Adding Types to Untyped Languages

Over the last 15 years, we have experienced a programming language renaissance. Numerous scripting languages have become widely used in industrial and open-source projects. They have supplemented the existing mainstream languages-C++ and Java and, in contexts such as systems administration and web programming, they have started to play a dominant role. While each scripting language comes with its own philosophy, their designers share an antipathy to types. As a result, these languages come without a static type system. Most script developers initially welcome this freedom, but soon discover that the lack of a type system deprives them of an essential maintenance tool. My keynote explains my team's approach to equip such languages with a type system. The goal of our work is to empower programmers so that they can gradually enrich scripts with types on a module-by-module basis as they perform maintenance work on the system. Naturally, we wish to ensure type soundness so that the type annotations are meaningful, and we wish to accommodate the programming idioms of the original language in order to keep the overhead of type enrichment low.