GOM: A Strongly Typed Persistent Object Model With Polymorphism

Abstract

In this paper the persistent object model GOM is described. GOM is an object-oriented data model that provides the most essential object features in a “lean” and coherent syntactical framework. These features include: object identity, object instantiation, subtyping and inheritance, operation refinement, dynamic (late) binding. One of the main goals in the design of GOM was type safety. In order to achieve this we developed a strongly typed language that enables the verification of type safety at compile time. It is shown in this paper how commonly encountered “traps” for strong typing are avoided in GOM by specifying a very clean subtyping semantics on the basis of substitutability and type signatures.

The typing rules that enforce strong typing at compile time somewhat restrain the flexibility of the object model because subtyping and inheritance has to be restricted—in particular—for collection-valued types. The solution to regain the expressiveness that was traded off for safety is by combining inheritance and explicitly controlled operation polymorphism. To make operations polymorphic signatures may contain type variables which are substituted by named types at compile time.