Theory and Practice of Action Semantics

Action Semantics is a framework for the formal descriptionof programming languages. Its main advantage over other frameworksis pragmatic: action-semantic descriptions (ASDs) scale up smoothly torealistic programming languages. This is due to the inherent extensibilityand modifiability of ASDs, ensuring that extensions and changes tothe described language require only proportionate changes in its description.(In denotational or operational semantics, adding an unforeseenconstruct to a language may require a reformulation of the entire description.)After sketching the background for the development of action semantics,we summarize the main ideas of the framework, and provide a simpleillustrative example of an ASD. We identify which features of ASDsare crucial for good pragmatics. Then we explain the foundations ofaction semantics, and survey recent advances in its theory and practicalapplications. Finally, we assess the prospects for further developmentand use of action semantics.The action semantics framework was initially developed at the Universityof Aarhus by the present author, in collaboration with David Watt(University of Glasgow). Groups and individuals scattered around fivecontinents have since contributed to its theory and practice

Logical and Computational Aspects of Programming With Sets/Bags/Lists

We study issues that arise in programming with primitive recursion over non-free datatypes such as lists, bags and sets. Programs written in this style can lack a meaning in the sense that their outputs may be sensitive to the choice of input expression. We are, thus, naturally led to a set-theoretic denotational semantics with partial functions. We set up a logic for reasoning about the definedness of terms and a deterministic and terminating evaluator. The logic is shown to be sound in the model, and its recursion free fragment is shown to be complete for proving definedness of recursion free programs. The logic is then shown to be as strong as the evaluator, and this implies that the evaluator is compatible with the provable equivalence between different set (or bag, or list) expressions. Oftentimes, the same non-free datatype may have different presentations, and it is not clear a priori whether programming and reasoning with the two presentations are equivalent. We formulate these questions, precisely, in the context of alternative presentations of the list, bag, and set datatypes and study some aspects of these questions. In particular, we establish back-and-forth translations between the two presentations, from which it follows that they are equally expressive, and prove results relating proofs of program properties, in the two presentations

Continuation semantics for PROLOG with cut

We present a denotational continuation semantics for PROLOG with cut. First a uniform language B is studied, which captures the control flow aspects of PROLOG. The denotational semantics for B is proven equivalent to a transition system based operational semantics. The congruence proof relies on the representation of the operational semantics as a chain of approximations and on a convenient induction principle. Finally, we interpret the abstract language B such that we obtain equivalent denotational and operational models for PROLOG itself

On the Action Semantics of Concurrent Programming Languages

Action semantics is a framework for semantic description of prograrnming languages. In this framework, actions are semantic entities, used to represent the potential behaviour of programs --- also the contributions that parts of programs make to such behaviour. The notation for expressing actions, called action notation, is combinator-based. It is used in much the same way that lambda-notation is used in denotational semantics. However, the essence of action notation is operational, rather than mathematical, and its meaning is formally defined by a structural operational semantics together with a bisimulation equivalence.This paper briefly motivates action semantics, and explains the basic concepts. It then illustrates the use of the framework by giving an action semantic description of a small example language. This language includes a simple form of concurrency: tasks that may synchronize by means of rendezvous. The paper also discusses the operational semantics of action notation, focusing on the primitive actions that represent asynchronous message transmission and process initiation

The action semantics of object-oriented languages

Action Semantics is a framework for defining the semantics of languages. It is intended to be accessible to a wider audience of Computer Scientists than traditional semantics frameworks (such as Denotational Semantics). There has been little work carried out to date on the techniques required to define object-oriented languages with Action Semantics. The work presented in this thesis examines four potential approaches to defining the Action Semantics of object-oriented languages. In order to illustrate the four approaches a simple language EIL (Example Inheritance Language) is given, and described using these four approaches. The language Smalltalk-80 has been selected for a case study of a practical application of one of the techniques described above. It is important to be able to relate Action Semantics definitions of object-oriented languages to similar definitions given in other frameworks. It is described how this can be achieved. An example is given for the Action Semantics and Denotational Semantics of Smalltalk. This thesis concludes that it is feasible to produce Action Semantics definitions of object-oriented languages