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ABSTRACT

The actor message-passing model of concurrent computation has inspired new ideas in the areas of knowledge-based systems, programming languages and their semantics, and computer systems architecture. This thesis extends and unifies the work of Carl Hewitt, Irene Greif, Henry Baker, and Giuseppe Attardi, who developed the mathematical content of the model. The ordering laws postulated by Hewitt and Baker can be proved using a notion of global time. The most general ordering laws are equivalent to an axiom of realizability in global time. Since nondeterministic concurrency is more fundamental than deterministic sequential computation, there may be no need to take fixed points in the underlying domain of a power domain. Power domains built from incomplete domains can solve the problem of providing a fixed point semantics for a class of nondeterministic programming languages in which a fair merge can be written. The locality laws postulated by Hewitt and Baker may be proved for the semantics of an actor-based language. Altering the semantics slightly can falsify the locality laws. The locality laws thus constrain what counts as an actor semantics.