Eliminating Reflection from Type Theory: To the Legacy of Martin Hofmann

Théo Winterhalter 1 Matthieu Sozeau 2 Nicolas Tabareau 1
1 GALLINETTE - Gallinette : vers une nouvelle génération d'assistant à la preuve
Inria Rennes – Bretagne Atlantique , LS2N - Laboratoire des Sciences du Numérique de Nantes
2 PI.R2 - Design, study and implementation of languages for proofs and programs
Inria de Paris, CNRS - Centre National de la Recherche Scientifique, UPD7 - Université Paris Diderot - Paris 7, PPS - Preuves, Programmes et Systèmes
Abstract : Type theories with equality reflection, such as extensional type theory (ETT), are convenient theories in which to formalise mathematics, as they make it possible to consider provably equal terms as convertible. Although type-checking is undecidable in this context, variants of ETT have been implemented, for example in NuPRL and more recently in Andromeda. The actual objects that can be checked are not proof-terms, but derivations of proof-terms. This suggests that any derivation of ETT can be translated into a typecheckable proof term of intensional type theory (ITT). However, this result, investigated categorically by Hofmann in 1995, and 10 years later more syntactically by Oury, has never given rise to an effective translation. In this paper, we provide the first syntactical translation from ETT to ITT with uniqueness of identity proofs and functional extensionality. This translation has been defined and proven correct in Coq and yields an executable plugin that translates a derivation in ETT into an actual Coq typing judgment. Additionally, we show how this result is extended in the context of homotopy to a two-level type theory.
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Théo Winterhalter, Matthieu Sozeau, Nicolas Tabareau. Eliminating Reflection from Type Theory: To the Legacy of Martin Hofmann. CPP 2019 - The 8th ACM SIGPLAN International Conference on Certified Programs and Proofs, Jan 2019, Lisbonne, Portugal. pp.91-103, ⟨10.1145/3293880.3294095⟩. ⟨hal-01849166v3⟩

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