# Re: RDF lists

From: pat hayes <phayes@ai.uwf.edu>
Date: Fri, 20 Sep 2002 08:58:57 -0500
Message-Id: <p05111b19b9b0d9fb4cb8@[65.217.30.172]>
To: "Jos De_Roo" <jos.deroo.jd@belgium.agfa.com>

```
>Pat, I think I'm fine with that
>
>how can we express that
>when given
>   _:l1 rdf:first :a .
>   _:l1 rdf:rest :b .
>
>   _:l2 rdf:first :a .
>   _:l2 rdf:rest :b .
>
>then _:l1 and _:l2 are tidy

Er...you can't. That is, there could be two lists with the same members.

Pat

>(we have that "for some" is actually
>"for one" in this case and I've missed
>a notation for that important fact)
>
>-- ,
>Jos De Roo, AGFA http://www.agfa.com/w3c/jdroo/
>
>
>
>
>                       pat
>hayes
>                       <phayes@ai.uwf.edu        To:
>w3c-rdfcore-wg@w3.org
>                       >
>cc:
>                       Sent by:                  Subject:  RDF
>lists
>
>w3c-rdfcore-wg-req
>
>uest@w3.org
>
>
>
>
>                       2002-09-20
>12:39
>
>AM
>
>
>
>
>
>
>
>
>
>Since RDF now has DAML_style lists (right?), I thought it would be
>good to get feedback on the proposed MT for them. So here it is: toss
>-----------
>Given a set S, we define a set of lists over S to be a set L
>containing the empty sequence <> and all structures of the form <s,
>l> where s is in S and l is in L.
>
>As with any recursive 'definition', this in fact is an equation with
>many possible solutions. The usual way to interpret this kind of
>definition is in terms of a minimal solution of the equation. That
>means that one would understand the set of lists to be the smallest
>collection of things that would satisfy the recursion, which would be
>all finitely deep lists which have no loops, i.e. structures of the
>form <s1 <s2 <...<sn <>>...>>. On this view, every list defines a
>finite sequence of elements of S. Other lists are possible, however,
>which would also satisfy the recursive definition: infinitely deep
>lists, for example, or looping lists of the form l = <s, l>.
>
>Although it is possible to axiomatize a recursive 'definition' as a
>logical assertion, there is no way to finitely axiomatize the least
>fixed-point solution. We could impose it as a semantic condition; but
>this condition, although intuitively sensible and in correspondence
>to the usual semantics for computational languages, may have some
>regrettable consequences when used, as here, in the context of a
>descriptive language. In particular, there would be no way to
>establish the completeness of any finitary inference process relative
>to such a semantics. We therefore avoid making this stipulation, and
>simply require that the set of lists in any interpretation be *some*
>set which satisfies the recursive definition. Note that this means
>that any set of lists will at least contain all the finite
>non-looping lists.
>
>The semantics of the rdf list vocabulary is then straightforward. In
>any RDF interpretation I, we assume that
>
>ICEXT(I(rdf:List)) is a set of lists over IR  @@Note the use of 'a'
>rather than 'the'.@@
>
>I(rdf:nil) = <>
>
><x, y> in IEXT(I(rdf:first)) iff x = <y, l> for some l in
>ICEXT(I(rdf:List))
>
><x, y> in IEXT(I(rdf:rest)) iff x = <s, y> for some s in IR
>
>We note in passing that this semantics requires that the universe IR
>is closed under the operation of constructing lists.
>
>Any interpretation I of any RDF graph of the form
>
>A1 rdf:type rdf:List .
>A1 rdf:first B1 .
>A1 rdf:rest A2 .
>A2 rdf:first B2 .
>A2 rdf:rest A3 .
>...
>An rdf:first Bn .
>An rdf:rest rdf:nil .
>
>has I(A1) = <I(B1), <I(B2), <... <I(Bn), <> >...>>. We will describe
>this as a sequence and write it as [I(B1), ... , I(Bn)]. Sequences
>are the ordered multisets of the elements of finite lists.
>
>--------
>
>
>--
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--
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```
Received on Friday, 20 September 2002 09:59:02 UTC

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