A Type System that Reconsiles Classes and Extents

We present a type system that naturally couples two diierent, and apparently contradictory, notions of inheritance that occur in object-oriented databases. To do this we distinguish between the type and a kind of a value. A type describes the entire structure of a value, while a kind describes only the availability of certain elds or methods. This distinction allows us to manipulate heterogeneous collections (collections of values with diiering types) in a statically type-checked language. Moreover, the type system is polymorphic and types may be inferred using an extension of the technique used in ML. This means that it is easy to express general-purpose operations for the manipulation of heterogeneous collections. We believe that this system not only provides a natural approach to static type-checking in object-oriented databases; it also ooers a technique for dealing with external databases in a statically typed language.