would be 
screwed.)

Possible objections.

The chief objection to this idea, seems to me, is that it makes a 
simple literal triple with no datatyping into a very weak assertion. 
In effect, the above graph without datatyping says almost nothing, 
since the D mappings could be anything. In fact, the arbitrariness of 
the rdfd:lex meaning illustrates that weakness very precisely. 
However, there are several responses to this.
First, its hard to see what else we could say about the meaning of a 
'bare' literal *if* we want to be able to then go on and later add 
datatyping so as to restrict its meaning in the various triples, 
since that datatyping can indeed make it mean virtually anything. 
Life is like that, tough.
Second, we could introduce a special property called something like 
rdfd:rigidliteral, which forces a literal to be interpreted 
literally, as it were. This acts like a datatype property, but what 
it says is that the literal really does denote itself: its a kind of 
pre-emptive datatype-exclusion device which produces a datatype clash 
with any datatype. The semantics is that it forces D to be the 
identity map in its object, and it denotes equality. Then we could 
get the current meaning by writing things like

<ex:Jenny> <ex:age> _:x .
_:x <rdfd:rigidliteral> "10" .

This is awkward, admittedly, but thats because we can't do the 
obvious thing and make literals into subjects. If we could then 
rdfd:rigidliteral  could be a class and we could write things like

<ex:Jenny> <ex:age> "10" .
"10" <rdf:type> <rdfd:rigidliteral> .

But we can't. Sigh.

Note, this doesn't screw up the idea of checking literal identity by 
string-matching.  Literals still denote themselves, and you could 
substitute one for another if you knew they were really the same 
literal. The only thing you have to be slightly careful of is when 
you are doing existential generalization; and even then, all you have 
to do is make sure you are using a new bnode each time you do it, by 
'detaching' the new bnode from the old literal node one triple at a 
time, rather than just over-writing the literal with the bnode. Once 
you have done this, the bnodes you have introduced are perfectly 
normal in all respects, and any rdfs:range assertions on the 
property, etc., will apply to them just as before.  For example:

<ex:age> <rdfs:range> <xsd:integer> .
<ex:Jenny> <ex:age> "10" .
_:f <dc:title> "10" .

does not say that the literal "10" denotes ten; it says that it *maps 
to* something which is an integer, is all. It entails:

<ex:age> <rdfs:range> <xsd:integer> .
<ex:Jenny> <ex:age> _:x .
_:x <rdf:type> <xsd:integer> .
_:x <rdfd:lex> "10" .

and _:x  denotes ten by the usual rules.

Also, of course, any graph entails itself, and all the usual stuff 
like that still works OK.

This all works like a dream with 'remote' datatyping on property 
ranges; eg, the following graph

<ex:Jenny> <ex:age> "10" .
_:f <dc:title> "10" .
_:f <rdf:type> <ex:movie> .
<ex:age> <rdfd:rangedatatype> <xsd:integer> .
<dc:title> <rdfd:rangedatatype> <xsd:string> .

is consistent and has no datatype clashes in it, and says that 
Jenny's age is ten and that some movie is titled "10".

However, notice that

<ex:Jenny> <ex:age> "10" .
_:f <dc:title> "10" .
_:f <xsd:string> "10" .
<ex:age> <rdfd:rangedatatype> <xsd:integer> .

(5 nodes, tidy) says that _:f is the string '10', which isn't right 
at all. In other words, datatype properties have to be handled with 
care. This would be OK:

<ex:Jenny> <ex:age> "10" .
_:f <dc:title> _:sss .
_:sss <xsd:string> "10" .
<ex:age> <rdfd:rangedatatype> <xsd:integer> .

Finally, the following, while weird:

<ex:Jenny> <ex:age> "10" .
<ex:Joe> <ex:age> _:x .
_:x <xsd:string> "10" .
<ex:age> <rdfd:rangedatatype> <xsd:integer> .

(6 nodes)  is in fact consistent and clash-free; but just barely, and 
only if <ex:age> can have both numbers and strings in its rdfs:range.

One way to rule things like this out, if someone wanted to do that, would be:

<rdfs:range> <rdfs:subPropertyOf> < rdfd:rangedatatype>  .

Pat



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Received on Thursday, 13 June 2002 23:04:50 EDT

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