Gerardo Costa

Weak and strong fairness in CCS

Abstract We examine the issue of weak and strong fairness in the framework of Milners CCS. Our approach is operational. We address the problem of giving sets of finite rules for generating all and only the admissible execution sequences when fairness is assumed. We achieve our aims by defining two calculi, one for weak and the other for strong fairness. Both calculi are extensions of standard CCS. In neither case do we appeal to random assignment or to transformations. A distinguishing feature of the weak fair calculus, unlike standard approaches which appeal to random assignment, is that it does not involve predictive choice.

A fair calculus of communicating systems

SummaryIn this paper we are concerned with an operational approach to fairness, the problem of defining and generating the fair execution sequences of a concurrent language. One solution invokes two semantic levels: one level (the positive) prescribes the finite and infinite execution sequences without regard to their fairness while the other (the negative) filters out the unfair ones. The first level is given as a set of generative rules whereas the second is encoded as a definition of fair execution sequence. Entirely positive approaches have been proposed which appeal to random assignment. Here we offer an alternative positive approach for a subset of Milners CCS. We show that rules can be given for generating just the fair sequences which avoid random assignment.

Abstract dynamic data types: A temporal logic approach

A concrete dynamic-data type is just a partial algebra with predicates such that for some of the sorts there is a special predicate defining a transition relation. An abstract dynamic-data type (ad-dt) is an isomorphism class of such algebras. To obtain specifications for ad-dt’s, we propose a logic which combines many-sorted first-order logic with branching-time combinators. We consider both an initial and a loose semantics for our specifications and give sufficient conditions for the existence of the initial models. Then we discuss structured specifications and implementation.

A Fair Calculus of Communicating Systems

We contrast a two level operational approach to fairness with a single level approach. The latter involves presenting a set of rules which generate all and only the fair execution sequences of a concurrent language. This we do for a subset of Milners CCS.

Distributive semantics for nondeterministic typed λ-calculi

Abstract In this paper we present a semantics for nondeterministic applicative languages based on the typed λ-calculus with a binary choice operator. It provides a natural interpretation of the choice operation and shows its relation with abstraction and application. A distinguishing feature of the semantics, that can also be considered as one of its motivations, is the preservation of deterministic equivalences, in a sense to be made precise in the paper. On the operational side, the semantics relies on a rewriting system, which follows an outermost strategy with sharing of arguments. The denotational counterpart is the distributivity of application with respect to the union operation. We present a full technical treatment of the case when sharing/distributivity is assumed with respect to ground arguments only and we have exemplified the approach on the language NDPCF , for which we show that a denotational semantics exists which is fully abstract with respect to the operational one. It is based on a construction of an intrinsically fully abstract model w.r.t. an interpretation of basic constant and function symbols.