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M. Gabbrielli

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I&C Journal 2000 Journal Article

A Timed Concurrent Constraint Language

F.S. de Boer
M. Gabbrielli
M.C. Meo

We study a timed concurrent constraint language, called tccp, which is obtained by a natural timed interpretation of the usual ccp constructs: action-prefixing is interpreted as the next-time operator and the parallel execution of agents follows the scheduling policy of maximal parallelism. Additionally, tccp includes a simple primitive which allows one to specify timing constraints. We define the operational semantics of tccp by means of a transition system and we define a denotational model which is fully abstract with respect to the usual notion of observables (that is, the results of terminating computations). Moreover, we study the semantics and expressive power of the notion of maximal parallelism underlying the computational model of tccp: We define a fully abstract semantics for a sublanguage of tccp, called ccpm, which essentially is concurrent constraint programming, provided that we interpret the parallel operator in terms of maximal parallelism rather than of interleaving. We show that tccp is strictly more expressive than ccpm which, in its turn, is strictly more expressive than ccp.

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I&C Journal 1995 Journal Article

Observable Behaviors and Equivalences of Logic Programs

M. Gabbrielli
G. Levi
M.C. Meo

We first introduce a general semantic scheme for logic programs which provides a uniform framework for defining different compositional semantics parametrically with respect to a given notion of observability. The equivalence of the operational (top-down) and fixpoint (bottom-up) construction of the semantics is ensured by the scheme (provided a congruence property is verified). We then define several observational equivalences on logic programs and investigate how they are related. The equivalences are based on various observables (successful derivations, computed answers, partial computed answers and call patterns) and on a notion of program composition. For each observational equivalence we study the relation with a suitable formal semantics by investigating correctness and full abstraction properties. All the semantics we consider are obtained as instances of the general scheme.

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