Core Higher-Order Session Processes: Tractable Equivalences and Relative Expressiveness

TL;DR

This paper introduces tractable bisimulations for the higher-order pi-calculus with session primitives, demonstrating their equivalence to contextual equivalence and analyzing the expressiveness of various subcalculi.

Contribution

It develops typed bisimulations for HOpi, studies a minimal subcalculus HO, and establishes encoding and expressiveness relationships among HOpi, HO, and the session pi-calculus.

Findings

Typed bisimulations coincide with contextual equivalence.

HO can encode HOpi extended with higher-order features.

HOpi, HO, and pi are equally expressive.

Abstract

This work proposes tractable bisimulations for the higher-order pi-calculus with session primitives (HOpi) and offers a complete study of the expressivity of its most significant subcalculi. First we develop three typed bisimulations, which are shown to coincide with contextual equivalence. These characterisations demonstrate that observing as inputs only a specific finite set of higher-order values (which inhabit session types) suffices to reason about HOp} processes. Next, we identify HO, a minimal, second-order subcalculus of HOpi in which higher-order applications/abstractions, name-passing, and recursion are absent. We show that HO can encode HOpi extended with higher-order applications and abstractions and that a first-order session pi-calculus can encode HOpi. Both encodings are fully abstract. We also prove that the session pi-calculus with passing of shared names cannot be encoded into HOpi without shared names. We show that HOpi, HO, and pi are equally expressive; the expressivity of HO enables effective reasoning about typed equivalences for higher-order processes.