On the Expressiveness of Mixed Choice Sessions

Kirstin Peters
Nobuko Yoshida

Session types provide a flexible programming style for structuring interaction, and are used to guarantee a safe and consistent composition of distributed processes. Traditional session types include only one-directional input (external) and output (internal) guarded choices. This prevents the session-processes to explore the full expressive power of the pi-calculus where the mixed choices are proved more expressive than the (non-mixed) guarded choices. To account this issue, recently Casal, Mordido, and Vasconcelos proposed the binary session types with mixed choices (CMV+). This paper carries a surprising, unfortunate result on CMV+: in spite of an inclusion of unrestricted channels with mixed choice, CMV+'s mixed choice is rather separate and not mixed. We prove this negative result using two methodologies (using either the leader election problem or a synchronisation pattern as distinguishing feature), showing that there exists no good encoding from the pi-calculus into CMV+, preserving distribution. We then close their open problem on the encoding from CMV+ into CMV (without mixed choice), proving its soundness and thereby that the encoding is good up to coupled similarity.

In Valentina Castiglioni and Claudio A. Mezzina: Proceedings Combined 29th International Workshop on Expressiveness in Concurrency and 19th Workshop on Structural Operational Semantics (EXPRESS/SOS 2022), Warsaw, Poland, 12th September 2022, Electronic Proceedings in Theoretical Computer Science 368, pp. 113–130.
Published: 6th September 2022.

ArXived at: https://dx.doi.org/10.4204/EPTCS.368.7 bibtex PDF

Comments and questions to: eptcs@eptcs.org
For website issues: webmaster@eptcs.org