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Re: RIF-in-RDF: Requirement 4

From: Michael Kifer <kifer@cs.stonybrook.edu>
Date: Sun, 25 Jul 2010 17:57:33 -0400
To: Sandro Hawke <sandro@w3.org>
CC: public-rif-wg <public-rif-wg@w3.org>
Message-ID: <20100725175733.0e0b2dc5@kiferserv>
Sandro,
I don't understand your argument. So, you are proposing that somebody would
explicitly write

foo[my:conjuncts->list(bar1 bar2 bar3)]

If so, why can't the same one write this:

foo[my:conjuncts->bar1]
foo[my:conjuncts->bar2]
foo[my:conjuncts->bar3]

?

michael



On Sun, 25 Jul 2010 16:49:52 -0400
Sandro Hawke <sandro@w3.org> wrote:

> Dave [1], Harold [2], and Michael [3] have all expressed a desire to
> have the RIF-in-RDF mapping more closely follow the XML syntax.  In
> particular, they suggest it use repeated properties instead of
> gathering all the values of the properties into a list.
> 
> I'm extremely sympathetic to this desire.  If you look back at the
> history of the web page, you'll see this is what my first version did,
> and then I stalled out for months as I realized it wouldn't work.
> Eventually I decided I just had to go ahead with the list-based
> approach that's currently in the document.
> 
> The compelling problem for me is that using repeated properties, as
> far as I know, it is not possible to reliably transform a RIF document
> using an incomplete reasoner.  I've called this "Requirement 4" in
> RIF-in-RDF [4].
> 
> Let me back up and explain what I'm trying to do and why I think it's
> important.
> 
> In my talks and writing about RIF to Semantic Web audiences, I explain
> that where I think RIF is essential is in data transformation.  With
> RIF, we can allow interoperation between vocabularies.  My standard
> example is that FOAF has a foaf:name property, and it also has
> foaf:firstName and foaf:lastName.  When you're producing FOAF data,
> which should you use?  When you're consuming FOAF data, which should
> you look for?  In both cases, if you want interoperability, you have
> to do both.  When there are only two options, and everyone knows about
> them, that's okay.  But what happens when the third, fourth, and fifth
> "standard" properties for representing names comes along?  It's a
> nightmare; the fact that the producer and consumer are both using RDF
> ends up not buying you very much at all.
> 
> But RIF can solve this problem.  By having the ontology documents for
> each of terms include some RIF (via rif:importWithProfile), the folks
> deploying new properties can express how they map data to alternative
> properties.  (In this case, with some string operations.)  Now,
> data-consuming systems which implement RIF can automatically get the
> data in exactly the vocabulary they want.
> 
> I think this is a very compelling use case.  In fact, without this
> mechanism (or an equivalent one) I don't see how the Semantic Web can
> work at all.  More recently, I've started using another example (which
> I mentioned on a recent telecon), where facebook's Open Graph Protocol
> uses RDF with a different style of modeling than most of the Semantic
> Web; here, again, RIF can provide interoperability via translation
> rules.
> 
> Now, imagine we have this all in place.  Lots of RDF data out there,
> using various vocabularies.  When you dereference the terms you find
> some RIF that lets you translate between them, so it's all roughly
> interoperable.  Of course, not every vocabulary can be mapped; some
> aren't well understood enough to formalize, etc.  But many can be
> translated.  This allows new vocabularies to be deployed, and the
> overall system to grow and evolve in place.
> 
> Now, remember the RIF extensibility requirement?  In the current
> design, we met it by providing may-ignore and must-understand
> extensions via annotations and new xml elements.  This works, but only
> in very broad strokes.  We have no "graceful" fallback.  Extensions
> can't offer syntactic sugar, and they certainly can't offer features
> which can be approximated.  This mechanism may not be good enough to
> allow extensions to really be deployed on the open Web.  We talked
> about all this years ago, but decided we didn't have time to work out
> all the details, and that it could wait.
> 
> So, as you may have guessed by now, I want to provide RIF
> extensibility the same way I want to provide FOAF name extensibility:
> with RIF translation (fallback) rules.
> 
> I'll walk through this, below, but here's the punchline: I think it
> works fine with the list-style of RIF-in-RDF, but I don't think it can
> be done with the repeated-properties style.  This is why I need the
> lists.
> 
> I have a few ideas of transformations I want right now...
> 
>   - automatically add universal quantification to free variables
>   - extend frames to allow for context/named-graphs (cf Decker's TRIPLE)
>   - convert some kinds of rules between PRD and BLD (trading off
>     between new() and logic functions)
>   - convert logic functions to builtin list operations (I think this
>     can be done; not sure) getting more of BLD into Core
>   - standard rewritings: get rid of conjunction in rule heads, disjunction
>     in rule bodies, Skolemize
>   - re-write out named-argument-uniterms
> 
> ... but they're all too complex to use as first illustrations.  For
> that I'll use something that ridiculous, but pleasantly simple:
> 
>   - Allow people to use the term my:Conjunction instead of rif:And.   Also,
>     use my:conjunct instead of rif:formula inside it.
> 
> Before actually writing the transformation rule, we have to decide
> what the transformations are going to look like in RIF.   Some options:
> 
>    1.  in place, new and old, overlapping; the new data (the output)
>        is distinguished by using different properties and/or classes.
>    2.  copy the whole document, with changes
>    3.  ...   maybe some other approaches?
> 
> Let's try (1) first, since it's more terse.  Our input looks like
> this:
> 
>       ...
>       <if>       <!-- or something else that can have an And in it -->
>          <my:Conjunction>
>              <my:conjunct>$1</my:conjunct>
>              <my:conjunct>$2</my:conjunct>
>              ...
>          </my:Conjunction>
>       </if>
>       ...
> 
> and we'll just "replace" the element names.
> 
> However, since we don't have a way to "replace" things in this
> "overlapping" style, we'll just add a second <if> property, and the
> serializer or consumer will discard this one, since it contains an
> element not allowed by the dialect syntax.
> 
> So, the rule will add new triples, but leave the old ones intact.
> The rule will leave us with this:
> 
> 
>       ...
>       <if>       <!-- or something else that can have an And in it -->
>          <my:Conjunction>
>              <my:conjunct>$1</my:conjunct>
>              <my:conjunct>$2</my:conjunct>
>              ...
>          </my:Conjunction>
>       </if>
>       <if>      <!-- the same property, whatever it was -->
>          <And>
>              <formula>$1</formula>
>              <formula>$2</formula>
>              ...
>          </And>
>       </if>
>       ...
> 
> Here's the rule:
> 
>  forall ?parent ?prop ?old ?conjunct ?new
>  if And(
>    ?parent[?prop->?old]
>    my:Conjunction#?old[my:conjunct->?conjunct]
>    ?new = wrapped(?old)  <!-- use a logic function to create a new node -->
>  ) then And (
>    ?parent[?prop->?new]
>    rif:And#?new[rif:formula->?conjunct]
>  )
> 
> This works fine, as long as the reasoning is complete.  However, if
> the reasoning is ever incomplete, we end up with undetectably
> incorrect results.  Rules that were "if and(a b c) then d" might get
> turned into "if and(a b) then d"!
> 
> I don't think it's sensible to expect reasoners to be complete.  It's
> great to have termination conditions arise from the rules; it's not
> good to require the reasoner to run until it knows all possible
> inferences have been made.  With the above approach, there's no
> termination condition other than "make all the inferences possible".
> 
> Alternatively, if we use the list encoding, the rule is very similar:
> 
>  forall ?parent ?prop ?old ?conjuncts ?new
>  if And(
>    ?parent[?prop->?old]
>    my:Conjunction#?old[my:conjuncts->?conjuncts]
>    ?new = wrapped(?old)
>  ) then And (
>    ?parent[?prop->?new]
>    rif:And#?new[rif:formulas->?conjuncts]
>  )
> 
> ... but now we can set a termination condition: if a RIF document in
> the desired dialect *can* be extracted, then you're done.
> 
> A few notes:
> 
>     * I've included the types (like rif:And) for now.  Whether to do
>       that is a separate issue (specifically ISSUE-101).
> 
>     * It's okay to have the rules produce multiple valid RIF
>       documents; you can stop after generating one, but you can also
>       continue.  If there's some kind of weighting on the rules (cf
>       XTAN's "impact" mechanism) you can search for a solution that's
>       better than some others.  It may be possible to efficiently
>       direct this search towards the best solution; I'm not sure.
> 
>     * I don't think the copy-the-whole-document approach to
>       translation helps at all.  There, instead of attaching the new
>       node to the same parent, we attach it to a new parent, and we
>       end up with a whole new tree.  But still, branches of the tree
>       are generated by separate rules applications, so an incomplete
>       reasoner may produce incomplete (wrong) output trees.
> 
> I think that's it.  I trust y'all will point out any confusing or
> incorrect elements of this argument.
> 
>       -- Sandro
> 
> [1] http://lists.w3.org/Archives/Public/public-rif-wg/2010Jul/0015
> [2] http://lists.w3.org/Archives/Public/public-rif-wg/2010Jul/0017
> [3] http://lists.w3.org/Archives/Public/public-rif-wg/2010Jul/0018
> [4] http://www.w3.org/2005/rules/wiki/RIF_In_RDF#Requirements
> 
> 
> 
> 
> 
Received on Sunday, 25 July 2010 21:58:08 GMT

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