Typechecking protocols with Mungo and StMungo: a session type toolchain for Java

Static typechecking is an important feature of many standard programming languages. However, static typing focuses on data rather than communication, and therefore does not help programmers correctly implement communication protocols in distributed systems. The theory of session types provides a basis for tackling this problem; we use it to develop two tools that support static typechecking of communication protocols in Java. The first tool, Mungo, extends Java with typestate definitions, which allow classes to be associated with state machines defining permitted sequences of method calls: for example, communication methods. The second tool, StMungo, takes a session type describing a communication protocol, and generates a typestate specification of the permitted sequences of messages in the protocol. Protocol implementations can be validated by Mungo against their typestate definitions and then compiled with a standard Java compiler. The result is a toolchain for static typechecking of communication protocols in Java. We formalise and prove soundness of the typestate inference system used by Mungo, and show that our toolchain can be used to typecheck a client for the standard Simple Mail Transfer Protocol (SMTP)

Safe Compositional Specification of Network Systems With Polymorphic, Constrained Types

In the framework of iBench research project, our previous work created a domain specific language TRAFFIC [6] that facilitates specification, programming, and maintenance of distributed applications over a network. It allows safety property to be formalized in terms of types and subtyping relations. Extending upon our previous work, we add Hindley-Milner style polymorphism [8] with constraints [9] to the type system of TRAFFIC. This allows a programmer to use for-all quantifier to describe types of network components, escalating power and expressiveness of types to a new level that was not possible before with propositional subtyping relations. Furthermore, we design our type system with a pluggable constraint system, so it can adapt to different application needs while maintaining soundness. In this paper, we show the soundness of the type system, which is not syntax-directed but is easier to do typing derivation. We show that there is an equivalent syntax-directed type system, which is what a type checker program would implement to verify the safety of a network flow. This is followed by discussion on several constraint systems: polymorphism with subtyping constraints, Linear Programming, and Constraint Handling Rules (CHR) [3]. Finally, we provide some examples to illustrate workings of these constraint systems.National Science Foundation (CCR-0205294

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