Assigning Types to Processes

Abstract

In wide area distributed systems it is now common for higher-order code to be transferred from one domain to another; the receiving host may initialise parameters and then execute the code in its local environment. We propose a fine-grained typing system for a higher-order ?-calculus which can be used to control the effect of such migrating code on local environments. Processes may be assigned different types depending on their intended use. This is in contrast to most of the previous work on typing processes where all processes are typed by a unique constant type, indicating essentially that they are well-typed relative to a particular environment. Our process type takes a form of an interface limiting the resources to which it has access, and the types at which they may be used. Allowing resource names to appear both in process types and process terms, as interaction ports, complicates the typing system considerably. For the development of a coherent typing system, we use a kinding technique, similar to that used by the subtyping of the system F, and order-theoretic properties of our subtyping relation. Various examples illustrate the use of our fine-grained typing system for distributed systems. As a specific application we define a new typed behavioural equivalence for the higher-order ?-calculus. The expressiveness of our types enables us to state and prove interesting identities between typed processes