Separating Sessions Smoothly

Fowler, Simon; Kokke, Wen; Dardha, Ornela; Lindley, Sam; Morris, John

doi:10.46298/lmcs-19(3:3)2023

Logical Methods in Computer Science

2023

DOI: 10.46298/lmcs-19(3:3)2023

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Separating Sessions Smoothly

Simon Fowler¹,

Wen Kokke²,

Ornela Dardha³

et al.

Abstract: This paper introduces Hypersequent GV (HGV), a modular and extensible core calculus for functional programming with session types that enjoys deadlock freedom, confluence, and strong normalisation. HGV exploits hyper-environments, which are collections of type environments, to ensure that structural congruence is type preserving. As a consequence we obtain an operational correspondence between HGV and HCP -- a process calculus based on hypersequents and in a propositions-as-types correspondence with classical … Show more

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Cited by 2 publications

References 53 publications

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Special Delivery: Programming with Mailbox Types

Fowler,

Attard,

Sowul

et al. 2023

Proc. ACM Program. Lang.

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The asynchronous and unidirectional communication model supported by mailboxes is a key reason for the success of actor languages like Erlang and Elixir for implementing reliable and scalable distributed systems. While many actors may send messages to some actor, only the actor may (selectively) receive from its mailbox. Although actors eliminate many of the issues stemming from shared memory concurrency, they remain vulnerable to communication errors such as protocol violations and deadlocks. Mailbox types are a novel behavioural type system for mailboxes first introduced for a process calculus by de’Liguoro and Padovani in 2018, which capture the contents of a mailbox as a commutative regular expression. Due to aliasing and nested evaluation contexts, moving from a process calculus to a programming language is challenging. This paper presents Pat, the first programming language design incorporating mailbox types, and describes an algorithmic type system. We make essential use of quasi-linear typing to tame some of the complexity introduced by aliasing. Our algorithmic type system is necessarily co-contextual, achieved through a novel use of backwards bidirectional typing, and we prove it sound and complete with respect to our declarative type system. We implement a prototype type checker, and use it to demonstrate the expressiveness of Pat on a factory automation case study and a series of examples from the Savina actor benchmark suite.

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Special Delivery: Programming with Mailbox Types

Fowler,

Attard,

Sowul

et al. 2023

Proc. ACM Program. Lang.

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Verified Lock-Free Session Channels with Linking

Somers,

Krebbers

2024

Proc. ACM Program. Lang.

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Type systems and program logics based on session types provide powerful high-level reasoning principles for message-passing concurrency. Modern versions employ bidirectional session channels that (1) are asynchronous so that send operations do not block, (2) have buffers in both directions so that both parties can send messages in parallel, and (3) feature a link operation (also called forward) to concisely write programs in process style. These features complicate a low-level lock-free implementation of channels and therefore increase the gap between the meta theory of prior work—which is verified w.r.t. a high-level semantics of channels (e.g., π-calculus)—and the code that runs on an actual computer. We address this problem by verifying a low-level lock-free implementation of session channels w.r.t. a high-level specification based on session types. We carry out our verification in a layered manner by employing the Iris framework for concurrent separation logic. We start with an abstract specification of (unidirectional) queues—of which we provide a linked-list and array-segment based implementation—and gradually build up to session channels with all of the aforementioned features. To make a layered verification possible we develop two logical abstractions—queues with ghost linking and pairing invariants—to reason about the atomicity and changing endpoints due to linking, respectively. All our results are mechanized in the Coq proof assistant.

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Separating Sessions Smoothly

Cited by 2 publications

References 53 publications

Special Delivery: Programming with Mailbox Types

Special Delivery: Programming with Mailbox Types

Verified Lock-Free Session Channels with Linking

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