Journal of Computing and Information Technology - CIT 14, 2006, 2, 161–174 161 doi:10.2498/cit.2006.02.07 Transclusions in an HTML-Based Environment* Josef Kolbitsch1 and Hermann Maurer2 1 Graz University of Technology, Austria 2Institute for Information Systems and Computer Media, Graz University of Technology, Austria Transclusions are an advanced technique for the inclusion remote document B that is virtually included of existing content into new documents without the need into document A seeFigure1 . to duplicate it. Although originally described in the early 1960s, transclusions have still not been made available to users and authors on the world wide web. This paper describes the prototype implementation of a system that allows users to write articles that may contain transclusions. The system offers a simple web-based interface where users can compose new articles. With a simple button the user has the ability to insert a transclu- sion from any HTML page available on the world wide web. While other approaches introduce new markups for the HTML specification, make use of technologies such as XML and XLink or employ authoring systems that internally support transclusions and can generate web pages as output, this implementation solely relies on the techniques provided by an HTML-based environment. Therefore HTML, Javascript, the Document Object Model, CGI scripts and HTTP are the core technologies utilised in the prototype. Keywords: hypertext, transclusions, xanalogical struc- ture, authoring systems, publishing systems, web-based applications. Fig. 1. Exemplary transclusion. Part of document B 1. Introduction top right is transcluded into document A top left . Bottom: the source code of document A does not contain the actual text of document 2, but only the data required to retrieve it from the original document. In 1965, Ted Nelson presented “a file structure for the complex, the changing and the indeter- minate”, in which he introduced the term hyper- text see 20 . One of the fundamental concepts The following sub-sections discuss the back- in Nelson’s notion of hypertext is a technique ground of transclusions and their use in HTML. called transclusions. Transclusions allow au- Section 2 addresses attempts to implement tran- thors to include portions of existing documents sclusion. Our design and implementation of into their own articles without duplicating them. transclusions in HTML-based environments are Basically, a transclusion in document A is a ref- detailed in section 3. Issues that were encoun- erence to a portion of the content of a potentially tered during the implementation are described This paper was supported by the Styria Professorship for Revolutionary Media Technologies. 162 Transclusions in an HTML-Based Environment in section 4. Finally, section 5 discusses several Apart from obvious improvements in author- aspects of our implementation. ing and publishing systems, transclusions can also offer a solution to copyright issues expe- rienced today on the world wide web: authors 1.1. Background of Transclusions include content into their documents by means of transclusions. Whenever a reader views a Transclusions are designed as complete replace- transclusion, a note about the rights associated ment for all cut-and-paste mechanisms in use. with the transcluded content is added, and a mi- Nelson argues that cut-and-paste is not what cropayment is made to the corresponding owner people actually want to do but that it is a re- 25 . Nelson names this model transcopyright striction imposed upon authors by the nature of see 24 . paper. Writers actually do not want to make a copy of an existing document, cut out the piece they want to reuse and paste it in their docu- 1.3. Transclusions and HTML ment. They want to include the original content and let readers know what the source and the The Hypertext Markup Language HTML, 7 context of the quote is e. g. , 21 . is a relatively simple language for describing Reference lists at the end of a scientific publi- platform-independent hypertext pages. In the cation, for instance, are usually not what is in- early stages of its development, the focus of tended by writers and desired by readers. They HTML was on style and graphical presenta- are rather a pragmatic solution to the problem tion rather than on functionality and underlying that both the source and the context of the quo- paradigms. Therefore, many innovative ideas tation are lost by copying-and-pasting a portion such as bidirectional hyperlinks and issues al- of content printed on paper. ready known at that time including broken links were not considered in the implementation e. What used to be physical restrictions of paper g. , 27 . was embraced by most computing systems in an attempt to resemble the work environments and In principle, transclusions are used in HTML. object common processes in offices cf., 33 . There- Designated markups including img, fore most current graphical operating systems and embed incorporate content such as im- make use of metaphors such as a desktop, fold- ages, Java applets and animations into HTML ers and documents; a document has to be put pages by means of linking. Thus, these el- in exactly one folder; there is a clipboard, and ements basically make use of the concept of content from a different document is included transclusions. using copy-and-paste mechanisms see 23 . Transclusions in HTML are very limited,though. Only certain media such as images can be virtu- 1.2. Implications of Transclusions ally included, whereas textual content, in gen- eral, cannot be transcluded. Moreover, the tran- Transclusions are, however, not only a mere re- sclusion mechanisms available in HTML can placement for copy-and-paste. They assure that only be applied to entire documents. Fine- the original context of a quotation is preserved grained transclusions such as a small spatial and can provide a visible link to the source of selection of an image are not implemented. the transclusion. Ted Nelson’s approach to real- ising this functionality is based on transpointing windows e. g. , 22 . 2. Attempts to Implement Transclusions Moreover, authors of documents can be noti- fied when their articles are transcluded. Thus Although the idea of transclusions was proposed they can, for instance, find out about other some forty years ago, only a few attempts to researchers in the same area. Authors using implement this advanced technique have been transclusions, on the other hand, can be in- made. The following sections give an overview formed automatically about modifications in of several notable approaches to the realisation source documents see 14 . of transclusions. Transclusions in an HTML-Based Environment 163 2.1. Xanadu Although the proposal seems rational, it has not been accepted, and no web browser to date has Transclusions are an integral part of Xanadu, the feature implemented. Ted Nelson’s original hypertext system e. g. , 21 . Their implementation relies on a docu- ment model, though, that is radically different 2.3. Transclusions with IFrames and from what is widely used today. In Xanadu, Embedded Objects documents versions do not contain content but references to the actual content. Content is both The recommendation for HTML 4 includes stored and referenced with the highest granular- markups for inline frames and embedded ob- ity possible – on the level of single characters. jects e. g. , 7 . Both inline frames and embed- All content is retained in content repositories. ded objects define areas within a given HTML document that can be used to display potentially Any document is made up of a list of references remote resources. Inline frames can merely to content stored in the system, e. g. , a document contain HTML pages and images, whereas ob- consists of “characters 124 to 729 and 1276 to jects may contain resources of arbitrary type. 1301 from the repository”. When content from document B is transcluded into document A, the Transcluding document A into document B can, corresponding references to the actual content for instance, be achieved by inserting an ob in the repositories are added to the reference list ject tag with a reference to document A into of document A. document B 16 . The capabilities of this technique are rather limited, though. Only en- Thus, the creation and retrieval of transclusions tire documents can be referenced. Moreover, in Xanadu are trivial list operations. Ted Nel- the context is lost because a link from the doc- son also details a number of functions related ument containing the embedded object to the to transclusions and the handling of situations source of the transclusion is not provided by in which documents are modified or large por- these markups. Therefore this approach is not tions of documents are deleted e. g. , 25 . well suited for realising transclusions. Basically, these functions can be seen as more complex list operations. 2.4. XML-Based Transclusions 2.2. Proposal to Amend the HTML The Extensible Markup Language, XML, is Specification a flexible language for describing documents that contain structured information see 31 . Since the idea of transclusions is already present In contrast to other markup languages such as in HTML for media such as images and multi- HTML, where both syntax and semantics are media animations, a sensible approach to text- determined, neither a set of tags nor the seman- based transclusions is to introduce a new tag that tics are defined in XML. Therefore, XML per allows users to transclude text. Therefore, 28 se does not contain a distinct markup for links; suggests an amendment to the HTML specifica- a separate linking language is used instead. tion. A new markup, text, is proposed with the intention to offer an element of the same XLink, the XML Linking Language e.g., 30 , provides a framework for describing the syntax embed significance as img or .