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Re: [PRD] Proposal for object representation (and ACTION-592 complete)

From: Christian de Sainte Marie <csma@ilog.fr>
Date: Tue, 03 Mar 2009 14:40:01 +0100
Message-ID: <49AD3331.20807@ilog.fr>
To: kifer@cs.sunysb.edu
CC: RIF WG <public-rif-wg@w3.org>
Michael,

Michael Kifer wrote:
> 
> these things are called path expressions and they are not necessarily
> single-valued. For instance, FLORA-2 uses them alongside the frames,
> and they are simply shortcuts for some forms of conjunctions of frames.

I must not have been clear about it: my proposal is about the representation of single-valued attributes, as is usual in object-oriented programming.

Or is your point that my choice of the point-based path notation for the PS in my strawman proposal is confusing, because that notation is also used in contexts where the expressions are not necessarily single valued?

If this is the case, I have no problem using a different PS, or keeping only the XML syntax :-)

It is like for the name of the construct (Getter): we can certainly find a better name and PS. But I would rather not waste our time finding them, before there is at least some consensus that we want to have such a construct.

> Regarding your puzzle ?x[attr->?x.attr], it is a shortcut for
> ?x[attr->?y] /\ ?x[attr->?y].

No, sorry again for not having been clearer: it is not. Because in my proposal, ?x.attr is explicitely single-valued.

The PRD2Core fallback from: P(?x.attr) (where ?x.attrr is a single valued basic term according to my strawman proposal) to: Exists ?y (?x[attr->?y /\ P(?y)) (where  the number of values of the "attr" slot in the frame is not constrained) works because there is no fallbacks for other PRD constructs that require the "single-valuedness" of the attribute "attr": so, the change is neutral if no such construct is used in the rules; and, in all the cases where the change risks not be neutral, that is, when a "single-valuedness"-requiring construct is used, the ruleset will be rejected as not PRD2Core-ifiable.

?x[attr->?x.attr] (where, again, ?x.attr is a single valued basic term, per my strawman proposal) is trivially true when the (single) value of ?x.attr is an individual or a literal. The question I had in mind when I used the shortcut ?x[attr->?x.attr], is when the (single) value of ?x.attr is, e.g., a set.

But the benefit of having a specific syntax for the single-valued attributes is that we do not have to specify the equivalence with other, existing constructs, such as the frame. Or should I rather say: the motivation for having a specific syntax for the single-valued attributes is that we do not have/want/know how to specify the equivalence with other, existing constructs, such as the frame?

> If you are treating x.attr procedurally then you might get into trouble.

Do you mean, in your acceptation of ?x.attr (not necessarily single-valued), or also in my strawman proposal (where it is single-valued by definition)?

If the latter: that would, indeed, be a rather big drawback; maybe to the point of making my proposal useless. Can you explain?

Cheers,

Christian
Received on Tuesday, 3 March 2009 13:40:55 GMT

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