1. A non-transitory, computer-readable storage medium storing program instructions that when executed on a computing device cause the computing device to perform: specializing a generic class for a particular type parameterization of a plurality of type parameterizations, wherein the generic class is based on a generic class declaration, wherein the generic class is specializable over the plurality of type parameterizations; wherein to specialize the generic class for the particular type parameterization, the program instructions further cause the computing device to perform: loading a class corresponding to a refinement class declaration for the generic class declaration, wherein the refinement class declaration is specific to the particular type parameterization, wherein the refinement class declaration comprises an alternate implementation for the generic class when the generic class is specialized for the particular type parameterization; and wherein the generic class is specializable for others of the plurality of type parameterizations without loading the class corresponding to the refinement class declaration.