Dynamic Typing in Polymorphic Languages

matching protects the identity of \hidden" representation types and prevents accidental matches in cases where several abstract types happen to have the same representation. On the other hand, transparent matching allows a more permissive style of programming, where a dynamically typed value of some abstract type is considered to be a value of a di erent version of \the same" abstract type. This exibility is critical in many situations. For example, a program may create disk les containing dynamic values, which should remain usable even after the program is recompiled, or two programs on di erent machines may want to exchange abstract data in the form of dynamically typed values. By viewing abstract types formally as existential types [17], we can see exactly where the di erence between these two solutions lies, and suggest a generalization of existential types that supports both. (Existential types can in turn be coded using universal types; with this coding, our design for dynamic types in the previous sections yields the second solution.) To add existential types to the variant of F de ned in the previous section, we extend the syntax of types and terms as in Figure 3.