- From: Michael Kifer <kifer@cs.sunysb.edu>
- Date: Fri, 13 Jun 2008 03:22:38 -0400
- To: Axel Polleres <axel.polleres@deri.org>
- Cc: Jos de Bruijn <debruijn@inf.unibz.it>, Rule Interchange Format Working Group WG <public-rif-wg@w3.org>
On Fri, 13 Jun 2008 00:22:56 +0200 Axel Polleres <axel.polleres@deri.org> wrote: > > Michael Kifer wrote: > > I was just checking if this was the intent. > > > > I think it is enough to define this function by a bunch of axioms > > > > iriToStr("a"^^rif:iri "a"^^xs:string). > > You mean basically that these axioms are implicit in RIF for all > strings? Well, why not... we get infinitly many axiomatic facts with > that in RIF (which is a thing which some people don't like about RDF...) > but so would we with Jos solution, indeed, I do not see what the > preceding paragraph adds. The two solutions are equivalent. Jos wanted it to be semantically defined and I proposed a simpler, syntactic definition (which has slight disadvantages). > I think this brings us to a proper solution: Do not rush to judgement. > We can get something which looks like a *real* casting function by the > same means, not having to go the nasty bypath of having a predicate by > replacing > > "iriToStr("a"^^rif:iri "a"^^xsd:string) is satisfied in every RIF > interpretation." > > with the following: > > "The following equalities hold in every RIF interpretation: > > xsd:string("a"^^rif:iri) = "a"^^xsd:string > > and > > rif:iri("a"^^xsd:string) = "a"^^xsd:iri > " > > why didn't we realize this earlier... We realized this, but did not propose, because it is wrong :-) For instance, the first group of equalities will prohibit different IRIs from denoting the same resource. (The second group seems ok, but I would not be surprised if it also has problems.) > This is more elegant than having a predicate for one cast and functions > for all others. Also, this form of equalities has the elegance that > additional equalities (e.g. by iris referrng to the same object) are not > a problem at all with this. > > Accordingly, I changed the respective section on casting from an d to > rif:iri to the following text: I am afraid you will have to change back :-) --michael > ---------------------- > > ==== <tt>rif:iri</tt> ==== > {| > | style="background-color: #80FF80" rowspan="1" colspan="1" | > Editor's note: Casting from and to is still under discussion in the > working group since <tt>rif:iri</tt> is not a datatype. For details, we > refer to [http://www.w3.org/mid/20080610143044.5698ABF57@nelson.w3.org > Issue-61]. The following is a strawman proposal which might still change > in future versions of this working draft. > |} > > <p>Additionally to the built-in cast functions for datatyes we allow > conversions from and to constants in the <tt>rif:iri</tt> symbol space > to and from <tt>xsd:string</tt>s following similar considerations as > conversions from and to <tt>xsd:anyURI</tt> in > <nowiki>[</nowiki>[[#ref-xpath-functions|XPath-Functions]]]. Technically > speaking, we cannot proceed as with the other cast functions, defining > the semantics via a fixed mapping '''''I'''''<sub>external</sub> for an > external schema <tt> ( ?arg<sub>1</sub>; rif:iri ( ?arg<sub>1</sub> ) > )</tt>, since <tt>rif:iri</tt> is not a datatype with a fixed value > space and fixed lexical-to value mapping. Instead, casts between > <tt>rif:iri</tt> and <tt>xsd:string</tt> are defined via an infinite set > of axiomatic equalities in every RIF interpretation as follows.</p> > > <p>The following equalities hold in every RIF interpretation for each > unicode string <i>a</i>:</p> > <ul> > <li><tt>xsd:string("</tt><i>a</i><tt>"^^rif:iri) = > "</tt><i>a</i><tt>"^^xsd:string</tt></li> > <li><tt>rif:iri("</tt><i>a</i><tt>"^^xsd:string) = > "</tt><i>a</i><tt>"^^xsd:iri</tt></li> > </ul> > > <p>Thus, although there is no explicit schema <tt> ( ?arg<sub>1</sub>; > rif:iri ( ?arg<sub>1</sub> ) )</tt> in RIF, casts between > <tt>xsd:string<tt>s and <tt>rif:iri</tt>s are still possible in RIF with > the intended semantics that the IRI represented by a particular string > can be cast to this very string and vice versa. > </p> > ---------------------- > > I like this solution :-) > > Axel > > > > > > and not bother with the preceding text in your message. It is much simpler that way. > > > > michael > > > > On Tue, 10 Jun 2008 19:22:10 +0200 > > Jos de Bruijn <debruijn@inf.unibz.it> wrote: > > > >> Well, it was the only reasonable alternative I could think of for > >> casting IRIs to strings. > >> > >> I am personally actually not convinced that we even need such a casting > >> function, but there are some people who think it is useful. > >> > >> Best, Jos > >> > >> Michael Kifer wrote: > >>> This makes iriToString a multivalued function (i.e., the same iri has several > >>> string interpretations). Is this what we want? > >>> > >>> > >>> --michael > >>> > >>> On Tue, 10 Jun 2008 16:40:04 +0200 > >>> Jos de Bruijn <debruijn@inf.unibz.it> wrote: > >>> > >>>> See [1] and preceding the messages in the thread for a description of > >>>> the semantic problems. In [1] I also proposed a casting predicate that > >>>> seems to work. > >>>> > >>>> In summary, the biggest semantic challenge in casting IRIs to strings is > >>>> that several different IRIs may be mapped to the same string. The > >>>> following was my proposal: > >>>> > >>>> "Let I be an interpretation, let u be an element in the domain of I, and > >>>> let {i1, ..., in} be the set of IRIs that denote u, i.e. for each ij (1 > >>>> <= j <= n), IC(ij)=u. IR(iriToString)(u,"ij")=t for (1 <= j <= n); > >>>> IR(iriToString)(u,s)=f for every element s not in {"i1", ..., "in"}." > >>>> > >>>> The rule set > >>>> iriToString("b"^^rif:iri,"b"^^xsd:string) > >>>> > >>>> is satisfied in every RIF interpretation. > >>>> > >>>> I think this predicates should be sufficient for most of the use cases. > >>>> > >>>> > >>>> Best, Jos > >>>> > >>>> Rule Interchange Format Working Group Issue Tracker wrote: > >>>>> ISSUE-61 (IRI-Casts): Casting to/from rif:iri [DTB] > >>>>> > >>>>> http://www.w3.org/2005/rules/wg/track/issues/ > >>>>> > >>>>> Raised by: Christopher Welty > >>>>> On product: DTB > >>>>> > >>>>> It is clear users of RIF dialects such as BLD will want to be able to convert (cast) instances of rif:iri to/from other datatypes, in particular strings and possibly rif:text. > >>>>> > >>>>> In general, a casting mechanism is not present in DTB and should be added.
Received on Friday, 13 June 2008 07:23:53 UTC