Visual formalisms revisited

The development of an interactive application is a complex task that has to consider data, behavior, inter- communication, architecture and distribution aspects of the modeled system. In particular, it presupposes the successful communication between the customer and the software expert. To enhance this communica- tion most modern software engineering methods rec- ommend to specify the different aspects of a system by visual formalisms. In essence, visual specifications are directed graphs that are interpreted in a particular way for each as- pect of the system. They are also intended to be com- positional. This means that, each node can itself be a graph with a separate meaning. However, the lack of a denotational model for hierarchical graphs often leads to the loss of compositionality. This has severe negative consequences in the development of realistic applications. In this paper we present a simple denotational model (which is by definition compositional) for the architecture and behavior aspects of a system. This model is then used to give a semantics to almost all the concepts occurring in ROOM. Our model also provides a compositional semantics for or-states in statecharts.