# Re: ISSUE-61 (IRI-Casts): Casting to/from rif:iri [DTB]

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>
Message-ID: <20080613032238.6ee3f8d2@cs.sunysb.edu>
```

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

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

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