Always-available static and dynamic feedback

Developers who write code in a statically typed language are denied the ability to obtain dynamic feedback by executing their code during periods when it fails the static type checker. They are further confined to the static typing discipline during times in the development process where it does not yield the highest productivity. If they opt instead to use a dynamic language, they forgo the many benefits of static typing, including machine-checked documentation, improved correctness and reliability, tool support (such as for refactoring), and better runtime performance. We present a novel approach to giving developers the benefits of both static and dynamic typing, throughout the development process, and without the burden of manually separating their program into statically and dynamically-typed parts. Our approach, which is intended for temporary use during the development process, relaxes the static type system and provides a semantics for many type-incorrect programs. It defers type errors to run time, or suppresses them if they do not affect runtime semantics. We implemented our approach in a publicly available tool, DuctileJ, for the Java language. In case studies, DuctileJ conferred benefits both during prototyping and during the evolution of existing code.