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Review of RIF Primer

From: Gary Hallmark <gary.hallmark@oracle.com>
Date: Sat, 25 Sep 2010 12:23:31 -0700
Message-ID: <4C9E4C33.8080301@oracle.com>
To: RIF WG <public-rif-wg@w3.org>
  Overall this is well-written but neglects PRD. I have suggestions and 
new text to set PRD on an equal footing with BLD/Core. [Suggestions in 
square brackets.] Replacement text not in square brackets. Section 
heading numbers are given for easier reference.

1.0 Introduction
There are many declarative rule languages, ...
Jess, Drools, IBM ILog, Oracle Business Rules, etc. Note that many rules 
languages are not purely declarative. For example, prolog provides a cut 
operator and production rule languages provide action with side-effects. 
However, all these rule languages have a core subset that is declarative.

This document focuses on the Basic Logic Dialect [RIF-BLD]
[change to]
This document focuses on the Core Logic Dialect [RIF-Core], a common 
subset of the Basic Logic Dialect [RIF-BLD] and the Production Rule 
Dialect [RIF-PRD]. A few of the features specific to BLD and to PRD are 
also considered.

[change to]
This document focuses on the RIF Core dialect and on the commonly used 
built-in functions and datatypes described in DTB. Existing RIF dialects 
[BLD] and [PRD] are also briefly considered. This document does not 
discuss [FLD].

This document does not include any discussion of the model-theoretic or 
operational semantics of any of the dialects of RIF. An intuitive 
understanding of the notions of pattern matching, assumption and 
consequence ought to be sufficient to understand the Primer and to 
enable a computer scientist to write rules in RIF. The reader who is 
interested in learning more about the model-theoretic semantics of 
RIF-BLD is encouraged to read the BLD document [BLD]. The reader who is 
interested in learning more about the operational semantics of RIF-PRD 
is encouraged to read the PRD document [PRD].

This Primer is targeted at getting computer scientists to quickly learn 
how to write rules in RIF, but not necessarily to cover all aspects of 
syntax. As a result, there may be some details of RIF syntax, 
specifically, those that are not necessary for writing most rules in 
RIF, which are not covered in this document. All details of syntax are 
covered in RIF-BLD and RIF-PRD.

[please use a more readable convention (consistently throughout)  for 
multi-word symbols. I suggest "-" separating the words. E.g. plays-role, 
role-in-film, Vivien-Leigh, Blanche-Dubois, etc.]

[There is no need to introduce "Convenience Syntax". Use RIF-PRD 
presentation syntax, which also uses If/Then instead of :- ]

Throughout this document we will use RIF-PRD Presentation Syntax in 
which this implication is written almost unchanged in mixfix notation

If A Then B.

Note that in RIF-BLD Presentation Syntax [RIF-BLD] --- this is written 
in infix notation as B :- A, where the antecedent A and consequent B are 
reversed, but the implication expresses the exact same meaning.

[Need to globally replace "RIF Convenience Syntax" with RIF-PRD 
Presentation Syntax and replace "RIF Presentation Syntax" with RIF-BLD 
Presentation Syntax. In general, search for "Convenience Syntax" and 
change each based on surrounding context.]

[following is misleading because it is not valid BLD or PRD syntax:]
This is semantically equivalent to the rule And(Rule1 Rule2)

[change to]
it is now possible to write the complete Basic Combination Rule in RIF PRD:

The precise conditions that allow one to draw a conclusion from a set of 
rules and facts in RIF is discussed in great detail in [RIF-BLD] and 
[RIF-PRD], which give a model-theoretic semantics and an operational 
semantics, respecively, for inference in RIF. A full discussion of 
model-theoretic and operational semantics is beyond the scope of this 

[retitle this section]
BLD Extensions to RIF Core: Functions and Equality
[In this section we use BLD Presentation Syntax, i.e. ":-" rather than 

[insert a new section between 5.1 and 5.2]
5.2 PRD Extensions to RIF Core: Negation, Priority, and Modification
So far, rule conditions have been positiive. RIF-PRD adds negation to 
rule conditions. For example, the following rule set computes awardless 
film actors:
[in an example box]
   Prefix(rdfs <http://www.w3.org/2000/01/rdf-schema#>)
   Prefix(imdbrel <http://example.com/imdbrelations#>)
   Prefix(dbpedia <http://dbpedia.org/ontology/>)

  Group( 2
    Forall ?Actor ?Film ?Role (
      If   And(imdbrel:plays-role(?Actor ?Role) 
imdbrel:role-in-film(?Role ?Film))
      Then dbpedia:starring(?Film ?Actor)

   Group( 1
     Forall ?Actor (
       If   Not(Exists ?Film ( And(
                   dbpedia:starring(?Actor ?Film)
                   imdbrel:win-award(?Actor ?Film)) ))
       Then dbpedia:awardless-film-actor(?Actor)

The numbers 1 and 2 associated with each group are priorities.
According to the operational semantics of PRD, rule groups are evaluated 
in order of descending priority. Priorities are important here to ensure 
that the dbpedia:starring relation is computed before the 
dbpedia:awardless-film-actor relation is computed.

RIF-PRD also allows rule actions to modify facts, as discussed later in 
section 6.2.

5.2 Datatypes and Builtins
[either change to 5.3, or better, because DTB is in Core, move out of 
Section 5 so that section 5 contains only extensions to Core (i.e. BLD 
and PRD)]

[The RIF example is invalid. ex:e1 # ex:Example[ex:a -> 1] is not legal 

This is a direct consequence of the fact that RIF-Core has the semantics 
of first-order logic.

Object-oriented languages, on the other hand, have the semantics of 
programming languages. The slot a is therefore understood as a variable 
which can be overwritten.
RIF-PRD has a modify action with operational semantics that can 
overwrite slot values. For example,
[example box]
  Prefix(ex <http://example.com/exampleconcepts#>)
  Group (
    Do (
      (?e1 new())
      Assert(?e1 # ex:Example)
      Assert(?e1[ex:a -> 1])
      Modify(?e1[ex:a -> 2])

At the end of the action block (identified using the keyword "Do"), the 
slot a has the value 2.

This design criterion led the designers of RIF Core, BLD, and PRD to 
make choices that maximize the number of rules systems that can 
effectively interchange rules through RIF, often at the expense of 
expressiveness. The absence of negation in Core and BLD, for example, is 
clear in the standard RIF logic dialects (Core, BLD), and this design 
choice in BLD was made because different rule systems implement negation 
in so many different ways, selecting a specific semantics for negation 
in BLD would have prevented interchange between all the languages that 
used a different semantics. Production rule systems, on the other hand, 
typically use the same kind of negation semantics, called inflationary 
negation. PRD presentation syntax provides an alternate keyword, INeg, 
that may be used instead of Not to explicitly indicate that inflationary 
negation is being used.
Received on Saturday, 25 September 2010 19:24:14 UTC

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