System-Level Types for Component-Based Design

We present a framework to extend the concept of type systems in programming languages to capture the dynamic interaction in component-based design, such as the communication protocols between components. In our system, the interaction types and the dynamic behavior of components are defined using interface automata - an automata-based formalism. Type checking, which checks the compatibility of a component with a certain interaction type, is conducted through automata composition. Our type system is polymorphic in that a component may be compatible with more than one interaction type. We show that a subtyping relation exists among various interaction types and this relation can be described using a partial order. This system-level type order can be used to facilitate the design of polymorphic components and simplify type checking. In addition to static type checking, we also propose to extend the use of interface automata to the on-line reflection of component states and to run-time type checking. We illustrate our framework using a component-based design environment, Ptolemy II, and discuss the trade-offs in the design of system-level type systems.