Extended Object-Typing for Selecting Specialized Behavior from Subclasses

In statically typed object-oriented languages, reusable libraries of pre-compiled components can be used in type-safe manner without requiring a compiler to type check an already compiled class. A particular problem with object-oriented languages is determining whether a method provided in a superclass will continue to type check correctly when inherited in a subclass. In this paper, we address the problem of type checking a method body such that the most specialized behaviour for an object is determined without restricting the type of methods in subclasses to be identical to those in superclasses and without introducing special keywords for such methods. We present a solution in a static type system via extensions to an object's typing information. Our approach gives a precise meaning to the variations of receiver type and argument type, and increases the expressibility of a persistent object-oriented programming language called POOL