Session type

Session types in type theory are used to ensure correctness in concurrent programs by guaranteeing that messages sent and received between processes follow the expected order and type. They help prevent communication errors, deadlocks, and ensure protocol conformance in concurrent and distributed systems. Session types can be adapted for both channel and actor systems.

Binary session types are used to check interactions between two processes, while multiparty session types handle interactions involving more than two participants. In multiparty session types, a global type describes the communication sequence among all participants, which is then projected into local types from each participant's perspective. This approach preserves sequencing information that would be lost with binary session types.

Binary session types are formally defined using operations such as send (!), receive (?), branches (&), selections (⊕), recursion (rec), and termination (end). For example, the type \( S = !bool.?int.end \) represents a session that sends a boolean, receives an integer, and then terminates.

Session types have been implemented in various programming languages, including Scala (e.g., lchannels, Effpi, STMonitor), Rust (e.g., Session-types, sesh), Python (Session Actors), Erlang (Monitored Session Erlang), OCaml (FuSe, session-ocaml), Haskell (Priority Sesh), Java (Java Typestate Checker), and Swift (Swift Sessions). These implementations support the practical application of session types in diverse programming contexts.