Branching Out: Existential External Choice in Effpi

2026-04-08

AI Summary

This work enhances the expressiveness of protocol modeling in message-passing programs by extending the strongly-typed framework Effpi with novel support for multi-channel external choice branches and timeout handling mechanisms. Building upon behavioral types and a variant of CCS encoding, the approach integrates a compiler plugin to enforce type-safe verification. The proposed extension substantially enriches the expressivity of protocol types, enabling the type-safe modeling and verification of complex interaction protocols such as the Raft consensus algorithm. This demonstrates the practical applicability and effectiveness of the framework in real-world distributed systems.

Abstract

Effpi is a framework for writing strongly-typed message-passing programs in Scala, where the compiler enforces the conformance of process implementations to specified protocol types. A compiler plugin is provided to verify properties of protocols, such as deadlock-freedom and liveness, by encoding the behavioural types into a variant of CCS. To address limitations in the expressiveness of the existing toolkit, we extend Effpi with external choice by introducing a branching operation. Upon accepting a message via a branch, protocols enforce a continuation which depends on the label (type) of the received message. We equip the branching operation with the ability to accept messages over more than one channel. Additionally, we introduce a "catch timeout" operation to allow processes to gracefully handle a lack of incoming messages. The enhanced expressiveness of Effpi is demonstrated through a number of examples, including an implementation of the Raft consensus algorithm.

Problem

• external choice
• message-passing
• protocol types
• branching
• expressiveness

Innovation

• external choice
• branching operation
• multi-channel reception
• timeout handling
• protocol types