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RE: ISSUE-126 (Revisit Datatypes): A new proposal for the real <-> float <-> double conundrum

From: Boris Motik <boris.motik@comlab.ox.ac.uk>
Date: Fri, 4 Jul 2008 15:18:32 +0100
To: "'Bijan Parsia'" <bparsia@cs.man.ac.uk>
Cc: "'OWL Working Group WG'" <public-owl-wg@w3.org>
Message-ID: <004201c8dde0$d955c9b0$7212a8c0@wolf>

OK, so what is the case for keeping the interpretation finite?

I really believe this is difficult to implement correctly, 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.

I firmly believe that, if we stick to the continuous 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.

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.


The simple solution 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).

Regards,

	Boris

> -----Original Message-----
> From: Bijan Parsia [mailto:bparsia@cs.man.ac.uk]
> Sent: 04 July 2008 14:59
> To: Boris Motik
> Cc: 'OWL Working Group WG'
> Subject: Re: ISSUE-126 (Revisit Datatypes): A new proposal for the real <-> float <-> double
> conundrum
> 
> On 2 Jul 2008, at 20:09, Boris Motik wrote:
> 
> > Hello,
> >
> > In light of our discussion at today's teleconf, I would like to put
> > forward a new proposal for resolving the dilemma regarding the
> > real vs. float vs. double problem.
> >
> > 1. We make all of xsd:float, xsd:double, and owl:real disjoint.
> 
> Agreed.
> 
> > 2a. We make the value space of xsd:float as containing the
> > following things:
> >
> > - the special values NaN, +inf, and -inf
> 
> Agreed.
> 
> > - the continuous range of real numbers between the real number min
> > = -(2^24-1)*2^(-149) and the real number max = (2^24-1)*2^104,
> > inclusive.
> 
> We should separate this out. I think there's a case to be made for
> keeping them finite.
> 
> > 2b. We make the set of constants of xsd:float exactly the same as
> > in XML Schema. Thus, the lexical representations are INF, -INF,
> > NaN, and all strings of the form "manEex", where "man" is the
> > decimal mantissa and "ex" is an integer.
> 
> Sounds good.
> 
> > By the way, I think that XML Schema specification (Section 3.2.4)
> > is incomplete here: we should additionally say what happens in the
> > case of constants "manEex" for which man*10^ex is *NOT
> > REPRESENTABLE* in the form m*2^e matching the conditions of
> > xsd:float. The
> > problem is that, since we are dealing with a logical language, we
> > need to specify how rounding is handled.
> 
> Agreed.
> 
> > We have two options:
> > - we disqualify such constants as syntactically incorrect, or
> > - we say that such constants stand for the nearest rounded float.
> 
> I prefer the former.
> 
> > 3. We do the similar thing for xsd:double.
> 
> Pace the continuity.
> 
> > 4. We would disallow the "pattern" facet for all numeric datatypes.
> 
> Agreed.
> 
> > In this way, we've stayed true to the XML Schema specification in
> > the sense that all datatypes are disjoint, and we provide for the
> > special values. The only departure from XML Schema is that we
> > officially need not worry about the discreteness of xsd:float and
> > xsd:double during reasoning. I find this last point quite
> > important: what is the point in producing a spec that nobody will
> > implement in its entirety?
> 
> But this is true now, yes? as one can provide arbitrarily large, but
> finite datatypes working with ranges of integers. Indeed, I wouldn't
> be surprised if it weren't pretty easy to do with strings as well.
> 
> Given that that's the case, I think we should leave it to be correct
> with respect to the type and just warn people. All things being
> equal, I'd rather restrict them syntactically then to mess with their
> semantics. The only reason to use a float or a double type in a
> complex way is when you care about the discreteness. I'd rather defer
> all that aspect to people who, e.g., want to deal with logical float
> based equations (e.g., using intervals, or what have you).
> 
> It's not an happy situation we're in, so I'm inclined to be
> conservative with regard to the semantics even at the cost of
> practical implementation of everything wrt those types.
> 
> > Let me know how you feel about this.
> 
> I'm really hesitant to specify a variant semantics in the absence of
> more concrete understanding of the use space. In a sense, we're faced
> with some legacy stuff. I'd prefer not to fix it too much in advance
> of a lot more knowledge. If it were the only case where we had large
> finite ranges of concrete values, I would be more amenable. Since we
> can define such anyway, I don't see that this is *so* much worse.
> (It's a bit worse as it's built in, so requires less effort for
> people to hit.)
> 
> Cheers,
> Bijan.
Received on Friday, 4 July 2008 14:20:12 UTC

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