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Guarded Kleene Algebra with Tests: Coequations, Coinduction, and Completeness

Authors Todd Schmid , Tobias Kappé , Dexter Kozen , Alexandra Silva

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Author Details

Todd Schmid
  • Department of Computer Science, University College London, UK
Tobias Kappé
  • Department of Computer Science, Cornell University, Ithaca, NY, USA
Dexter Kozen
  • Department of Computer Science, Cornell University, Ithaca, NY, USA
Alexandra Silva
  • Department of Computer Science, University College London, UK

Funding

  • Kappé, Tobias: DARPA grant HR001120C0107 (Pronto)
  • Kozen, Dexter: NSF grant CCF-2008083
  • Silva, Alexandra: ERC Consolidator Grant AutoProbe (101002697) and a Royal Society Wolfson Fellowship

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Todd Schmid, Tobias Kappé, Dexter Kozen, and Alexandra Silva. Guarded Kleene Algebra with Tests: Coequations, Coinduction, and Completeness. In 48th International Colloquium on Automata, Languages, and Programming (ICALP 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 198, pp. 142:1-142:14, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021) https://doi.org/10.4230/LIPIcs.ICALP.2021.142

Abstract

Guarded Kleene Algebra with Tests (GKAT) is an efficient fragment of KAT, as it allows for almost linear decidability of equivalence. In this paper, we study the (co)algebraic properties of GKAT. Our initial focus is on the fragment that can distinguish between unsuccessful programs performing different actions, by omitting the so-called early termination axiom. We develop an operational (coalgebraic) and denotational (algebraic) semantics and show that they coincide. We then characterize the behaviors of GKAT expressions in this semantics, leading to a coequation that captures the covariety of automata corresponding to these behaviors. Finally, we prove that the axioms of the reduced fragment are sound and complete w.r.t. the semantics, and then build on this result to recover a semantics that is sound and complete w.r.t. the full set of axioms.

Subject Classification

ACM Subject Classification
  • Theory of computation → Program reasoning
Keywords
  • Kleene algebra
  • program equivalence
  • completeness
  • coequations

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