Introduction

The original OWL Web Ontology Language as defined by W3C has several syntaxes:

the normative exchange syntax in RDF/XML [2], -the so-called abstract syntax for OWL DL [7], and an XML syntax [6].

The new version of OWL, OWL 1.1, currently being developed by the W3C OWL working group (http://www.w3.org/2007/OWL/wiki/OWL Working Group), is likely to also have several different syntaxes:

-RDF/XML, as before, -a functional-style syntax, replacing the abstract syntax [1], and a revised XML syntax [3].

These syntaxes have differing purposes; for transfer within the Semantic Web, to assist in formal definition of the semantics, for transfer as XML. None of them, however, are designed for ease of use by humans when building or analyzing ontologies. Instead, it was thought that these vital tasks would be performed using tools that provide a graphical interface for human users. Indeed there are tools that provide such a graphical interface, including Protégé (http://protege.stanford.edu/overview/) and Swoop (http://code.google.com/ p/swoop/). However, it was soon understood that some parts of creating an ontology require the creation and entry of complex descriptions, for example when creating a class. A useful way of creating such complex descriptions is via a linear syntax. Similarly presenting these complex descriptions can usefully be done by using a linear syntax (generally with nice indentation).

The original Manchester OWL syntax [4] was created to meet this need. It was designed mainly as a syntax for OWL descriptions that would be easier to write and understand, particularly for non-logicians. The Manchester OWL syntax is not, however, a natural language syntax for OWL. The Protégé OWL plugin used the Manchester OWL syntax both when entering and displaying complex descriptions. The syntax could also be used to dump and read OWL DL ontologies.

Syntax Philosophy

The Manchester OWL syntax tries to minimize syntactic constructs that are difficult to enter or understand, particularly in description. Complex descriptions use short, intuitive English words instead of logical symbols and the usual precedence rules to minimize parentheses.

For example, The Manchester OWL syntax is much more compact and more readable.

Student or

The new functional-style syntax for OWL 1.1 has moved from the framelike abstract syntax for OWL DL and is, instead, based on axioms. This move was taken because of problems encountered with the original abstract syntax having to do with performing operations on OWL ontologies, such as removing axioms. The functional-style syntax thus corresponds very closely to a potential mechanism for storing ontologies in tools, the structural specification of OWL 1.1 ontologies [1].

The Manchester OWL syntax, purposefully, retains a object-centered framelike organization for ontologies, as a frame organization supports a very natural way of creating, editing, and viewing ontologies, similar to the presentation of ontologies in most ontology-building tools, e.g., Protégé 4. IN this organization, all the information about an object (a class or property or individual) is collected together, both to aid in perusal of ontology documents and to mirror the way graphical user interfaces like Protégé present information about parts of ontologies.

Frames and the parts of a frame are uniformly introduced by keywords that end in colons, allowing for their easy recognition. It is suggested that when printing ontologies that indentation be used to help with readability, as follows:

Class: Person

SubClassOf: hasAge exactly 1 and hasGender exactly 1 and hasGender only {female , male} Individual: John Types: Person Facts: hasWife Mary, hasSon Bill, hasDaughter Susan, hasAge 33, hasGender male SameAs: Jack

OWL 1.1

The advent of OWL 1.1 required some changes to the Manchester OWL syntax.

The new constructs in descriptions, qualitative cardinality restrictions and self restrictions, were easy to handle, just requiring additional syntax, similar to the existing syntax. Allowing CURIES [5] simply was a matter of extending the syntax of what used to be local names. Punning mostly required no changes, but see below for one problem with punning.

OWL 1.1 allows annotations in many more places than were allowed previously. These annotations are allowed in the Manchester OWL syntax. They are introduced by an Annotations: keyword, in a way consistent with the other constructs in the syntax.

The following example shows a class frame with two annotations (a label and a comment) on the frame itself, one annotation on the subclass axiom to Person and two annotations on the subclass axiom to owl:Thing.

Class: Student

Annotations: rdfs:label "Student"@en rdfs:comment "The class of students" SubClassOf:

Annotations: dc:creator "Peter" Person, Annotations: dc:creator "Matthew" dc:date "12 May 2007" owl:Thing Because annotations break up the flow of information, they can be harmful to readability.

The syntax here allows for annotations on annotations which are not (yet) part of OWL 1.1. Extending annotations in this way is one of the issues that the OWL working group is considering. Some constructs in OWL 1.1, such as the axioms for n-way disjointness of classes and properties and n-way difference and sameness of individuals, are not obviously part of any particular object. In fact, they are equally about each of the n classes, properties, or individuals involved. Attaching these constructss to any one frame would be misleading. The Manchester OWL syntax thus breaks from its general object-centricity with specific syntax for these axioms, as in the following example. The frames for classes, properties, and individuals play a similar role to declarations in OWL 1.1, and thus the syntax does not have a separate construct for declarations. However, it can be the case that an ontology adds information to an imported class, property, or individual. In this case the frame for that class, property, or individual is not a declaration. Global analysis is thus required to determine which declarations are required in an ontology.

Moving between the Manchester OWL syntax and the functional-style syntax and structural specification for OWL 1.1 requires moving between frames and axioms. This is actually not difficult at all, as each frame construct has a number of top-level pieces that each correspond directly to an axiom.

For example, the frame Punning between object and data properties (i.e., an object property and a data property with the same name) could cause ambiguity between object and data property restrictions. Overcoming this ambiguity would require much extra vocabulary, resulting in lessened readability. Therefore the Manchester OWL syntax does not support dumping of ontologies with punned object and data properties. Duplicating the syntax, as in the OWL 1.1 functional-style syntax, was another possible solution, but it would have interfered with readability too much.

Individual

The Manchester OWL Syntax

Here is a grammar for the Manchester OWL Syntax, with some low-level productions (e.g., digits) removed. In the grammar, terminals are enclosed in single quotes ('') and non-terminals are in bold. Optional parts are in brackets ([]) and repeated parts are in braces (). Some productions are given informally or by references, indicated with italics. The OWL Primer (http://www.w3.org/2007/OWL/wiki/Primer), under consideration in the W3C OWL Working Group, currently uses the Manchester OWL syntax as its primary syntax because this syntax is the most readable OWL syntax. The OWL Working Group may also decide to produce a Working Group note on the Manchester OWL syntax.

Descriptions atomic::= OWLClassURI | '{' individualURI { ',' individualURI } '}' | '('

The OWL 1.1 language is still under development. The syntax in this paper attempts to track the current version of OWL 1.1 with a bit of anticipation related to annotations. The Manchester OWL syntax will be updated as OWL 1.1 is updated. Protégé 4 currently implements a slightly older version of the syntax (and has a couple of bugs in this area as well).