The Dynamic Practice and Static Theory of Gradual Typing

Abstract

We can tease apart the research on gradual types into two `lineages': a pragmatic, implementation-oriented dynamic-first lineage and a formal, type-theoretic, static-first lineage. The dynamic-first lineage's focus is on taming particular idioms - `pre-existing conditions' in untyped programming languages. The static-first lineage's focus is on interoperation and individual type system features, rather than the collection of features found in any particular language. Both appear in programming languages research under the name "gradual typing", and they are in active conversation with each other. What are these two lineages? What challenges and opportunities await the static-first lineage? What progress has been made so far?