Hume box calculus: robust system development through software transformation

Hume is a contemporary programming language oriented to systems with strong resource bounds, based on autonomous concurrent “boxes” interacting across “wires”. Hume’s design reflects the explicit separation of coordination and computation aspects of multi-process systems, which greatly eases establishing resource bounds for programs. However, coordination and computation are necessarily tightly coupled in reasoning about Hume programs. Furthermore, in Hume, local changes to coordination or computation, while preserving input/output correctness, can have profound and unforeseen effects on other aspects of programs such as timing of events and scheduling of processes. Thus, traditional program calculi prove inappropriate as they tend to focus exclusively either on the coordination of interacting processes or on computation within individual processes. The Hume box calculus offers a novel approach to manipulating multi-process systems by accounting seamlessly for both coordination and computation in individual rules. Furthermore, the “Hierarchical Hume” extension enables strong locality of the effects of program manipulation, as well as providing a principled encapsulation mechanism. In this paper, we present an overview of the Hume box calculus and its applications in program development. First of all, a base set of rules for introducing, changing, composing, separating and eliminating Hume boxes and wires, possibly within hierarchies, is presented. Next additional strategies are derived and a constructive approach to program development is illustrated through two examples of system elaboration from truth tables. Finally, at a considerably higher level, the use of the Hume box calculus to verify a generic transformation from a single box to an equivalent multi-box program, offering a balanced parallel implementation, is discussed.