Schema Specification of RuleML 0.88

The upper layer of the RuleML hierarchy of rules is discussed in our main page's design section. In that terminology, the system of RuleML XSDs presented here only covers derivation rules, not reaction rules.

This is because we think it is important to start with a subset of simple rules, test and refine our principal strategy using these, and then work 'up' to the more general categories of rules in the hierarchy. For this we choose Datalog, a language corresponding to relational databases (ground facts without complex domains or 'constructors') augmented by views (possibly recursive rules), and work a few steps upwards to further declarative rules as allowed in (equational) Horn logic. We also introduce a URL/URI module corresponding to webizing. The 'UR'-Datalog join of both of these classes then permits inferences over RDF-like 'resources' and can be re-specialized to RDF triples.

Version 0.88 incorporates the following changes motivated by First-Order Logic RuleML:

The following syntactical changes have also taken place in 0.88:

For more information, see the Changes section. The grammar of RuleML 0.88 (i.e. the content model of each individual tag) is now available as a single document called Content Models for RuleML in addition to being encoded within the XML Schemas. Issues relevant to RuleML 0.88 were already discussed in the Issues List of FOL RuleML, in particular issues 1. and 9.

For updated information, see the Revisions section.

The following changes (from 0.87) affect the syntactic XML structure of RuleML documents in a 1-to-1 manner.

Top level 'Rulebase' is replaced with KQML-like performatives 'Assert' or 'Query', wrapping an 'And', which specify that this conjunction is asserted or queried, respectively. This allows the separation of declarative content from such procedural performatives. For example,

... ]]>

becomes

... ]]>

A 'Query' child,

... ... ... ]]>

now becomes top-level:

... ]]>

'Fact' tags above Atoms are no longer used. Thus,

keep Mary object ]]>

is now simply

keep Mary object ]]>

There are now attributes 'innerclose' (for 'And') and 'closure' (for 'Atom and 'Implies'). So, for example,

... : ... ]]>

is equivalent to

... : ... ]]>

Explicit quantifier 'Forall' is now optionally provided for FOL purposes (as requested) along with 'declare' and 'formula' roles. For example, assuming X is the only variable occuring free in the Atom,

X ... ]]>

'Forall' can be role-skipped into,

X ... ]]>

which can be further shortened to:

... ]]>

The type tag 'Slot' has been changed into a role tag 'slot' (note the lowercase initial letter as per case-distinguished Type/role convention of 0.87). The outer 'role' tag is discarded, being no longer necessary to achieve an alternating type/role "striped syntax" (important for compatibility with RDF). Also, inner 'name' and 'filler' roles are now replaced by positionality of the multiple-valued 'slot' role tag. For example,

color blue ]]>

is now

color blue ]]>

'Imp' has been expanded to 'Implies'. This change suggests that the body be supplied before the head, body => head, so these roles are now skippable. So, for A() => B(),

A B ]]>

is equivalent to

A B ]]>

Similarly, 'Eq' has been expanded to 'Equal'. For example,

D 10 ]]>

is now written as

D 10 ]]>

Other miscellaneous changes include 'role' being discarded (along with 'name' and 'filler') because 'slot' is now itself a role, and 'clause' and 'wff' being replaced by 'formula' (which is also usable within 'Forall').

Affecting the semantics, 'rbaselab' and 'rlab' are unified into 'oid', which is also used for general object identification. For example,

gpKR fact 1 offer Ecobile special 20000 ]]>

Conversely, the 'href' attribute is split into 'wref' (web reference) and its complementary 'wlab' (web label) attribute (only valid on Inds within an oid). For example,

gpKR fact 1 offer Ecobile special 20000 ]]>

The following revisions have been made:

To precisely reflect the closure conventions of LP, top-level And (directly below Assert), corresponding to "Rulebase" from previous versions, may no longer have an innerclose attribute with a value of "existential" (only "universal").

New connective "Equivalent" is now permitted below top-level And, as well as within Forall, as syntactic sugar for two converse Implies expressing equivalence. However, to avoid conjunctions etc. in the head, only atomic equivalence is permitted. The accompanying "torso" role tag (neutralizing head and body) is also introduced. Equivalent is disallowed in the urcbindatagroundfact sublanguage (along with, as before, Implies).

Under Query, And & Or may now have a closure attribute with a value of "existential" (but not "universal"), reflecting LP's querying closure conventions.

New explicit quantifier "Exists", usable only directly below Query, is introduced. In addition to the quantified variables, it may contain an And, Or, Atom or a nested Exists. Exists is disallowed in bindatagroundlog and, by inheritance, urcbindatagroundlog (along with Forall, as before).

The modularization used for Versions 0.7 and 0.8 was inverted since RuleML 0.85 to be more intuitive. The motivating factors behind this switch were simplicity (a single root with two distinct branches), consistency (inheritance in a single direction, for obvious super/subclass relationships) and efficiency (non-redundant implementation).

However, a limitation within XML Schema prevented this approach from being easily implemented (resulting in the unstable XSDs in 0.85), so the modularization was re-analyzed and a new model resulted. This updated model reflects both the -- now fully-validating -- (XSD) implementation and expressiveness layering of RuleML, simultaneously capturing both the abstract and concrete.

DTDs have limited support for modularity, but it can be achieved in a roundabout way using macro-like parameter entities. In particular, the contents of an external file can be included using an externally-linked parameter entity. For example, the following includes the contents of datalog.dtd:

%datalog_include;]]>

Simple inclusion is not enough, though: overriding is also necessary. Previously, this was managed using INCLUDE/IGNORE sections: the section that declared the element which had to be changed was simply IGNOREd, then the element was re-declared.

In version 0.85, this clumsy method of overriding was replaced with a much more elegent solution wherein every element's content model was explicitly defined by a parameter entity. The rulebase label '_rbaselab' (now 'oid'), for example, became declared as follows:

]]>

Since parameter entities can overwrite one another (even across files), this content model could be easily replaced with another specified in a different DTD altogether, much like re-assigning a global variable in traditional programming languages. For example, the content model of the rulebase label '_rbaselab' is just "(ind)" in urcbindatagroundfact.dtd (as above), but was extended to permit a complex term (thus, becoming "(ind | cterm)") in hornlog.dtd:

]]>

(Note that this overriding entity must be defined before the inclusion of other files.)

A visual demonstration of this process can be found on slide 25 of the Object-Oriented RuleML: Re-Modularized and XML Schematized via Content Models presentation.

The content model-based approach to modularization also works for XML Schema, using groups (and attributeGroups) instead of parameter entities. For example (now using 0.88 syntax, where '_rbaselab' is now 'oid'),

]]>

becomes

]]>

There is no need for workarounds in XSD: <redefine> makes the specified changes and includes everything else. For example,

%include;]]>

becomes

]]>

A stable XML Schema representation of RuleML version 0.88 has been created using an approach that is consistent with that used in the (version 0.85) DTDs.

Numerous sample RuleML documents have been prepared and maintained as examples and for testing purposes. They are stored in the Examples directory. (Examples from previous versions of RuleML are also maintained, e.g. 0.87 examples). Note that additional examples taken from the First-Order Logic RuleML document have been added in 0.88: appendix1a.ruleml, appendix1b.ruleml and appendix2b.ruleml.

Continuing the practice begun in 0.86, an XSLT stylesheet named 087-to-088.xslt has been created to automatically update RuleML 0.87 documents to 0.88. Note that all examples have already been updated. For instance, taking the own example from 0.87 as input, the stylesheet outputs the following: own-088.ruleml. (See a comparison of these two files generated using HTML Diff.) Additional translation output samples are found in the Upgrader directory.

An XSLT processor which may be used to perform these transformations on a whole directory at once is Saxon, using the following command:

The stylesheet has also been tested using XML Spy (version 2005 sp1).

An XSLT stylesheet has been developed (088_normalizer.xslt) for normalizing the syntax used in a given RuleML 0.88 instance, reconstructing all skipped role tags to be in a fully-expanded, normal form. For example, the compact version of the 'own' example,

buy person merchant object keep person object own person object ... keep Mary XMLBible ]]>

is normalized to the expanded version:

buy person merchant object keep person object own person object ... keep Mary XMLBible ]]>

(See a comparison of these two files generated using HTML Diff.)

Additional examples are located in the Normalizer directory.

To ensure validation stability, the 0.88 XSDs have been tested (using corresponding instance documents/examples) with numerous validators/tools. A summary of these validation results follows:

W3C XML Schema Validator (XSV) v 2.8-1
All examples and schemas validate perfectly except for the binary sublanguages due to a known bug introduced since 2.7-1. See Appendix 3 for instructions on validating an example against its XML Schema and the corresponding output.

Altova XMLSpy v 2005 sp2
All examples and schemas validate perfectly.

Saxon v SA 8.3
All examples and schemas validate perfectly. Sample output:

Appended below is the XML Schema (version 0.88) for a Datalog subset of RuleML (Appendix 1). Also appended below is a simple example rulebase that conforms to that XSD (Appendix 2), and instructions for how to validate the example against the schema (Appendix 3).

The entire family of 0.88 XSDs, specified in a modular fashion, are available here: http://www.ruleml.org/0.88/xsd. The XSD files in the main directory represent actual sublanguages of RuleML, whereas those in the modules subdirectory are elementary components included in the main XSDs. For more information, see the Modularization section.

More sample files -- each referring to the most specific XSD which validates it -- can be found at http://www.ruleml.org/0.88/exa.

Source: datalog.xsd

Source: own.ruleml