We propose an Operational Game Semantics for a (call-by-value) concurrent higher-order language with first-order store (references may contain other references or first-order values such as integers or booleans; however they may not store higher-order values such as functions). We adapt the game-semantic notion of visibility, which semantically captures the absence of higher-order references and developed for sequential higher-order languages, to a concurrent setting. We thus define a complete-trace preorder, and prove it sound for the contextual preorder, by introducing a synchronization-based composition of semantic configurations and establishing an observational adequacy result. We also prove completeness for the subset of the language in which functions return first-order values. In contrast to the case of sequential visibility, in the labeled transition semantics we have to account for the presence of multiple active threads with possibly different visibilities, and of a tree-like structure for managing the dependencies among the threads so created. Moreover, we have to reason on families of traces, rather than single traces, as in concurrent setting the order among certain actions cannot be enforced.

Mixed choice multiparty message passing is an expressive concurrency programming paradigm where components use non-determinism to choose between concurrent options for sending and receiving messages. This flexibility makes it possible to program advanced algorithms, such as leader election protocols, succinctly. We present Mixtris, a mechanised higher-order separation logic for reasoning about functional correctness of higher-order imperative programs with mixed choice multiparty message passing in a shared memory setting. Mixtris builds upon recent work on separation logic for (non-mixed choice) multiparty message-passing programs, by drawing inspiration from session type systems for mixed choice multiparty message-passing programs. Mixtris is the first program logic for mixed choice multiparty message passing. We prove soundness of Mixtris using a novel model of our mixed choice multiparty protocols. We demonstrate how Mixtris can be used to formally reason about challenging examples, including some leader election protocols such as Chang and Roberts’s ring leader election protocol. All the results in the paper (both meta-theory and examples) have been formalised in the Rocq proof assistant on top of the Iris program logic framework.

The π-calculus is the paradigmatical name-passing calculus. While being purely name-passing, it allows the representation of higher-order functions and store. We study how π-calculus processes can be controlled so that computations can only involve storage of first-order values. The discipline is enforced by a type system that is based on the notion of visibility, coming from game semantics. We discuss the impact of visibility on the behavioural theory. We propose characterisations of may-testing and barbed equivalence, based on (variants of) trace equivalence and labelled bisimilarity, in the case where computation is sequential, and in the case where computation is well-bracketed.