From: Bijan Parsia <bparsia@cs.man.ac.uk>

Date: Fri, 4 Jul 2008 15:53:23 +0100

Message-Id: <2FFE3324-ABEE-4D7C-BA69-D661E3EA3539@cs.man.ac.uk>

Cc: "'OWL Working Group WG'" <public-owl-wg@w3.org>

To: "Boris Motik" <boris.motik@comlab.ox.ac.uk>

Date: Fri, 4 Jul 2008 15:53:23 +0100

Message-Id: <2FFE3324-ABEE-4D7C-BA69-D661E3EA3539@cs.man.ac.uk>

Cc: "'OWL Working Group WG'" <public-owl-wg@w3.org>

To: "Boris Motik" <boris.motik@comlab.ox.ac.uk>

On 4 Jul 2008, at 15:18, Boris Motik wrote: > OK, so what is the case for keeping the interpretation finite? I believe I've already made the case. The case rests on two points: 1) Semantically, they are finite. 2) Implementations already have to deal with large finite datatypes via user defined ranges on integers. Thus, this makes nothing *in principle* worse. Now, having such a datatype built in raises an affordance toward dangerous practice. But I'd rather handle that with best practices and advice, rather than mucking with the semantics in absence of extensive experience. > I really believe this is difficult to implement correctly, I've no doubt of that whatsoever. I've conceded it. > and I repeat my example (slightly modified). > > (1) PropertyRange( a:prop > DatatypeRestriction( xsd:float > minExclusive "n1"^^xsd:float > maxExclusive "n2"^^xsd:float > ) > ) > (2) n1 is a constant that corresponds to the number 1 * 2^-149 > (3) n2 is a constant that corresponds to the number 3 * 2^-149 > > (4) ClassAssertion( MinCardinality( 2 a:prop rdfs:literal) a:i ) > > This ontology is unsatisfiable: the range of a:prop contains only > one object, but (4) requires existence of two different objects. > The difficulty in detecting this is that you need to count how many > numbers are there between n1 and n2. How are you going to do that? > The binary representation of floats is really cumbersome to deal with. Er...floats (and doubles) are inherently binary. I'm confused. The floats have (at least) three issues: 1) Exactness of operations (irrelevant since we don't have equations) 2) Finiteness of the set (fixed range and discrete) 3) Difficulties with the representation (which I always associated with 1) 2 always struck me as the big problem. I pointed out that we already have this problem with integers and user defined types. I'm confused by your new point. Please clarify whether it's 2 or 3 you are concerned with. If 2, please explain how it's different from other finite type ranges. > I firmly believe that, if we stick to the continuous Discrete, I believe you mean. > implementation of floats, NO reasoner on this earth will implement > this test > case correctly. I am open to others showing me wrong. > > Furthermore, I firmly believe that such inferences are irrelevant > for practice. We have a continuous floating point type already (decimal). We can always defined a fixed range (with < and > facets) and a length (with the digit limiting thing), so we can always recover a float like type from decimals. Given that we have a continuous floating point type already, the reason for picking float can rest on two bits: The weird extra constants (e.g., NaN) The discreteness. These two things are not separate, of course. The discreteness (combined with inexactness of representation) are a driver for the constants. > Finally, I don't see a point in producing a spec for which we know > nobody will implement correctly and that is irrelevant for > practice. I know neither of these things. I suspect it's likely. But I'd rather guide people away from using floats than to distort their meaning. As I said, I can imagine wanting to reason about floats as floats (e.g., for analyzing certain computational processes). I don't know if that would work out or not, frankly, but given that we have ways to work around floats (e.g., 'use decimal, people!') I don't see the point of mucking with the semantics. > The simple solution It's a simple solution, certainly. > is to make floats continuous and be done with it. Implementations > become trivial and users will never notice the > difference anyway (partly because they won't care and partly > because implementations will assume a continuous interpretation > anyway). I'm uncomfortable with this because it's based on some (plausible) assumptions that I don't know are true. I'd rather encourage people to use the more feasible types, or to use floats in non-dangerous ways. If I were designing things from the ground up, I wouldn't have included floats at all. Cheers, Bijan.Received on Friday, 4 July 2008 14:51:10 GMT

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