Context-free session types describe structured patterns of communication on heterogeneously typed channels, allowing the specification of protocols unconstrained by tail recursion. The enhanced expressive power provided by non-regular recursion comes, however, at the cost of the decidability of subtyping, even if equivalence is still decidable. We present an approach to subtyping context-free session types based on a novel kind of observational preorder we call XYZW-simulation, which generalizes XY-simulation (also known as covariant-contravariant simulation) and therefore also bisimulation and plain simulation. We further propose a subtyping algorithm that we prove to be sound, and present an empirical evaluation in the context of a compiler for a programming language. Due to the general nature of the simulation relation upon which it is built, this algorithm may also find applications in other domains.
@InProceedings{silva_et_al:LIPIcs.CONCUR.2023.11, author = {Silva, Gil and Mordido, Andreia and Vasconcelos, Vasco T.}, title = {{Subtyping Context-Free Session Types}}, booktitle = {34th International Conference on Concurrency Theory (CONCUR 2023)}, pages = {11:1--11:19}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-299-0}, ISSN = {1868-8969}, year = {2023}, volume = {279}, editor = {P\'{e}rez, Guillermo A. and Raskin, Jean-Fran\c{c}ois}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.CONCUR.2023.11}, URN = {urn:nbn:de:0030-drops-190055}, doi = {10.4230/LIPIcs.CONCUR.2023.11}, annote = {Keywords: Session types, Subtyping, Simulation, Simple grammars, Non-regular recursion} }
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