Impredicativity, Cumulativity and Product Covariance in the Logical Framework Dedukti

Authors Thiago Felicissimo, Théo Winterhalter

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Author Details

Thiago Felicissimo
  • Université Paris-Saclay, INRIA project Deducteam, Laboratoire Méthodes Formelles, ENS Paris-Saclay, France
Théo Winterhalter
  • Université Paris-Saclay, INRIA project Deducteam, Laboratoire Méthodes Formelles, ENS Paris-Saclay, France


The authors thank François Thiré and Yoan Géran for helpful remarks about a first draft, Gaspard Férey, Jean-Pierre Jouannaud, Frédéric Blanqui and Gilles Dowek for informative discussions around the subject of this paper, and the anonymous reviewers for their helpful comments.

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Thiago Felicissimo and Théo Winterhalter. Impredicativity, Cumulativity and Product Covariance in the Logical Framework Dedukti. In 9th International Conference on Formal Structures for Computation and Deduction (FSCD 2024). Leibniz International Proceedings in Informatics (LIPIcs), Volume 299, pp. 21:1-21:23, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2024)


Proof assistants such as Coq implement a type theory featuring three important features: impredicativity, cumulativity and product covariance. This combination has proven difficult to be expressed in the logical framework Dedukti, and previous attempts have failed in providing an encoding that is proven confluent, sound and conservative. In this work we solve this longstanding open problem by providing an encoding of these three features that we prove to be confluent, sound and to satisfy a restricted (but, we argue, strong enough) form of conservativity. Our proof of confluence is a contribution by itself, and combines various criteria and proof techniques from rewriting theory. Our proof of soundness also contributes a new strategy in which the result is shown in terms of an inverse translation function, fixing a common flaw made in some previous encoding attempts.

Subject Classification

ACM Subject Classification
  • Theory of computation → Type theory
  • Theory of computation → Equational logic and rewriting
  • Dedukti
  • Rewriting
  • Confluence
  • Dependent types
  • Cumulativity
  • Universes


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