Within the last ten years there has been a upsurge of interest in hypertext as a medium for on-line access to written documents. As a central objective of Natural Language Generation (NLG) is on-line creation of written documents, it is hardly surprising that a number of generation systems have been developed which use hypertext as their interface.
Conventional hypertext is static, in the sense that---even though each reader may sample different parts of it---once a hyper-document has been constructed by its authors, its content and form do not change. However, applying NLG to hypertext allows the possibility of dynamic hypertext, where the document is created as demanded by the user. This approach allows the hypertext pages seen by the user to be customised in relation to the browsing context.
In some cases, customisation is only in relation to content---the page brings together information from various on-line sources, but presents it in a single standard way. Other systems, using more advanced NLG techniques, allow the text to be customised in terms of both content and presentation form, being sensitive to such factors as the user model (characteristics of the user), the discourse history (a record of information presented so far), and the system's goals (what the system means to achieve).
Amongst the latter systems, the degree of flexibility supported by the text generation component varies considerably. In some systems (such as StrathTutor [Kibby and Mayes 1988]) and Trellis [Stotts and Furuta 1989]), the hypertext pages are hand-crafted in advance, but the way they are linked together varies from session to session depending on the interests of the user. In other systems, the hypertext pages themselves are generated at run-time, as well as the links between them. In many current systems, a mixture of these two methods is employed.
Clearly, there are many different ways of introducing flexibility into a hypertext system. Our concern in this paper is to outline the most important of these, indicating which have already been explored, and which others deserve closer attention within the NLG community.
A number of hypertext systems incorporating NLG technology have already been developed. In this paper, we will make reference to the five systems listed below.
In addition to these systems, we will introduce our own prototype system ILEX-0, which is being developed specifically to examine issues arising in the domain of dynamic hypertext. The system is designed to simulate the interaction of a museum tour guide and a browsing visitor; this interaction is described in more detail in Section 2. In Section 3, the browsing task provides the context for a general discussion of the factors which influence the ability of a hypertext system to function dynamically, and in the light of this discussion, some observations about existing dynamic hypertext systems. Section 4 then draws some conclusions concerning the real potential of dynamic hypertext.