Thesis Proposal: Data Representations and Polymorphic Languages

Languages that provide polymorphism tend to place restrictions on object representations, separate compilation, and/or performance. For instance, most Ada implementations require that polymorphism be resolved before compilation. Consequently, separate compilation is sacriiced, since eliminating poly-morphism is a cross-module transformation. Some languages, like C++ and Modula-3, restrict the representations of polymorphic objects to \pointer" types. These languages provide good separate compilation but restrict programmers from writing certain types of polymorphic code. In addition, some runtime overhead might be paid due to the extra pointer indirection. Languages such as Scheme, Lisp, SML, and Haskell, place no explicit restrictions on the representations of objects. However, almost all implementations use the \pointer" restriction to implement the polymorphic features. In fact, most implementations make every object a pointer. This allows the languages to support very exible forms of polymorphism and separate compilation. However, the performance of such languages suuers due to the space and time overheads introduced by the indirection. The goal of this work is to show that polymorphic languages can be implemented without restrictions on object representations or separate compilation. Furthermore, by lifting these restrictions, programmers and compilers can choose representations that lead to superior performance.