A Modular Language for Concurrent Programming

How will programmers respond to the long-promised con-currency revolution, which now appears both inevitable and imminent? One common answer is “by adding threads to objects”. This paper presents an alternative answer that we believe will reduce rather than add complexity to the soft-ware of the future. Building on the ideas of an earlier gener-ation, we propose a modern programming language based on message passing. A module cannot invoke a method in an-other module, but can only send data to it. Modules may be constructed from other modules, thus permitting processes within processes. Our goal is to provide the flexibility and expressiveness of concurrent programming while limiting, as much as possible, the complexity caused by nondeterminism. The principle innovations reported in the paper derive from bringing together ideas—some well known, but others al-most forgotten—found in the historical software literature, and combining these ideas to solve problems facing modern software developers. In addition, at least one idea reported here appears to be novel, namely the introduction of an in-terface hierarchy based not on data elements or methods, but on path expressions, on the actual flow of control within a module. It is more natural to classify components of a process-oriented system by control flow rather than data content. Another novel feature is the integration of unit tests into the source of each component, thus reducing the possibilities for testing to get out of step with development. 1