Relating structure and power: Comonadic semantics for computational resources

Samson Abramsky,Nihil Shah

doi:10.1093/logcom/exab048

Abstract

AbstractCombinatorial games are widely used in finite model theory, constraint satisfaction, modal logic and concurrency theory to characterize logical equivalences between structures. In particular, Ehrenfeucht–Fraïssé games, pebble games and bisimulation games play a central role. We show how each of these types of games can be described in terms of an indexed family of comonads on the category of relational structures and homomorphisms. The index $k$ is a resource parameter that bounds the degree of access to the underlying structure. The coKleisli categories for these comonads can be used to give syntax-free characterizations of a wide range of important logical equivalences. Moreover, the coalgebras for these indexed comonads can be used to characterize key combinatorial parameters: tree depth for the Ehrenfeucht–Fraïssé comonad, tree width for the pebbling comonad and synchronization tree depth for the modal unfolding comonad. These results pave the way for systematic connections between two major branches of the field of logic in computer science, which hitherto have been almost disjoint: categorical semantics and finite and algorithmic model theory.

Highlights

There is a remarkable divide in the field of logic in Computer Science, between two distinct strands: one focussing on semantics and compositionality (“Structure”), the other on expressiveness and complexity (“Power”)
We develop a novel approach to relating categorical semantics, which exemplifies the first strand, to finite model theory, which exemplifies the second
We show how our game comonads can be used to give syntax-free characterizations of a range of logical equivalences, which play a central rôle in finite model theory and modal logic

Summary

Introduction

There is a remarkable divide in the field of logic in Computer Science, between two distinct strands: one focussing on semantics and compositionality (“Structure”), the other on expressiveness and complexity (“Power”). It is remarkable because these two fundamental aspects of our field are studied using almost disjoint technical languages and methods, by almost disjoint research communities. We develop a novel approach to relating categorical semantics, which exemplifies the first strand, to finite model theory, which exemplifies the second.

Objectives

Results

Conclusion