- From: Graham Klyne <Graham.Klyne@MIMEsweeper.com>
- Date: Wed, 24 Oct 2001 17:23:31 +0100
- To: Pat Hayes <phayes@ai.uwf.edu>
- Cc: w3c-rdfcore-wg@w3.org
At 09:10 PM 10/23/01 -0500, Pat Hayes wrote: >> ex:shoe shoe:size "10" . [...] >>Now, if I have knowledge (ex-RDF) that >> _:a shoe:size "10" . >>and >> _:a shoe:size "10.0" . >>are equivalent statements, then I can infer from the truth of the above >>example that >> ex:shoe shoe:size "10.0" . >>is also true, without knowing that the literals used have a numeric data >>type. > >If you really do know that equivalence holds for *any* _:a, but that's a >universal quantification. How you gonna say that in RDF? Er, I'm not. I did say "ex-RDF". >But let me provide a tweak. Suppose that the equivalence between 10 and >10.0 is provided only in the datatype mapping itself. A datatype is >defined semantically by a mapping from lexical space to value space, so it >is quite possible for that mapping to make this kind of identification. In >the model theory we currently have, the inference you want (from ex:shoe >shoe:size "10" . to ex:shoe shoe:size "10.0" .) would be valid only if >something provided enough typing information to enable to reasoner to know >that both these literal occurrences were numbers, say. BUt even then, a >*reasoner* would have to somehow be able to make the inference by >accessing information about the datatype mapping, and the MT doesn't say >anything about HOW to do that. It just says the inference is >'datatype-entailed'. Yes, I think all this can work with knowledge of datatypes (which I think is also "ex-RDF" in that it's not fully describable "in" RDF (as currently understood). My niggle was that implementations would need full knowledge of datatyping to use this kind of information, which might be a large swallow for some implementations... >>You mentioned in another message the idea of a "default" datatype, in >>which literal strings denote themselves; maybe the above issue is >>resolved by requiring that this default datatype be present in every >>interpretation? Or: any RDF that is true in some datatyped >>interpretation is also true in a corresponding interpretation with >>default datatyping. > >Thats not true, but what IS true is that any datatyping can be 'added' to >the default in a certain sense. In the string-default (it only works for >strings) the datatyping of a character string is the character string, ie >the datatyping is the identity mapping. So eg > ex:shoe shoe:size "10" . >says that the size of the shoe is the character string '10'. Which isn't >actually right, presumably; ... I suppose not. I was trying to chase down the idea that it meant the value was something that could be represented by the string. I see now that default identity interpretation idea doesn't work. And all roads seem to lead back to some form of datatyping in the sense of a data type that sets limits on the possible things denoted by a literal. The reason that I've latched on to this shoe size example is that, for many purposes, the literal value is all one needs to know about this property. Crudely, I know that a show size of "11" fits me, and other shoe sizes do not. I can also recognize that "11.0" may be an alternative name for the size that fits me. I may also know that size "12" is too large, and size "10" is too small. But beyond this, I have no idea what the number actually means, and for me to reason about shoe sizes I don't need to know this. Thus, while the shoe size may nominally be a numeric literal, I don't need a full understanding of numeric values to make basic decisions about my shoe size. >but what is right is what we would get by reinterpreting I(shoe:size) to >mean 'the size of the shoe is something denoted by the string', (ie rather >than the string itself) which is really the bNode version of the literal >(Sergey's S3). So what is true is that if a graph has a satisfying >interpretation I under some datatyping T , then it has a closely related >satisfying interpetation I' under the default datatyping, such that if you >kind of semantically compose T with I' on these literal-anonymous nodes, >then you get the original I+T back again. So one can think of S3 as this >kind of 'interim' datatyping where you insist on interpreting literals as >themselves, and treat the places where they occur as saying that something >exists with that name. It provides a kind of general-purpose >put-the-typing-decision-off strategy. Yes. After writing the above, I think this is very close to what I'm trying to describe. A tangential question: in model-theoretic terms, is the datatyping system T to be considered as a part of an interpretation, or something that is separate from an interpretation so that interpretation and datatyping system are both needed to determine the truth of a graph? My current sense is that I+T should be regarded as an interpretation, so that all the language about being true in an interpretation and entailment still works. >I realize this is incoherent, but will try to write it up coherently >before Friday. That will be interesting. #g ------------------------------------------------------------ Graham Klyne MIMEsweeper Group Strategic Research <http://www.mimesweeper.com> <Graham.Klyne@MIMEsweeper.com> ------------------------------------------------------------
Received on Wednesday, 24 October 2001 13:07:43 UTC