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Giovanni Lagorio

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3 papers

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I&C Journal 2012 Journal Article

Featherweight Jigsaw — Replacing inheritance by composition in Java-like languages

Giovanni Lagorio
Marco Servetto
Elena Zucca

We present FJig, a simple calculus where basic building blocks are classes in the style of Featherweight Java, declaring fields, methods and one constructor. However, inheritance has been generalized to the much more flexible notion originally proposed in Brachaʼs Jigsaw framework. That is, classes play also the role of modules, that can be composed by a rich set of operators, all of which can be expressed by a minimal core. Fields and methods can be declared of four different kinds (abstract, virtual, frozen, local) determining how they are affected by the operators. We keep the nominal approach of Java-like languages, that is, types are class names. However, a class is not necessarily a structural subtype of any class used in its defining expression. While this allows a more flexible reuse, it may prevent the (generalized) inheritance relation from being a subtyping relation. So, the required subtyping relations among classes are declared by the programmer and checked by the type system. The calculus allows the encoding of a large variety of different mechanisms for software composition in class-based languages, including standard inheritance, mixin classes, traits and hiding. Hence, FJig can be used as a unifying framework for analyzing existing mechanisms and proposing new extensions. We provide two different semantics of an FJig program: flattening and direct semantics. The difference is analogous to that between two intuitive models to understand inheritance: the former where inherited methods are copied into heir classes, and the latter where member lookup is performed by ascending the inheritance chain. Here we address equivalence of these two views for a more sophisticated composition mechanism.

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GandALF Workshop 2010 Workshop Paper

Coinductive subtyping for abstract compilation of object-oriented languages into Horn formulas

Davide Ancona
Giovanni Lagorio

In recent work we have shown how it is possible to define very precise type systems for object-oriented languages by abstractly compiling a program into a Horn formula f. Then type inference amounts to resolving a certain goal w. r. t. the coinductive (that is, the greatest) Herbrand model of f. Type systems defined in this way are idealized, since in the most interesting instantiations both the terms of the coinductive Herbrand universe and goal derivations cannot be finitely represented. However, sound and quite expressive approximations can be implemented by considering only regular terms and derivations. In doing so, it is essential to introduce a proper subtyping relation formalizing the notion of approximation between types. In this paper we study a subtyping relation on coinductive terms built on union and object type constructors. We define an interpretation of types as set of values induced by a quite intuitive relation of membership of values to types, and prove that the definition of subtyping is sound w. r. t. subset inclusion between type interpretations. The proof of soundness has allowed us to simplify the notion of contractive derivation and to discover that the previously given definition of subtyping did not cover all possible representations of the empty type.

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TCS Journal 2006 Journal Article

A flexible model for dynamic linking in Java and C #

Sophia Drossopoulou
Giovanni Lagorio
Susan Eisenbach

Dynamic linking supports flexible code deployment, allowing partially linked code to link further code on the fly, as needed. Thus, end-users enjoy the advantage of automatically receiving any updates, without any need for any explicit actions on their side, such as re-compilation, or re-linking. On the down side, two executions of a program may link in different versions of code, which in some cases causes subtle errors, and may mystify end-users. Dynamic linking in Java and C # are similar: the same linking phases are involved, soundness is based on similar ideas, and executions which do not throw linking errors give the same result. They are, however, not identical: the linking phases are combined differently, and take place in different order. Consequently, linking errors may be detected at different times by Java and C # runtime systems. We develop a non-deterministic model, which describes the behaviour of both Java and C # program executions. The non-determinism allows us to describe the design space, to distill the similarities between the two languages, and to use one proof of soundness for both. We also prove that all execution strategies are equivalent with respect to terminating executions that do not throw link errors: they give the same results.

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