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Re: RIF and QL

From: Frank McCabe <frank.mccabe@us.fujitsu.com>
Date: Wed, 25 Jan 2006 14:11:06 -0800
Message-Id: <54806D8A-DC36-483D-9795-516C4EA3761B@us.fujitsu.com>
To: public-rif-wg@w3.org

On Jan 25, 2006, at 12:11 PM, Ed Barkmeyer wrote:

>
> François Bry wrote:
>
>> I would like to submit the following views concerning the declarative
>> fragment of RIF (ie the following is not about updates, actions,
>> (re)active rules, etc. that RIF might have to offer):
>> 1. RIF should be a "lingua franca" (literally "French speech",  
>> meaning a
>> language used as common or commercial tongue among peoples of diverse
>> speech).
>
> Agreed.
> [Aside: For the record, "lingua franca" was originally a "pidgin"  
> commercial language of the Mediterranean basin in the 13th-17th  
> centuries.  It meant "language of the Franks", where "Franks" was  
> (in 1300) a general term for Western Europeans  (most of whom were,  
> or were ruled by, descendants of the German "Frankish" tribes that  
> overran France, Spain and Italy in the 5th-7th centuries).  I  
> understand that it was simplified medieval Italian, with words from  
> German, Provençal, Catalan and Arabic.  Regrettably, François, not  
> French, or at least not the langue d'oil.]


Notwithstanding some of the technical issues raised, I think that  
hoping (or even requiring) yet another lingua franca is a hopeless  
endeavor. For a number of reasons:
1. There is already too much content out there that would need  
'translating' from what would be 'legacy' notations to the lingua  
franca. That translation effort is probably not going to happen, and  
to base the RIF on requiring it is silly.
2. Each system that has adopted a particular notation/semantics/meta  
model has, presumably, good reasons for making that choice. But,  
there is no single semantics that will cover all possible choices; so  
you will disenfranchise a lot of people - no matter what your choice.
3. Ownership of information is paramount in the Internet. This  
includes the semantics as well as the axiom set (i.e., some people  
may want to live inside that 'blast radius' of fuzzy logic:)
4. Creating a new lingua franca (or even lingua über toutes to mix  
languages) sets us up for another fight when someone decides that yet  
another language is going to be the LF.

I believe that laying the foundations for rules interoperability -  
even in the face of semantic heterogeneity - is a more appropriate task.
Off the cuff, this suggests that
1. the original semantics of a rule set be exposed in any encoding of  
the rule set in RIF
2. a technique for 'communicating' between different logics is a key  
technology for the RIF
3. a technique for preserving the provenance of information during  
inference would also be required.

I am sure that much more will be needed. But these some of the are  
simple ;-< engineering aspects that may be important.

Frank


>
>> 2. The RIF lingua franca should abstract out "the diverse speech" by
>> dis-considering (a) idiosyncracies, (b) computational choices, as  
>> well
>> as (c) some semantic choices of query languages and rule
>> languages/engines. These three points is motivated below.
>
> I agree in principle, but I think the details of this are debatable.
>
>> First, let draw a line between "rule format" and "rule language",  
>> both
>> being formal languages (ie something else than human/natural
>> languages):
>> - A rule format has a (unambigous) syntax and a declarative semantics
>> (preferably in the form of a model theory) but does not necessarily
>> have a computational semantics.
>> - A rule language also has a (unambiguous) syntax and a  
>> declarative semantics but also a computational semantics (and  
>> preferably a
>> prototype implementing this computational semantics).
>
> 1.  I agree that it is important to distinguish these ideas by  
> distinct terms.  I have a different understanding of "rule format",  
> but I am willing to use François terms if everyone agrees.  The  
> important thing is that we distinguish between "declarative" and  
> "computational" models.
>
> 2.  I think there is a 3rd distinct idea that we also need a term  
> for.  It is the one I would have called "rule format":
>  - A "rule xxx" is an unambiguous syntax for the expression of rules.
> That is, this is just the syntax, to which one or model theories  
> must be applied to get a declarative "rule format" (to use  
> François' term).  If François has taken "format", then we need  
> another word for "xxx".
>
> 3.  I don't think it is "preferable" to have a prototype as the  
> definition of the computational semantics, even though that has  
> been the traditional experience, e.g. with Prolog and Jess.  It is  
> possible to provide a formal definition of "computational  
> semantics" at the level needed to describe "procedural",  
> "stratification" and "meta-rule" behaviors, namely the explicit  
> computational behavior of each such element.  At the same time, the  
> "effective semantics" of a set of these computational elements  
> applied to a given ruleset is not specified, and may in some cases  
> be determinable only by simulation/implementation.
>
>> Second, let us recall the difference between declarative and
>> computational semantics. The difference is that the former might
>> ignore computational aspects such as an evaluation order for
>> conjunctions and disjunctions, ensuring termination through memoing,
>> and optimizations.
>
> Agreed.
>
>> Third, let discuss the three points (a), (b), and (c):
>> (a) Idiosyncracies (like writing "\+" instead of "not" in Prolog)  
>> should
>> obviously be dis-considered for otherwise we shall end up with a
>> dictionary of notations. Such a dictionary might be useful but should
>> not be an objective of the RIF WG.
>
> Yes.  The whole point of the RIF is to standardize one XML  
> representation of (FOL) "not".  Note that we may need more than one  
> "not" operator if a single ruleset can contain monotonic "not" and  
> non-monotonic (negation-as-failure) "not".
>
>> (b) Computational choices must dis-considered for otherwise we shall
>> end up with defining a rule language, not only a rule format, and
>> furthermore a rule language extending several other rule  
>> languages. It
>> is doubtful whether this is at all possible. It is undoubtedly
>> impossible within the time scale the RIF WG has.
>
> The real issue here is: How much and what kind of interoperability  
> do we want to guarantee?  Some computational elements may be needed  
> to define the semantics of a ruleset in some model theories, namely  
> those that attribute a notion of "time" (distinct points in time)  
> to evaluation of rules.
>
>> (c) More subtile is that some (not all!) semantic choices of query
>> languages and rule languages/engines should be dis-considered. Let us
>> consider three examples.
>> First, consider a rule with fuzzy truth values like:
>> R1 = if a:0.3 and b:0.2 then c:0.25
>> Obviously, how the fuzzy truth values are combined is an important
>> part of the rule's semantics. As it is well-known, how fuzzy truth
>> values are combined is one of the essential aspects that distinguish
>> fuzzy rule languages. Therefore, it is desirable that RIF gives a way
>> to express that rule R1 has been specified after fuzzy truth
>> combination C.
>> However, rule R1 can also be used after the following (weaker)  
>> meaning:
>> R1' = if a is possible and b is possible then c is possible.
>> Arguably, RIF is needed precisely for conveying R1 making it  
>> possible for
>> the recipient to interprete it a R1'.
>
>
> To put it bluntly, I don't think the RIFWG should get within the  
> blast radius of fuzzy logics.
>
>> Second, consider the following rules with non-monotonic negation:
>> R2 = if not a then b
>> R3 = if not b then a
>> The ruleset {R2, R3} might have been specified in a context where  
>> it is
>> used under the well-founded semantics. Under the well-founded  
>> semantics,
>> in the absence of other a- and b-headed rules, {R2, R3} means that  
>> a and
>> b are unknown.
>> However, the ruleset {R2, R3} can also be used after the stable model
>> semantics where it means a or b is true.
>
> This is somewhat inaccurate, but the point is well-taken: The  
> meaning of the ruleset is highly dependent on what François calls  
> "computational semantics".  I would have said, however, that the  
> distinction between stable-state semantics and "well-founded"  
> semantics ("stratified dependency"?) is so significant as to be  
> reflected in the model theory.
>
>> Third, consider a rule stating that a computer science student not  
>> attending the lecture on RIF must attend the lecture on Prolog.
>> This rule can be used for making a list of those student who  
>> should attend the Prolog lecture. In this case, the rule negation  
>> is non-monotonic negation.
>> This rule can also be used  in determining consequences of the  
>> regulation without considering students and student registrations  
>> to lectures.  In this case, the rule negation is monotonic negation.
>
> We...ell, the rule is given in somewhat ambiguous natural language.
> What exactly is meant by "must attend"?  One possibility is:
>   (ALL ?s)(IMPLIES (NOT (attends-RIF ?s)) (attends-Prolog ?s) )
> Another is:
>   IF (NOT (attends-RIF ?s)) THEN (Attend-Prolog ?s);
> where attends-RIF is a query, and Attend-Prolog is an action.
> And still another is:
>   (IMPLIES (NOT (attends-RIF ?s)) (Obligation (attends-Prolog ?s)))
> And the interpretation of the deontic "Obligation" modal may be:
>   (IMPLIES (NOT (attends-Prolog ?s)) (InvalidState ?s))
>
>> These example show that there might be cases where a Web actor  
>> need the
>> one semantics, another another semantics of non-monotonic negation.
>
> Perhaps I am being thick-headed, but I cannot draw that conclusion  
> from these examples.  All I see is that identical syntax can lead  
> to radically different interpretation if the model theory  
> underlying the sentence isn't somehow made explicit.
>
>> This
>> would be the case if the first actor, using the well-founded  
>> semantics,
>> is interested in derivable facts (ie investigates only  
>> necessities), while
>> the other actor is also interested in derivable disjunctions (eg for
>> investigating possibilities).
>
> IMO if the first actor intended a particular interpretation, and  
> the second actor presumes another interpretation, the second  
> interpretation can only be consistent by dumb luck.  In general, if  
> the second actor has no idea what model theory underlies the  
> ruleset given by the first actor, the second actor may impose his  
> own model theory on it, but any inferences he makes are likely to  
> be inconsistent with the original.  And the ambiguous third example  
> supports this.
>
> Conversely, if the second actor knows what model theory underlies  
> the ruleset given by the first actor, the second actor may know how  
> to interpret that model theory (or some subset of it), and thus the  
> ruleset, to give meaningful results in the second actor's preferred  
> model theory.
>
> So I don't deny the value of having the possibility of different  
> model theories.  I just want the ruleset to be explicit about the  
> model theory that would give it the intended interpretation.
>
>> Of course, RIF cannot be free of declarative semantics. A RIF "and"
>> must mean a logical "and", a RIF fuzzy truth value must have a  
>> meaning
>> and a RIF "non-monotonic not" must mean a "non-monotonic not". But
>> these meanings must leave ways open to re-interpretations --
>> admittedly within a yet to define reasonable scope.
>
> (It may be that François and I agree on this.  I just don't  
> understand his way of presenting this concept.)
>
>> 3. The RIF lingua franca should offer a way to procedurally attach
>> SPARQL queries, and queries expressed in other query languages, in  
>> rule
>> bodies.
>
> s/bodies/antecedents/ and I will agree.  (François later says  
> "bodies" = "condition parts".)
>
> I understand "procedurally attach" here to be the common term in  
> rule engine land for what Harold called "external functions".  What  
> François describes below is about "external functions" in general,  
> but there is a special problem with the syntax of query languages  
> vs the syntactic elements of the RIF.
>
>> RIF could support such procedural attachments under the following
>> assumptions:
>> - Expressions in a query language may appear in rule bodies
>> (= condition parts) with an interface stating sets of bindings they
>> provide for logical variables (occurring elsewhere in the rule).
>
> I need to understand what François has in mind by "expression in a  
> query language".  I can understand how something like:
>  (SPARQL 'RDFbase '<query text> <rule-expression> ... <rule- 
> expression>)
> could work as an "external query", where:
> - SPARQL designates a particular query service (or a class of query  
> service for which the rule engine is to find a server in  
> conjunction with the "RDFbase" parameter)
> - RDFbase designates the KB on which the service is to operate for  
> this query
> - <query text> is a query stated in the language of the designated  
> service that may contain references to "external parameters" using  
> the "external parameter syntax" *for that query language*
> - <rule-expression> is an operand expression in the rule language  
> (RIF) syntax.  The expression is evaluated by the rule engine  
> before invocation of the external query service, and the result of  
> the evaluation is passed in the position in which the expression  
> occurs, i.e. an "actual parameter".
>
> If the query language provides for references to external  
> parameters "by-position", the actual parameter substitution rule is  
> obvious.  If the references are "by-name", then the sequence of  
> rule-expression parameters becomes a sequence of '<name> <rule- 
> expression>, but the rule engine need not know that.
>
> What François describes below is the assignment of responsibility  
> for the RIF service invocation syntax, the query service (WSDL)  
> definitions, and the <query text> syntax (I think).
>
>> - It is the responsibility of the query language
>> designers/implementers to provide with such an interface.
>
> That is, the SPARQLers (and their competitors) have to define a  
> webservice interface for their query engines.
>
>> - It is the responsibility of the RIF designer to specify the format
>> of the interface.
>
> That is, RIF defines the form of a query invocation *as it appears  
> in a rule*.
>
> What is not stated is that the RIF-compliant engine has to know how  
> to map the RIF query invocation to the SPARQL webservice  
> invocation, and to the SQL/CLI invocation, and to the KQML  
> webservice invocation, etc.  Each of those mappings will probably  
> be somewhat different.  So an engine cannot really be blind to the  
> nature of the Query Service.  It can be blind to the syntax of the  
> query language.  (The engine could, of course, define its own  
> "standard mapping for unknown query services", and expect the user  
> to provide an intercept routine that converts that invocation into  
> the proper invocation for the user's favorite query service.)
>
>> - It is the responsibility of users of RIF, ie RIF programmers, to
>> ensure that the procedurally attached queries make sense. Especially,
>> RIF does not have to provide with a declarative semantics
>> considering/encompassing that of query languages.
>
> Of course.  But there is also a serious constraint on "external  
> queries":
> What is the relationship between the knowledge base for the  
> "external query" and the knowledge base for the rule engine?
>
> If the "external query" interrogates an *external* KB, then the  
> "external query function" has encapsulated (and therefore probably  
> well-defined) behavior.  But if the "external query" operates on  
> the same KB *concurrently with* the rule engine, and the  
> "action"/"consequent" part of a rule can modify the KB, the issue  
> of *timing* becomes a part of the interpretation model.  In  
> database land, this leads to all kinds of meta-rules and  
> "quarantining" and "transaction semantics" and other "things too  
> fierce to mention".
>
> IMO, RIF should define a query invocation syntax and assume that  
> the referenced KB is entirely external, or at least not modified in  
> any relevant way by the actions of the ruleset.  If this is not the  
> case in some instance, in the words of Algol68, "the further  
> elaboration of [ruleset inferencing] is not defined by this standard."
>
> -Ed
>
> P.S.  This is an area in which we have to deal with ?'s Corollary  
> to the Gödel Theorem:  "You can't allow everything you want without  
> allowing things you don't want."
>
> -- 
> Edward J. Barkmeyer                        Email: edbark@nist.gov
> National Institute of Standards & Technology
> Manufacturing Systems Integration Division
> 100 Bureau Drive, Stop 8263                Tel: +1 301-975-3528
> Gaithersburg, MD 20899-8263                FAX: +1 301-975-4482
>
> "The opinions expressed above do not reflect consensus of NIST,
>  and have not been reviewed by any Government authority."
>
Received on Thursday, 26 January 2006 03:44:37 GMT

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