Interactive rule-based specification with an application to visual language definition

In a rule-based approach the computation steps of a system are specified by rules that completely define how the system's state may change. For open systems a more liberal approach is required, where the state changes are only partly specified, and-interactively-other components may contribute further information on how the transformation is defined completely. In this paper we introduce a formal model for interactive rule-based specifications, where states are modelled as partial algebras and transformations are given by internal algebra rewritings and arbitrary external components. As an application we discuss how visual languages can be defined in this framework. Thereby the internal (logical) representations of visual expressions are transformed by rewriting rules, whereas their layouts are obtained interactively by external components like a constraint solver or a user working with a display and a mouse.