Half & Half : Multiple

Software often goes through a variety of extensions during its lifetime: adding new fields or new variants to a data structure, retroactively creating new type abstractions, and adding new operations on a data structure. As characterized by the extensibility problem, it should be possible to apply any combination of these types of extensions in any order. Mainstream object-oriented languages, however, do not well support the latter two. This paper proposes two language mechanisms that facilitate extending existing type hierarchies: multimethod dispatch and retroactive abstraction. For these two mechanisms to coexist, it is necessary to allow method dispatch on parameters of interface types, which presents problems with static type-checking. We present a type-safe solution that combines the two mechanisms by limiting multimethod type checks to package boundaries and by compiling certain packages with multimethods into sealed Jar files.