A Type-Safe Model of Adaptive Object Groups

Services are autonomous, self-describing, technology-neutral software units that can be described, published, discovered, and composed into software applications at runtime. Designing software services and composing services in order to form applications or composite services requires abstractions beyond those found in typical object-oriented programming languages. This paper explores service-oriented abstractions such as service adaptation, discovery, and querying in an object-oriented setting. We develop a formal model of adaptive object-oriented groups which offer services to their environment. These groups fit directly into the object-oriented paradigm in the sense that they can be dynamically created, they have an identity, and they can receive method calls. In contrast to objects, groups are not used for structuring code. A group exports its services through interfaces and relies on objects to implement these services. Objects may join or leave different groups. Groups may dynamically export new interfaces, they support service discovery, and they can be queried at runtime for the interfaces they support. We define an operational semantics and a static type system for this model of adaptive object groups, and show that well-typed programs do not cause method-not-understood errors at runtime.Comment: In Proceedings FOCLASA 2012, arXiv:1208.432

Simple Reference Immutability for System F-sub

Reference immutability is a type based technique for taming mutation that has long been studied in the context of object-oriented languages, like Java. Recently, though, languages like Scala have blurred the lines between functional programming languages and object oriented programming languages. We explore how reference immutability interacts with features commonly found in these hybrid languages, in particular with higher-order functions -- polymorphism -- and subtyping. We construct a calculus System F-sub-M which encodes a reference immutability system as a simple extension of F-sub and prove that it satisfies the standard soundness and immutability safety properties.Comment: 25 page

Designing type inference for typed object-oriented languages

Type-checked object-oriented languages have typically been designed with extremely simple type systems. However, there has recently been intense interest in extending such languages with more sophisticated types and subtyping relationships. JAVA and C# are mainstream languages that have been successfully extended with generic classes and methods; SCALA, FORTRESS, and X10 are new languages that adopt more advanced typing features, such as arrows, tuples, unions, intersections, dependent types, and existentials. Presently, the type inference performed by these languages is unstable and evolving. This thesis explores problems arising in the design of a type inference specification for such languages. We first present a formal description of subtyping in the context of a variety of advanced typing features. We then demonstrate how our formal subtyping algorithm can be easily re-expressed to produce a type inference algorithm, and observe that this algorithm is general enough to address a variety of important type-checking problems. Finally, we apply this theory to a case study of the JAVA language's type system. We express JAVA'S types and inference algorithm in terms of our formal theory and note a variety of opportunities for improvement. We then describe the results of applying an improved type inference implementation to a selection of existing JAVA code, noting that, without introducing significant backwards-incompatibility problems for these programs, we've managed to significantly reduce the need for annotated method invocations

Towards Strong Normalization for Dependent Object Types (DOT)

The Dependent Object Types (DOT) family of calculi has been proposed as a new theoretic foundation for Scala and similar languages, unifying functional programming, object oriented programming and ML-style module systems. Following the recent type soundness proof for DOT, the present paper aims to establish stronger meta-theoretic properties. The main result is a fully mechanized proof of strong normalization for D_<:, a variant of DOT that excludes recursive functions and recursive types. We further discuss techniques and challenges for adding recursive types while maintaining strong normalization, and demonstrate that certain variants of recursive self types can be integrated successfully

Hidden Type Variables and Conditional Extension for More Expressive Generic Programs

Generic object-oriented programming languages combine parametric polymorphism and nominal subtype polymorphism, thereby providing better data abstraction, greater code reuse, and fewer run-time errors. However, most generic object-oriented languages provide a straightforward combination of the two kinds of polymorphism, which prevents the expression of advanced type relationships. Furthermore, most generic object-oriented languages have a type-erasure semantics: instantiations of type parameters are not available at run time, and thus may not be used by type-dependent operations. This dissertation shows that two features, which allow the expression of many advanced type relationships, can be added to a generic object-oriented programming language without type erasure: 1. type variables that are not parameters of the class that declares them, and 2. extension that is dependent on the satisfiability of one or more constraints. We refer to the first feature as hidden type variables and the second feature as conditional extension. Hidden type variables allow: covariance and contravariance without variance annotations or special type arguments such as wildcards; a single type to extend, and inherit methods from, infinitely many instantiations of another type; a limited capacity to augment the set of superclasses after that class is defined; and the omission of redundant type arguments. Conditional extension allows the properties of a collection type to be dependent on the properties of its element type. This dissertation describes the semantics and implementation of hidden type variables and conditional extension. A sound type system is presented. In addition, a sound and terminating type checking algorithm is presented. Although designed for the Fortress programming language, hidden type variables and conditional extension can be incorporated into other generic object-oriented languages. Many of the same problems would arise, and solutions analogous to those we present would apply

Advanced flow-based type systems for object-oriented languages

Ph.DDOCTOR OF PHILOSOPH