Giuseppe Castagna

CDuce: an XML-centric general-purpose language

We present the functional language CDuce, discuss some design issues, and show its adequacy for working with XML documents. Distinctive features of CDuce are a powerful pattern matching, first class functions, overloaded functions, a very rich type system (arrows, sequences, pairs, records, intersections, unions, differences), precise type inference for patterns and error localization, and a natural interpretation of types as sets of values. We also outline some important implementation issues; in particular, a dispatch algorithm that demonstrates how static type information can be used to obtain very efficient compilation schemas..

A theory of contracts for Web services

Contracts are behavioral descriptions of Web services. We devise a theory of contracts that formalizes the compatibility of a client with a service, and the safe replacement of a service with another service. The use of contracts statically ensures the successful completion of every possible interaction between compatible clients and services. The technical device that underlies the theory is the filter, which is an explicit coercion preventing some possible behaviors of services and, in doing so, make services compatible with different usage scenarios. We show that filters can be seen as proofs of a sound and complete subcontracting deduction system which simultaneously refines and extends Hennessys classical axiomatization of the must testing preorder. The relation is decidable, and the decision algorithm is obtained via a cut-elimination process that proves the coherence of subcontracting as a logical system. Despite the richness of the technical development, the resulting approach is based on simple ideas and basic intuitions. Remarkably, its application is mostly independent of the language used to program the services or the clients. We outline the practical aspects of our theory by studying two different concrete syntaxes for contracts and applying each of them to Web services languages. We also explore implementation issues of filters and discuss the perspectives of future research this work opens.

A calculus for overloaded functions with subtyping

We present a simple extension of typed λ-claculus where functions can be overloaded by adding different “pieces of code”. In short, the code of an overloaded function is formed by several branches of code; the branch to execute is chosen, when the function is applied, according to a particular selection rule which depends on the type of the argument. The crucial feature of the present approach is that a subtyping relation is defied among types, such that the type of a term generally decreases during computation, and this fact induces a distinction between the “compile-time” type and the “run-time”type of a term. We study the case of overloaded functions where the branch selection depends on the run-time type of the argument, so that overloading cannot be eliminated by a static analysis of code, but is an essential feature to be dealt with during computation. We prove Confluence and Strong Normalization for this calculus as well as a generalized Subject-Reduction theorem (but proofs are ommitted in this abstract. The definition of this calculus is driven by the understanding of object-oriented features and the connections between our calculus and object-orientedness are extensively stressed. We show that this calculus provides a foundation for typed object-oriented languages which solves some of the problems of the standard record-based approach. It also provides a type-discipline for a relevant fragment of the “core framework” of CLOS.

Foundations of session types

We present a streamlined theory of session types based on a simple yet general and expressive formalism whose main eatures are semantically characterized and where each design choice is semantically justified. We formally define the semantics of session types and use it to devise the subsessioning relation. We give a coinductive characterization of subsessioning and describe algorithms to decide all the key relations defined in the article. We demonstrate the generality and expressive power of our framework by providing a session-based type system for a pi-calculus variant that does not rely on any specialized construct for session-based communication. The type system is shown to guarantee absence of communication errors and global progress.

Contracts for Mobile Processes

Theories identifying well-formed systems of processes--those that are free of communication errors and enjoy strong properties such as deadlock freedom--are based either on session types, which are inhabited by channels, or on contracts, which are inhabited by processes. Current session type theories impose overly restrictive disciplines while contract theories only work for networks with fixed topology. Here we fill the gap between the two approaches by defining a theory of contracts for so-called mobile processes, those whose communications may include delegations and channel references.

A gentle introduction to semantic subtyping

Subtyping relations are usually defined either syntactically by a formal system or semantically by an interpretation of types into an untyped denotational model. In this work we show step by step how to define a subtyping relation semantically in the presence of functional types and dynamic dispatch on types, without the complexity of denotational models, and how to derive a complete subtyping algorithm. It also provides a recipe to add set-theoretic union, intersection, and negation types to your favourite language.The presentation is voluntarily kept informal and discursive and the technical details are reduced to a minimum since we rather insist on the motivations, the intuition, and the guidelines to apply the approach.

The Seal Calculus Revisited: Contextual Equivalence and Bisimilarity

We present a new version of the Seal Calculus, a calculus of mobile computation. We study observational congruence and bisimulation theory, and show how they are related.

On global types and multi-party sessions

We present a new, streamlined language of global types equipped with a trace-based semantics and whose features and restrictions are semantically justified. The multi-party sessions obtained projecting our global types enjoy a liveness property in addition to the traditional progress and are shown to be sound and complete with respect to the set of traces of the originating global type. Our notion of completeness is less demanding than the classical ones, allowing a multiparty session to leave out redundant traces from an underspecified global type.

Decidable bounded quantification

The standard formulation of bounded quantification, system F≤, is difficult to work with and lacks important syntactic properties, such as decidability. More tractable variants have been studied, but those studied so far either exclude significant classes of useful programs or lack a compelling semantics. We propose here a simple variant of F≤ that ameliorates these difficulties. It has a natural semantic interpretation, enjoys a number of important properties that fail in F≤, and includes all of the programming examples for which F≤ has been used in practice.

A meta-language for typed object-oriented languages

In [3] we defined the λ&-calculus, a simple extension of the typed λ-calculus to model typed object-oriented languages. To develop a formal study of type systems for object-oriented languages we define, in this paper, a meta-language based on λ& and we show by a practical example how to use it to prove properties of a language. To this purpose we define a toy object-oriented language and its type-checking algorithm; then we translate this toy language into our meta-language. The translation gives the semantics of the toy language and a theorem on the translation of well-typed programs proves the correction of the type-checker of the toy language.