- From: Danny Ayers <danny.ayers@gmail.com>
- Date: Fri, 29 Sep 2006 00:38:22 +0200
- To: "Tim Berners-Lee" <timbl@w3.org>
- Cc: "Elisa F. Kendall" <ekendall@sandsoft.com>, "Robert Dodier" <robert.dodier@gmail.com>, semantic-web@w3.org, "Evan Wallace" <ewallace@cme.nist.gov>
On 28/09/06, Tim Berners-Lee <timbl@w3.org> wrote: > While > box1 exterms:mass [ex:value 2.4; exterms:units > exunits:kilograms] . > > might seem like a straightforward way, it turns out it is a trap! > > Presumably this one could write too > > box1 exterms:mass [ex:value 2.4; exterms:units > exunits:kilograms] ; > exterms:mass [ex:value 2400.0; exterms:units exunits:grams] . > > You would expect mass to be a functional property, to the two Bnodes > must > be owl:sameAs each other, so we can conclude that the masses are the > same: > > box1 exterms:mass [ > ex:value 2.4; > exterms:units exunits:kilograms; > ex:value 2400.0; > exterms:units exunits:grams] > > bzzzt! tilt! that means we have a mass with two values and two units. Nice catch. exterms:mass in the original example would maybe have been better named exterms:massMeasure, to avoid the (natural) expectation of it being a functional property. > A much *better way* is to use the unit as a connecting predicate. > > box1 exterms:mass [ exunits:kilograms 2.4 ]. > > You can then write quite validly > > box1 exterms:mass [ exunits:grams 2400.0; > exunits:kilograms 2.4 ]. > > This takes less space and can be easily manipulated, and doesn't > throw up > horrible errors. > > A unit kg is the relationship between 3kg and 3. Yup, that looks a lot better. [snip] > PPS: can't you even say, using units predicates as multiplication or > division of scalar quantities > > @prefix : <....exunits#>. > @keywords. > > box1 height 3 ^ m ; > speed 3 ^ m!s ; > acceleration 9.81 ^m!s!s . Not sure I grok what you've got there, is ! a predicate (on the literal) as "per"? (Is this all N3?) In the defence of, er, the devil, I'd have to note that the source of the example, the RDF Primer[1], doesn't cover the layer which includes functional properties (is the approach a reasonable model/approximation? Ok, your point above says not). Can't help but notice too that the doc "has been endorsed by the Director as a W3C Recommendation"... (does the n-ary relationships note [2] cover this I wonder?) Cheers, Danny. > > [1] http://www.w3.org/TR/swbp-n-aryRelations/ > > [2] http://www.w3.org/TR/rdf-primer/#rdfvalue -- http://dannyayers.com
Received on Thursday, 28 September 2006 22:38:29 UTC