The importance of typed languages for building robust software systems is, by now, an undisputed fact. Years of research have led to languages with richly expressive, yet easy to use, type systems for high-level programming languages. Types provide not only a conceptual framework for language designers, but also a ord positive bene ts to the programmer, principally the ability to express and enforce levels of abstraction within a program. Early compilers for typed languages followed closely the methods used for their untyped counterparts. The role of types was limited to the earliest s- ges...