Semantic subtyping for objects and classes

In this paper we propose an integration of structural subtyping with boolean connectives and semantic subtyping to define a Java-like programming language that exploits the benefits of both techniques. Semantic subtyping is an approach for defining subtyping relation based on set-theoretic models, rather than syntactic rules. On the one hand, this approach involves some non trivial mathematical machinery in the background. On the other hand, final users of the language need not know this machinery and the resulting subtyping relation is very powerful and intuitive. While semantic subtyping is naturally linked to the structural one, we show how our framework can also accommodate the nominal subtyping. Several examples show the expressivity and the practical advantages of our proposal

Brand Objects for Nominal Typing

Combinations of structural and nominal object typing in systems such as Scala, Whiteoak, and Unity have focused on extending existing nominal, class-based systems with structural subtyping. The typical rules of nominal typing do not lend themselves to such an extension, resulting in major modifications. Adding object branding to an existing structural system integrates nominal and structural typing without excessively complicating the type system. We have implemented brand objects to explicitly type objects, using existing features of the structurally typed language Grace, along with a static type checker which treats the brands as nominal types. We demonstrate that the brands are useful in an existing implementation of Grace, and provide a formal model of the extension to the language

Modular session types for objects

Session types allow communication protocols to be specified type-theoretically so that protocol implementations can be verified by static type checking. We extend previous work on session types for distributed object-oriented languages in three ways. (1) We attach a session type to a class definition, to specify the possible sequences of method calls. (2) We allow a session type (protocol) implementation to be modularized, i.e. partitioned into separately-callable methods. (3) We treat session-typed communication channels as objects, integrating their session types with the session types of classes. The result is an elegant unification of communication channels and their session types, distributed object-oriented programming, and a form of typestate supporting non-uniform objects, i.e. objects that dynamically change the set of available methods. We define syntax, operational se-mantics, a sound type system, and a sound and complete type checking algorithm for a small distributed class-based object-oriented language with structural subtyping. Static typing guarantees that both sequences of messages on channels, and sequences of method calls on objects, conform to type-theoretic specifications, thus ensuring type-safety. The language includes expected features of session types, such as delegation, and expected features of object-oriented programming, such as encapsulation of local state.Comment: Logical Methods in Computer Science (LMCS), International Federation for Computational Logic, 201

KafKa: Gradual Typing for Objects

A wide range of gradual type systems have been proposed, providing many languages with the ability to mix typed and untyped code. However, hiding under language details, these gradual type systems embody fundamentally different ideas of what it means to be well-typed. In this paper, we show that four of the most common gradual type systems provide distinct guarantees, and we give a formal framework for comparing gradual type systems for object-oriented languages. First, we show that the different gradual type systems are practically distinguishable via a three-part litmus test. We present a formal framework for defining and comparing gradual type systems. Within this framework, different gradual type systems become translations between a common source and target language, allowing for direct comparison of semantics and guarantees