Comparing Three Semantics for Linda-like Languages1

A simple calculus based on generative communication is introduced; among its primitives, it contains a conditional input operation that tests for presence (or absence) of an output, reminiscent of the inp predicate of Linda. We study three different operational semantics for the output operation, called instantaneous, ordered and unordered. The associated behavioural semantics are obtained as the coarsest congruence contained in the corresponding strong barbed semantics. We prove that when the output operation is instantaneous, the obtained semantics is a sort of asynchronous bisimulation; on the contrary, for the ordered semantics, as well as for the unordered one, the resulting semantics is a small variant of the classic (synchronous) bisimulation. Further rusults are related to the expressiveness of the language. The language is Turing powerful under both the instantaneous and ordered semantics, but there exists a series of problems having solution under the instantaneous interpretation that cannot be solved under the ordered one. Instead, the language is not Turing powerful under the unordered semantics. Thus, we conclude that there exists a precise expressiveness hierarchy among the three semantics. 2. Work supported by Esprit Working Group n.24512 COORDINA. Full version of [BGZ97c].