kremer@cpsc.ucalgary.ca

Dexter1: A variation of the Dexter Hypertext Reference Model

This work is primarily based on:
Halasz, F., Schwartz, M. "The Dexter Hypertext Reference Model." NIST Hypertext Standardization Workshop, Gaithersburg, MD. January 16-18, 1990.
WARNING: Work in progress.
In particular, although the Z specification of the original Dexter Model is complete, the Dexter1 variation on it is currently just a placeholder: it is just a copy of the original Dexter Model. This will be rectified as soon as I can get around to it.

Contents

A note about the Z notation used here

The version of the Z notation used here is the ZSL language developed by Xiaoping Jia at dePaul University. The ZSL language is used in conjunction with the ZTC (Z Type Checker), version 2.0, also from Xiaoping Jia. One of the nice things about ZTC is that it will take this page (either saved as HTML source or as pure text) as input. Since the type checker only interprets lines beginning with a tab character, it is easy to write a combination HTML and ZSL file.

A note about the symbols used in these pages

There are two closely replated specifications listed here. To aid comparison between the two corresponding elements between the two have been linked. Links to objects in the original Dexter specification have the symbol while links to objects in the extended Dexter1 specification have the symbol .

To make incremental changes though specification refinements more obvious, the symbol is used for new additions to the specification since the last refinement and the symbol is used for changed components. Both these symbols usually appear along the right-hand margin.

Objectives of this work

The Dexter1 variation to the Dexter Model is motivated by the need to use hypertexts in a less restrictive environment than the Dexter Model embraces. In addition, the use of hypertext for knowledge representation is a strong motivator.

Open world

The original model assumes a hypertext is a closed environment (a file?) which does not refer to objects outside its borders. It should be obvious to anyone reading this on the web that a more open view of the world can exist -- the web constitutes an extreme example of an open hypertext system.

It is with this view in mind that the Dexter1 Model is both relaxed and extended to account for references outside a hypertext's boundries. This implies that the hypertext cannot always control the deletion or accessability of a referenced objects, and must now account for "dangling" links.

Knowledge representation

In this work, knowledge representation is view as important role of hypertext, so the support of existing knowledge representation systems is seen as critical. Further, the author sees the hypertext being much more than the medium in which one can navigate among pages of some KR language's syntax, but the hypertext itself should embody the syntax of the KR language. In this view, the objects of KR language become hypertext nodes (in Dexter: atoms) and the relations become hyperlinks. As relations in some knowledge representation languages (e.g. Sowa 84) may be unary, binary, or n-ary, so the Dexter Model is relaxed to accomodate the unary links. Furthermore, both objects and relations are typed knowledge representations languages, and this leads to the introduction of a typing system into the Dexter1 Model.

Specification Elements Listing

kremer@cpsc.ucalgary.ca