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Gradual session types

Research output: Contribution to journalArticle

  • Atsushi Igarashi
  • Peter Thiemann
  • Yuya Tsuda
  • Vasco T. Vasconcelos
  • Philip Wadler

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Original languageEnglish
Article numbere17
Pages (from-to)1-56
Number of pages56
JournalJournal of Functional Programming
Publication statusPublished - 18 Nov 2019


Session types are a rich type discipline, based on linear types, that lifts the sort of safety claims that come with type systems to communications. However, web-based applications and microservices 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 Gradual GV as a gradually typed extension of the functional session type system GV. Following a standard framework of gradual typing, Gradual GV consists of an external language, which relaxes the type system of GV using dynamic types; an internal language with casts, for which operational semantics is given; and a cast-insertion translation from the former to the latter. 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.

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