Prof. Ole Lehrmann Madsen
Computer Science Department
University of Aarhus

Virtual classes and concurrency abstractions in BETA

One of the unique characteristics of BETA is the unification of abstraction mechanisms such as class, procedure, function, process type, generic class, interface, exception type, etc. into one abstraction mechanism: the pattern.

By abstraction mechanism, we mean a language construct that defines a template or pattern for creating instances. A class is a pattern for creating objects, a procedure is a pattern for creating procedure activations, a process type is a pattern for creating processes, etc. Besides keeping the language small, the unification has given a systematic treatment of all abstraction mechanisms and lead to a number of new possibilities: inheritance is also available for procedures, and virtual classes may be seen as an object-oriented version of generics.

At the primitive level BETA supports simple coroutines, a fork-construct and semaphores. By means of virtual classes and the innermechanism for method combination, it is possible to define high-level concurrency abstractions - including schedulers for active objects - on top of these primitives.

The talk will mainly present the virtual class mechanism and give examples of how to define concurrency abstractions like monitor, Ada-like rendezvous, futures and a non-preemptive form for scheduling called alternation.