Peter Thiemann (University of Freiburg, Germany) gives the fourth talk in the third panel, Contracts and Sessions, on the 3rd day of the ICFP conference. Co-written by Atsushi Igarashi (Kyoto University, Japan), Vasco Vasconcelos (University of Lisbon, Portugal), Philip Wadler (University of Edinburgh, UK).

Session types are a rich type discipline, based on linear types, that lift the sort of safety claims that come with type systems to communications. However, web-based applications and micro services are often written in a mix of languages, with type disciplines in a spectrum between static and dynamic typing. Gradual session types address this mixed setting by providing a framework which grants seamless transition between statically typed handling of sessions and any required degree of dynamic typing.

We propose GradualGV as an extension of the functional session type system GV with dynamic types and casts. We demonstrate type and communication safety as well as blame safety, thus extending previous results to functional languages with session-based communication. The interplay of linearity and dynamic types requires a novel approach to specifying the dynamics of the language.