Flexible Types for a Concurrent Model

Subtyping is undoubtedly useful for the support of incremental reenement and reuse of software components , a crucial feature of object-oriented languages. Types and subtyping for concurrent languages are not yet explored suuciently and tend to be less exible than desired. We propose a principle of dealing with very exible types in concurrent languages. The corresponding type model ensures that \message not un-derstood" errors cannot occur even if object behavior can be dynamically modiied in some ways.