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Abstract. We present a minimal core calculus that captures interesting constructs of the Scala programming language: nested classes, abstract types, mixin composition, and path dependent types. We show that the problems of type assignment and subtyping in this calculus are decidable.

Transactions are commonly described as being ACID: Allor-nothing, Consistent, Isolated and Durable. However, although these words convey a powerful intuition, the ACID properties have never been given a precise semantics in a way that disentangles each property from the others. Among the benefits of such a semantics would be the ability to trade-off the value of a property against the cost of its implementation. This paper gives a sound equational semantics for the transaction properties. We define three categories of actions, A-actions, I-actions and D-actions, while we view Consistency as an induction rule that enables us to derive system-wide consistency from local consistency. The three kinds of action can be nested, leading to different forms of transactions, each with a well-defined semantics. Conventional transactions are then simply obtained as ADI-actions. From the equational semantics we develop a formal proof principle for transactional programs, from which we derive the induction rule for Consistency.

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PiLib is a library written in Scala that implements the concurrency constructs of the π-calculus. Some features of the programming language Scala, examined in the paper, make it possible to use almost the same syntax as in the π-calculus. The advantages of this library with respect to a simple π-calculus interpreter are that we can transmit any value along names, we can get control over them using the type system, and we have access to the full power of Scala in terms of expressiveness and libraries.