We adopt the untyped imperative object calculus of Abadi and Cardelli as a minimal setting in which to study problems of compilation and program equivalence that arise when compiling objectoriented languages. We present both a big-step and a small-step substitution-based operational semantics for the calculus. Our rst two results are theorems asserting the equivalence of our substitutionbased semantics with a closure-based semantics like that given by Abadi and Cardelli. Our third result is a direct proof of the correctness of compilation to a stack-based abstract machine via a small-step decompilation algorithm. Our fourth result is that contextual equivalence of objects coincides with a form of Mason and Talcott's CIU equivalence; the latter provides a tractable means of establishing operational equivalences. Finally, we prove correct an algorithm, used in our prototype compiler, for statically resolving method osets. This is the rst study of correctness of an object-oriented abstract machine, and of operational equivalence for the imperative object calculus.

Software executing on distributed systems consists of many asynchronous, autonomous components which interact in order to coordinate local activity. The need for such coordination, as well as requirements such as heterogeneity, scalability, security and availability, considerably increase the complexity of code in distributed applications. Moreover, changing requirements, as well as changes in hardware platforms, lead to software that is constantly evolving and complicates reuse. To support development and evolution of distributed applications requires techniques which allow coordination code to be specified, customized, and maintained independently of application components; goals which cannot be realized solely through object-oriented techniques. This thesis demonstrates that meta-level specification of interaction policies enables modular description of component interaction policies, as well as customization of policy implementations. We present the high-level language Dil for spec...

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Machine The semantics presented in this section is essentially Sestoft's \mark 1" abstract machine for laziness [Sestoft 1997]. In that paper, he proves his abstract machine 6 A. K. Moran and D. Sands h fx = Mg; x; S i ! h ; M; #x : S i (Lookup) h ; V; #x : S i ! h fx = V g; V; S i (Update) h ; M x; S i ! h ; M; x : S i (Unwind) h ; x:M; y : S i ! h ; M [ y = x ]; S i (Subst) h ; case M of alts ; S i ! h ; M; alts : S i (Case) h ; c j ~y; fc i ~x i N i g : S i ! h ; N j [ ~y = ~x j ]; S i (Branch) h ; let f~x = ~ Mg in N; S i ! h f~x = ~ Mg; N; S i ~x dom(;S) (Letrec) Fig. 1. The abstract machine semantics for call-by-need. semantics sound and complete with respect to Launchbury's natural semantics, and we will not repeat those proofs here. Transitions are over congurations consisting of a heap, containing bindings, the expression currently being evaluated, and a stack. The heap is a partial function from variables to terms, and denoted in an identical manner to a coll...

This paper aims at providing confluence and determinism properties in concurrent processes, more specifically within the paradigm of object-oriented systems. Such results should allow one to program parallel and distributed applications that behave in a deterministic manner, even if they are distributed over local or wide area networks. For that purpose, an object calculus is proposed. Its key characteristics are asynchronous communications with futures, and sequential execution within each process. While

Action Semantics is a framework for the formal description of programming languages. Its main advantage over other frameworks is pragmatic: action-semantic descriptions (ASDs) scale up smoothly to realistic programming languages. This is due to the inherent extensibility and modifiability of ASDs, ensuring that extensions and changes to the described language require only proportionate changes in its description.

We study trace and may-testing equivalences in the asynchronous versions of CCS and -calculus. We start from the operational definition of the may-testing preorder and provide for it finitary and fully abstract trace-based characterizations, along with a complete in-equational proof system. We also touch upon two variants of this theory, by first considering a more demanding equivalence notion (must-testing) and then a richer version of asynchronous CCS. The results throw light on the difference between synchronous and asynchronous communication and on the weaker testing power of asynchronous observations. Keywords: Asynchronous Communications, Process Algebras, Semantics. This paper is an extended and revised version of [8] and [9]. 1 Contents 1 Introduction 3 2 Asynchronous CCS 5 2.1 Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.2 Operational semantics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.3 May-testing semant...

We define three semantic models for actor computation starting with a generalization to open systems of Clinger's event diagram model, and forming two abstractions: interaction diagrams and interaction paths. An algebra is defined on each semantic domain with operations for parallel composition, hiding of internal actors, and renaming. We use these models to provide semantics for descriptions of actor components based on actor theories and show that the semantics is a component algebra homomorphism. 1

Object-oriented distributed computing is becoming increasingly important for critical infrastructure in society. In standard object-oriented models, objects synchronize on method calls. These models may be criticized in the distributed setting for their tight coupling of communication and synchronization; network delays and instabilities may locally result in much waiting and even deadlock. The Creol model targets distributed objects by a looser coupling of method calls and synchronization. Asynchronous method calls and high-level local control structures allow local computation to adapt to network instability. Object variables are typed by interfaces, so communication with remote objects is independent from their implementation. The inheritance and subtyping relations are distinct in Creol. Interfaces form a subtype hierarchy, whereas multiple inheritance is used for code reuse at the class level. This paper presents the Creol syntax, operational semantics, and type system. It is shown that runtime type errors do not occur for well-typed programs.

We present a semantic framework for actor systems based on rewriting logic. This framework accounts for fairness and provides a variety of semantics for actor system components that have good composability properties. 1 Introduction We are interested in developing a semantic foundation for open distributed systems that supports specifying, composing, and reasoning about components of open distributed systems. We take the actor model of computation [12,1,2] as our starting point. Actors are independent computational agents that interact solely via message passing. An actor can create other actors; send messages; and modify its own local state. An actor can only effect the local state of other actors by sending them messages, and it can only send messages to its acquaintances -- addresses of actors it was given upon creation, it received in a message or actors it created. Actor semantics requires computations to be fair. We take two views of actors: as individuals and as elements of com...