Dynamic interfaces

We define a small class-based object-oriented language in which the availability of methods depends on an object’s abstract state: objects’ interfaces are dynamic. Each class has a session type which provides a global specification of the availability of methods in each state. A key feature is that the abstract state of an object may depend on the result of a method whose return type is an enumeration. Static typing guarantees that methods are only called when they are available. We present both a type system, in which the typing of a method specifies pre- and post-conditions for its object’s state, and a typechecking algorithm, which infers the pre- and post-conditions from the session type, and prove type safety results. Inheritance is included; a subtyping relation on session types, related to that found in previous literature, characterizes the relationship between method availability in a subclass and in its superclass. We illustrate the language and its type system with an example based on a Java-style iterator and a hierarchy of classes for accessing files, and conclude by outlining several ways in which our theory can be extended towards more practical languages