Static Type Inference for Parametric Classes

Central features of object-oriented programming are method inheritance and data abstraction attained through hierarchical organization of classes. Recent studies show that method inheritance can be nicely sup-ported by ML style type inference when extended to labeled records. This is based on the fact that a func-tion that selects a field f of a record can be given a polymorphic type that enables it to be applied to any record which contains a field f. Several type systems also provide data abstraction through abstract type dec-larations. However, these two features have not yet been properly integrated in a statically checked polymorphic type system. This paper proposes a static type system that achieves this integration in an ML-like polymorphic lan-guage by adding a class construct that allows the pro-grammer to build a hierarchy of classes connected by multiple inheritance declarations. Moreover, classes can be parameterized by types allowing “generic ” defini-tions. The type correctness of class declarations is stat-ically checked by the type system. The type system also infers a principal scheme for any type correct program containing methods and objects defined in classes.