Progressive types

Abstract

As modern type systems grow ever-richer, it can become increasingly onerous for programmers to satisfy them. However, some programs may not require the full power of the type system, while others may wish to obtain these rich guarantees incrementally. In particular, programmers may be willing to exploit the safety checks of the underlying runtime system as a substitute for some static guarantees. Progressive types give programmers this freedom, thus creating a gentler and more flexible environment for using powerful type checkers. In this paper we discuss the idea, motivate it with concrete, real-world scenarios, then show the development of a simple progressive type system and present its (progressive) soundness theorem.