## Section: New Results

### Type Theory and Proof Assistants

Participants : Simon Boulier, Eric Finster, Gaëtan Gilbert, Pierre-Marie Pédrot, Nicolas Tabareau, Théo Winterhalter.

#### Type Theory

##### Effects in Type Theory.

In [16] , we define the exceptional translation, a syntactic translation of the Calculus of Inductive Constructions (CIC) into itself, that covers full dependent elimination. The new resulting type theory features call-by-name exceptions with decidable type-checking and canonicity, but at the price of inconsistency. Then, noticing parametricity amounts to Kreisel's realisability in this setting, we provide an additional layer on top of the exceptional translation in order to tame exceptions and ensure that all exceptions used locally are caught, leading to the parametric exceptional translation which fully preserves consistency. This way, we can consistently extend the logical expressivity of CIC with independence of premises, Markov's rule, and the negation of function extensionality while retaining η-expansion. As a byproduct, we also show that Markov's principle is not provable in CIC. Both translations have been implemented in a Coq plugin, which we use to formalise the examples.

##### Eliminating Reflection from Type Theory.

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 [18], 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.

##### Foundations of Dependent Interoperability.

Full-spectrum dependent types promise to enable the development of correct-by-construction software. However, even certified software needs to interact with simply-typed or untyped programs, be it to perform system calls, or to use legacy libraries. Trading static guarantees for runtime checks, the dependent interoperability framework provides a mechanism by which simply-typed values can safely be coerced to dependent types and, conversely, dependently-typed programs can defensively be exported to a simply-typed application. In [2], we give a semantic account of dependent interoperability. Our presentation relies on and is guided by a pervading notion of type equivalence, whose importance has been emphasised in recent work on homotopy type theory. Specifically, we develop the notion of type-theoretic partial Galois connections as a key foundation for dependent interoperability, which accounts for the partiality of the coercions between types. We explore the applicability of both monotone and antitone type-theoretic Galois connections in the setting of dependent interoperability. A monotone partial Galois connection enforces a translation of dependent types to runtime checks that are both sound and complete with respect to the invariants encoded by dependent types. Conversely, picking an antitone partial Galois connection instead lets us induce weaker, sound conditions that can amount to more efficient runtime checks. Our framework is developed in Coq; it is thus constructive and verified in the strictest sense of the terms. Using our library, users can specify domain-specific partial connections between data structures. Our library then takes care of the (sometimes, heavy) lifting that leads to interoperable programs. It thus becomes possible, as we shall illustrate, to internalise and hand-tune the extraction of dependently-typed programs to interoperable OCaml programs within Coq itself.

##### Equivalences for Free: Univalent Parametricity for Effective Transport.

Homotopy Type Theory promises a unification of the concepts of equality and equivalence in Type Theory, through the introduction of the univalence principle. However, existing proof assistants based on type theory treat this principle as an axiom, and it is not yet clear how to extend them to handle univalence internally. In [7], we propose a construction grounded on a univalent version of parametricity to bring the benefits of univalence to the programmer and prover, that can be used on top of existing type theories. In particular, univalent parametricity strengthens parametricity to ensure preservation of type equivalences. We present a lightweight framework implemented in the Coq proof assistant that allows the user to transparently transfer definitions and theorems for a type to an equivalent one, as if they were equal. Our approach handles both type and term dependency. We study how to maximise the effectiveness of these transports in terms of computational behaviour, and identify a fragment useful for certified programming on which univalent transport is guaranteed to be effective. This work paves the way to easier-to-use environments for certified programming by supporting seamless programming and proving modulo equivalences.

##### Special Issue on Homotopy Type Theory and Univalent Foundations.

The preface [4] introduces the first special issue out of a series of workshops on Homotopy Type Theory and Univalent Foundations. This recent area of research finds its roots in the seminal work of Martin Hofmann and Thomas Streicher on the structure of Martin-Löf identity types. But the main research program has been foreseen by Vladimir Voevodsky, who, from its initial motivation of formalising his results in homotopy theory, has initiated what is now called the univalent foundations program. Borrowing ideas from homotopy theory, the goal of the univalent foundations program is to leverage dependent Type Theory to a formal framework that could replace Set Theory for the foundations of mathematics. This special issue gathers research contributions of some of the most prominent researchers of the field.

##### Goodwillie's Calculus of Functors and Higher Topos Theory

In [1], we develop an approach to Goodwillie's calculus of functors using the techniques of higher topos theory. Central to our method is the introduction of the notion of fiberwise orthogonality, a strengthening of ordinary orthogonality which allows us to give a number of useful characterisations of the class of n-excisive maps. We use these results to show that the pushout product of a ${P}_{n}$-equivalence with a ${P}_{m}$-equivalence is a ${P}_{m+n+1}$-equivalence. Then, building on our previous work, we prove a Blakers-Massey type theorem for the Goodwillie tower. We show how to use the resulting techniques to rederive some foundational theorems in the subject, such as delooping of homogeneous functors.

#### Proof Assistants

##### Typed Template Coq – Certified Meta-Programming in Coq.

Template-Coq [19], [10] is a plugin for Coq, originally implemented by Malecha, which provides a reifier for Coq terms and global declarations , as represented in the Coq kernel, as well as a denotation command. Initially, it was developed for the purpose of writing functions on Coq's AST in Gallina. Recently, it was used in the CertiCoq certified compiler project, as its front-end language, to derive parametricity properties, and to extract Coq terms to a CBV λ-calculus. However, the syntax lacked semantics, be it typing semantics or operational semantics, which should reflect, as formal specifications in Coq, the semantics of Coq's type theory itself. The tool was also rather bare bones, providing only rudimentary quoting and unquoting commands. We generalise it to handle the entire Calculus of Inductive Constructions (CIC), as implemented by Coq, including the kernel's declaration structures for definitions and inductives, and implement a monad for general manipulation of Coq's logical environment. We demonstrate how this setup allows Coq users to define many kinds of general purpose plugins, whose correctness can be readily proved in the system itself, and that can be run efficiently after extraction. We give a few examples of implemented plugins, including a parametricity translation. We also advocate the use of Template-Coq as a foundation for higher-level tools.

##### Definitional Proof-Irrelevance without K.

Definitional equality—or conversion—for a type theory with a decidable type checking is the simplest tool to prove that two objects are the same, letting the system decide just using computation. Therefore, the more things are equal by conversion, the simpler it is to use a language based on type theory. Proof-irrelevance, stating that any two proofs of the same proposition are equal, is a possible way to extend conversion to make a type theory more powerful. However, this new power comes at a price if we integrate it naively, either by making type checking undecidable or by realising new axioms—such as uniqueness of identity proofs (UIP)—that are incompatible with other extensions, such as univalence. In [3], taking inspiration from homotopy type theory, we propose a general way to extend a type theory with definitional proof irrelevance, in a way that keeps type checking decidable and is compatible with univalence. We provide a new criterion to decide whether a proposition can be eliminated over a type (correcting and improving the so-called singleton elimination of Coq) by using techniques coming from recent development on dependent pattern matching without UIP. We show the generality of our approach by providing implementations for both Coq and Agda, both of which are planned to be integrated in future versions of those proof assistants.