C e n t r u m v o o r W i s k u n d e e n I n f o r m a t i c a Software ENgineeringMinimal ownership for active objects D.G. Clarke, T. Wrigstad, J. Östlund, E.B. Johnsen REPORT SEN-R0803 JUNE 2008Software Engineering Minimal ownership for active objects ABSTRACT Active objects offer a structured approach to concurrency, encapsulating both unshared state and a thread of control. For efficient data transfer, data should be passed by reference whenever possible, but this introduces aliasing and undermines the validity of the active objects. This paper proposes a minimal variant of ownership types that preserves the required race freedom invariant yet enables data transfer by reference between active objects (that is, without copying) in many cases, and a cheap clone operation where copying is necessary. Our approach is general and should be adaptable to several existing active object systems. Abstract Active objects offer a structured approach to concurrency, encapsulating both unshared state and a thread of control. For efficient data transfer, data should be passed by reference whenever possible, but this introduces aliasing and undermines the validity of the active objects. This paper proposes a minimal variant of ownership types that preserves the required race freedom invariant yet enables data transfer by reference between active objects (that is, without copying) in many cases, and a cheap clone operation where copying is necessary. Our approach is general and should be adaptable to several existing active object systems.

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Many large software systems originate from untyped scripting language code. While good for initial development, the lack of static type annotations can impact code-quality and performance in the long run. We present an approach for integrating untyped code and typed code in the same system to allow an initial prototype to smoothly evolve into an efficient and robust program. We introduce like types, a novel intermediate point between dynamic and static typing. Occurrences of like types variables are checked statically within their scope but, as they may be bound to dynamic values, their usage is checked dynamically. Thus like types provide some of the benefits of static typing without decreasing the expressiveness of the language. We provide a formal account of like types in a core object calculus and evaluate their applicability in the context of a new scripting language.

Programming in an object-oriented language demands a fine balance between flexibility and control. At one level, objects need to interact freely to achieve our implementation goals. At a higher level, architectural constraints that ensure the system can be understood by new developers and can evolve as requirements change must be met. To resolve this tension, researchers have developed type systems expressing ownership and behavioural restrictions such as immutability. This work reports on our consolidation of the resulting discoveries into a single programming language. Our language, Joe3, imposes little additional syntactic overhead, yet can encode powerful patterns such as fractional permissions and the reference modes of Flexible Alias Protection.