Behavioural Reactivity and Real Time Programming in XML: Functional Programming meets SMIL animation

XML and its associated languages are emerging as powerful authoring tools for multimedia and hypermedia web content. Furthermore, intelligent presentation generation engines have begun to appear, as have models and platforms for adaptive presentations. However, XML-based models are limited by their lack of expressiveness in presentation and animation. As a result, authors of dynamic, adaptive web content must often use considerable amounts of script or code. The use of such script or code has two serious drawbacks. First, such code undermines the declarative description possible in the original presentation language, and second, the scripting/coding approach does not readily lend itself to authoring by non-programmers. In this paper we describe a set of XML language extensions, inspired by features from the functional programming world, which are designed to widen the class of reactive systems which could be described in languages such as SMIL. The features which we discuss extend the power of declarative modeling for the web by allowing the introduction of web media items which may dynamically react to continuously varying inputs, both in a continuous way and by triggering discrete, user-defined, events. The two extensions described herein are discussed in the context of SMIL Animation and SVG, but could be applied to many XML-based languages.