14 Search Results for "Escardó, Martín H."

Document
DOI: 10.4230/LIPIcs.LICS.2026.59
Generalized Decidability via Brouwer Trees

Authors: Tom de Jong, Nicolai Kraus, Aref Mohammadzadeh, and Fredrik Nordvall Forsberg

Published in: LIPIcs, Volume 380, 41st Annual Symposium on Logic in Computer Science (LICS 2026)

Abstract
In the setting of constructive mathematics, we suggest and study a framework for decidability of properties, which allows for finer distinctions than just "decidable, semidecidable, or undecidable". We work in homotopy type theory and use Brouwer tree ordinals to specify the level of decidability of a property. In this framework, we express the property that a proposition is α-decidable, for an ordinal α, and show that it generalizes decidability and semidecidability. Further generalizing known results, we show that α-decidable propositions are closed under binary conjunction, and discuss for which α they are closed under binary disjunction. We prove that if each P(i) is semidecidable, then the countable meet ∀ i ∈ ℕ. P(i) is ω²-decidable, and similar results for countable joins and iterated quantifiers. We also discuss the relationship with countable choice. All our results are formalized in Cubical Agda.
Cite as

Tom de Jong, Nicolai Kraus, Aref Mohammadzadeh, and Fredrik Nordvall Forsberg. Generalized Decidability via Brouwer Trees. In 41st Annual Symposium on Logic in Computer Science (LICS 2026). Leibniz International Proceedings in Informatics (LIPIcs), Volume 380, pp. 59:1-59:27, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2026)

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@InProceedings{dejong_et_al:LIPIcs.LICS.2026.59,
  author =	{de Jong, Tom and Kraus, Nicolai and Mohammadzadeh, Aref and Nordvall Forsberg, Fredrik},
  title =	{{Generalized Decidability via Brouwer Trees}},
  booktitle =	{41st Annual Symposium on Logic in Computer Science (LICS 2026)},
  pages =	{59:1--59:27},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-434-5},
  ISSN =	{1868-8969},
  year =	{2026},
  volume =	{380},
  editor =	{Faggian, Claudia and Katoen, Joost-Pieter},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.LICS.2026.59},
  URN =		{urn:nbn:de:0030-drops-268466},
  doi =		{10.4230/LIPIcs.LICS.2026.59},
  annote =	{Keywords: Decidability in constructive mathematics, homotopy type theory, ordinals, Brouwer trees, countable choice}
}
Document
DOI: 10.4230/LIPIcs.CSL.2026.19
A Unifying Conservation Theorem

Authors: Giulio Fellin

Published in: LIPIcs, Volume 363, 34th EACSL Annual Conference on Computer Science Logic (CSL 2026)

Abstract
The relationship between classical and constructive logics has long been illuminated by a series of conservation results, beginning with Kolmogorov’s negative translation and Glivenko’s double negation theorem, and later extended by Kuroda and Segerberg to first-order and minimal logics respectively. These results reveal how certain classical principles can be interpreted or recovered within weaker constructive frameworks, either via translations or through minimal extensions that satisfy specific logical properties. In this paper, we propose a unifying generalisation of these conservation theorems, that consolidates and expands the abstract methods introduced in earlier studies, offering a unified perspective on the interplay between classical provability and constructive reasoning.
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Giulio Fellin. A Unifying Conservation Theorem. In 34th EACSL Annual Conference on Computer Science Logic (CSL 2026). Leibniz International Proceedings in Informatics (LIPIcs), Volume 363, pp. 19:1-19:23, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2026)

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@InProceedings{fellin:LIPIcs.CSL.2026.19,
  author =	{Fellin, Giulio},
  title =	{{A Unifying Conservation Theorem}},
  booktitle =	{34th EACSL Annual Conference on Computer Science Logic (CSL 2026)},
  pages =	{19:1--19:23},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-411-6},
  ISSN =	{1868-8969},
  year =	{2026},
  volume =	{363},
  editor =	{Guerrini, Stefano and K\"{o}nig, Barbara},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.CSL.2026.19},
  URN =		{urn:nbn:de:0030-drops-254431},
  doi =		{10.4230/LIPIcs.CSL.2026.19},
  annote =	{Keywords: double negation, negative translation, conservation, minimal logic, Glivenko’s theorem}
}
Document
DOI: 10.4230/LIPIcs.ITP.2025.33
Scott’s Representation Theorem and the Univalent Karoubi Envelope

Authors: Arnoud van der Leer, Kobe Wullaert, and Benedikt Ahrens

Published in: LIPIcs, Volume 352, 16th International Conference on Interactive Theorem Proving (ITP 2025)

Abstract
Lambek and Scott constructed a correspondence between simply-typed lambda calculi and Cartesian closed categories. Scott’s Representation Theorem is a cousin to this result for untyped lambda calculi. It states that every untyped lambda calculus arises from a reflexive object in some category. We present a formalization of Scott’s Representation Theorem in univalent foundations, in the (Rocq-)UniMath library. Specifically, we implement two proofs of that theorem, one by Scott and one by Hyland. We also explain the role of the Karoubi envelope - a categorical construction - in the proofs and the impact the chosen foundation has on this construction. Finally, we report on some automation we have implemented for the reduction of λ-terms.
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Arnoud van der Leer, Kobe Wullaert, and Benedikt Ahrens. Scott’s Representation Theorem and the Univalent Karoubi Envelope. In 16th International Conference on Interactive Theorem Proving (ITP 2025). Leibniz International Proceedings in Informatics (LIPIcs), Volume 352, pp. 33:1-33:20, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2025)

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@InProceedings{vanderleer_et_al:LIPIcs.ITP.2025.33,
  author =	{van der Leer, Arnoud and Wullaert, Kobe and Ahrens, Benedikt},
  title =	{{Scott’s Representation Theorem and the Univalent Karoubi Envelope}},
  booktitle =	{16th International Conference on Interactive Theorem Proving (ITP 2025)},
  pages =	{33:1--33:20},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-396-6},
  ISSN =	{1868-8969},
  year =	{2025},
  volume =	{352},
  editor =	{Forster, Yannick and Keller, Chantal},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ITP.2025.33},
  URN =		{urn:nbn:de:0030-drops-246318},
  doi =		{10.4230/LIPIcs.ITP.2025.33},
  annote =	{Keywords: Lambda calculi, algebraic theories, categorical semantics, Karoubi envelope, formalization, Rocq-UniMath, univalent foundations}
}
Document
DOI: 10.4230/LIPIcs.CALCO.2025.7
A Coinductive Representation of Computable Functions

Authors: Alvin Tang and Dirk Pattinson

Published in: LIPIcs, Volume 342, 11th Conference on Algebra and Coalgebra in Computer Science (CALCO 2025)

Abstract
We investigate a representation of computable functions as total functions over 2^∞, the set of finite and infinite sequences over {0,1}. In this model, infinite sequences are interpreted as non-terminating computations whilst finite sequences represent the sum of their digits. We introduce a new definition principle, function space corecursion, that simultaneously generalises minimisation and primitive recursion. This defines the class of computable corecursive functions that is closed under composition and function space corecursion. We prove computable corecursive functions represent all partial recursive functions, and show that all computable corecursive functions are indeed computable by translation into the untyped λ-calculus.
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Alvin Tang and Dirk Pattinson. A Coinductive Representation of Computable Functions. In 11th Conference on Algebra and Coalgebra in Computer Science (CALCO 2025). Leibniz International Proceedings in Informatics (LIPIcs), Volume 342, pp. 7:1-7:15, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2025)

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@InProceedings{tang_et_al:LIPIcs.CALCO.2025.7,
  author =	{Tang, Alvin and Pattinson, Dirk},
  title =	{{A Coinductive Representation of Computable Functions}},
  booktitle =	{11th Conference on Algebra and Coalgebra in Computer Science (CALCO 2025)},
  pages =	{7:1--7:15},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-383-6},
  ISSN =	{1868-8969},
  year =	{2025},
  volume =	{342},
  editor =	{C\^{i}rstea, Corina and Knapp, Alexander},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.CALCO.2025.7},
  URN =		{urn:nbn:de:0030-drops-235662},
  doi =		{10.4230/LIPIcs.CALCO.2025.7},
  annote =	{Keywords: Computability, Coinduction}
}
Document
DOI: 10.4230/LIPIcs.FSCD.2025.19
Internal Effectful Forcing in System T

Authors: Martín H. Escardó, Bruno da Rocha Paiva, Vincent Rahli, and Ayberk Tosun

Published in: LIPIcs, Volume 337, 10th International Conference on Formal Structures for Computation and Deduction (FSCD 2025)

Abstract
The effectful forcing technique allows one to show that the denotation of a closed System T term of type (ι ⇒ ι) ⇒ ι in the set-theoretical model is a continuous function (ℕ → ℕ) → ℕ. For this purpose, an alternative dialogue-tree semantics is defined and related to the set-theoretical semantics by a logical relation. In this paper, we apply effectful forcing to show that the dialogue tree of a System T term is itself System T-definable, using the Church encoding of trees.
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Martín H. Escardó, Bruno da Rocha Paiva, Vincent Rahli, and Ayberk Tosun. Internal Effectful Forcing in System T. In 10th International Conference on Formal Structures for Computation and Deduction (FSCD 2025). Leibniz International Proceedings in Informatics (LIPIcs), Volume 337, pp. 19:1-19:17, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2025)

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@InProceedings{escardo_et_al:LIPIcs.FSCD.2025.19,
  author =	{Escard\'{o}, Mart{\'\i}n H. and da Rocha Paiva, Bruno and Rahli, Vincent and Tosun, Ayberk},
  title =	{{Internal Effectful Forcing in System T}},
  booktitle =	{10th International Conference on Formal Structures for Computation and Deduction (FSCD 2025)},
  pages =	{19:1--19:17},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-374-4},
  ISSN =	{1868-8969},
  year =	{2025},
  volume =	{337},
  editor =	{Fern\'{a}ndez, Maribel},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.FSCD.2025.19},
  URN =		{urn:nbn:de:0030-drops-236344},
  doi =		{10.4230/LIPIcs.FSCD.2025.19},
  annote =	{Keywords: Effectful forcing, Continuity, System T, Constructive Mathematics}
}
Document
Invited Talk
DOI: 10.4230/LIPIcs.FSCD.2025.1
Computation First: Rebuilding Constructivism with Effects (Invited Talk)

Authors: Liron Cohen

Published in: LIPIcs, Volume 337, 10th International Conference on Formal Structures for Computation and Deduction (FSCD 2025)

Abstract
Constructive logic and type theory have traditionally been grounded in pure, effect-free model of computation. This paper argues that such a restriction is not a foundational necessity but a historical artifact, and it advocates for a broader perspective of effectful constructivism, where computational effects, such as state, non-determinism, and exceptions, are directly and internally embedded in the logical and computational foundations. We begin by surveying examples where effects reshape logical principles, and then outline three approaches to effectful constructivism, focusing on realizability models: Monadic Combinatory Algebras, which extend classical partial combinatory algebras with effectful computation; Evidenced Frames, a flexible semantic structure capable of uniformly capturing a wide range of effects; and Effectful Higher-Order Logic (EffHOL), a syntactic approach that directly translates logical propositions into specifications for effectful programs. We further illustrate how concrete type theories can internalize effects, via the family of type theories TT^□_C. Together, these works demonstrate that effectful constructivism is not merely possible but a natural and robust extension of traditional frameworks.
Cite as

Liron Cohen. Computation First: Rebuilding Constructivism with Effects (Invited Talk). In 10th International Conference on Formal Structures for Computation and Deduction (FSCD 2025). Leibniz International Proceedings in Informatics (LIPIcs), Volume 337, pp. 1:1-1:20, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2025)

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@InProceedings{cohen:LIPIcs.FSCD.2025.1,
  author =	{Cohen, Liron},
  title =	{{Computation First: Rebuilding Constructivism with Effects}},
  booktitle =	{10th International Conference on Formal Structures for Computation and Deduction (FSCD 2025)},
  pages =	{1:1--1:20},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-374-4},
  ISSN =	{1868-8969},
  year =	{2025},
  volume =	{337},
  editor =	{Fern\'{a}ndez, Maribel},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.FSCD.2025.1},
  URN =		{urn:nbn:de:0030-drops-236167},
  doi =		{10.4230/LIPIcs.FSCD.2025.1},
  annote =	{Keywords: Effectful constructivism, realizability, type theory, monadic combinatory algebras, evidenced frame}
}
Document
DOI: 10.4230/LIPIcs.FSCD.2025.34
Monad Translations for Higher-Order Logic

Authors: Thomas Traversié

Published in: LIPIcs, Volume 337, 10th International Conference on Formal Structures for Computation and Deduction (FSCD 2025)

Abstract
Classical logic can be embedded into intuitionistic logic by inserting double negations in formulas. Several translations generalize this idea by using monad operators instead of double negations. They eliminate particular axioms, for instance the principle of excluded middle or the principle of explosion, and therefore can be used to embed classical logic into intuitionistic logic or intuitionistic logic into minimal logic. Such translations have been defined for first-order logic. In this paper, we define a translation, parameterized by monad operators, for higher-order logic. In particular, the property that any formula and its translation are equivalent in the presence of the eliminated axiom holds under functional extensionality and propositional extensionality. We apply this translation to embed higher-order classical (respectively intuitionistic) logic into higher-order intuitionistic (respectively minimal) logic. By adapting Friedman’s trick, we show that coherent formulas correspond to a constructive fragment of higher-order classical logic, meaning that we can transform classical proofs into intuitionistic proofs without modifying the proven statements.
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Thomas Traversié. Monad Translations for Higher-Order Logic. In 10th International Conference on Formal Structures for Computation and Deduction (FSCD 2025). Leibniz International Proceedings in Informatics (LIPIcs), Volume 337, pp. 34:1-34:14, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2025)

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@InProceedings{traversie:LIPIcs.FSCD.2025.34,
  author =	{Traversi\'{e}, Thomas},
  title =	{{Monad Translations for Higher-Order Logic}},
  booktitle =	{10th International Conference on Formal Structures for Computation and Deduction (FSCD 2025)},
  pages =	{34:1--34:14},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-374-4},
  ISSN =	{1868-8969},
  year =	{2025},
  volume =	{337},
  editor =	{Fern\'{a}ndez, Maribel},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.FSCD.2025.34},
  URN =		{urn:nbn:de:0030-drops-236495},
  doi =		{10.4230/LIPIcs.FSCD.2025.34},
  annote =	{Keywords: Higher-order logic, Intuitionistic logic, Kuroda’s translation, Monad}
}
Document
DOI: 10.4230/LIPIcs.TYPES.2024.3
A Foundation for Synthetic Stone Duality

Authors: Felix Cherubini, Thierry Coquand, Freek Geerligs, and Hugo Moeneclaey

Published in: LIPIcs, Volume 336, 30th International Conference on Types for Proofs and Programs (TYPES 2024)

Abstract
The language of homotopy type theory has proved to be an appropriate internal language for various higher toposes, for example for the Zariski topos in Synthetic Algebraic Geometry. This paper aims to do the same for the higher topos of light condensed anima of Dustin Clausen and Peter Scholze. This seems to be an appropriate setting for synthetic topology in the style of Martín Escardó. We use homotopy type theory extended with 4 axioms. We prove Markov’s principle, LLPO and the negation of WLPO. Then we define a type of open propositions, inducing a topology on any type such that any map is continuous. We give a synthetic definition of second countable Stone and compact Hausdorff spaces, and show that their induced topologies are as expected. This means that any map from e.g. the unit interval 𝕀 to itself is continuous in the usual epsilon-delta sense. With the usual definition of cohomology in homotopy type theory, we show that H¹(S,ℤ) = 0 for S Stone and that H¹(X,ℤ) for X compact Hausdorff can be computed using Čech cohomology. We use this to prove H¹(𝕀¹,ℤ) = 0 and H¹(𝕊¹,ℤ) = ℤ where 𝕊¹ is the set ℝ/ℤ. As an application, we give a synthetic proof of Brouwer’s fixed-point theorem.
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Felix Cherubini, Thierry Coquand, Freek Geerligs, and Hugo Moeneclaey. A Foundation for Synthetic Stone Duality. In 30th International Conference on Types for Proofs and Programs (TYPES 2024). Leibniz International Proceedings in Informatics (LIPIcs), Volume 336, pp. 3:1-3:20, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2025)

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@InProceedings{cherubini_et_al:LIPIcs.TYPES.2024.3,
  author =	{Cherubini, Felix and Coquand, Thierry and Geerligs, Freek and Moeneclaey, Hugo},
  title =	{{A Foundation for Synthetic Stone Duality}},
  booktitle =	{30th International Conference on Types for Proofs and Programs (TYPES 2024)},
  pages =	{3:1--3:20},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-376-8},
  ISSN =	{1868-8969},
  year =	{2025},
  volume =	{336},
  editor =	{M{\o}gelberg, Rasmus Ejlers and van den Berg, Benno},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.TYPES.2024.3},
  URN =		{urn:nbn:de:0030-drops-233659},
  doi =		{10.4230/LIPIcs.TYPES.2024.3},
  annote =	{Keywords: Homotopy Type Theory, Synthetic Topology, Cohomology}
}
Document
DOI: 10.4230/LIPIcs.TYPES.2024.1
Formalizing Equivalences Without Tears

Authors: Tom de Jong

Published in: LIPIcs, Volume 336, 30th International Conference on Types for Proofs and Programs (TYPES 2024)

Abstract
This expository note describes two convenient techniques in the context of homotopy type theory for proving - and formalizing - that a given map is an equivalence. The first technique decomposes the map as a series of basic equivalences, while the second refines this approach using the 3-for-2 property of equivalences. The techniques are illustrated by proving a basic result in synthetic homotopy theory.
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Tom de Jong. Formalizing Equivalences Without Tears. In 30th International Conference on Types for Proofs and Programs (TYPES 2024). Leibniz International Proceedings in Informatics (LIPIcs), Volume 336, pp. 1:1-1:6, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2025)

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@InProceedings{dejong:LIPIcs.TYPES.2024.1,
  author =	{de Jong, Tom},
  title =	{{Formalizing Equivalences Without Tears}},
  booktitle =	{30th International Conference on Types for Proofs and Programs (TYPES 2024)},
  pages =	{1:1--1:6},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-376-8},
  ISSN =	{1868-8969},
  year =	{2025},
  volume =	{336},
  editor =	{M{\o}gelberg, Rasmus Ejlers and van den Berg, Benno},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.TYPES.2024.1},
  URN =		{urn:nbn:de:0030-drops-233632},
  doi =		{10.4230/LIPIcs.TYPES.2024.1},
  annote =	{Keywords: 3-for-2 property, 2-out-of-3 property, definitional equality, equivalence, formalization of mathematics, synthetic homotopy theory, type theory}
}
Document
DOI: 10.4230/LIPIcs.FSCD.2021.8
Predicative Aspects of Order Theory in Univalent Foundations

Authors: Tom de Jong and Martín Hötzel Escardó

Published in: LIPIcs, Volume 195, 6th International Conference on Formal Structures for Computation and Deduction (FSCD 2021)

Abstract
We investigate predicative aspects of order theory in constructive univalent foundations. By predicative and constructive, we respectively mean that we do not assume Voevodsky’s propositional resizing axioms or excluded middle. Our work complements existing work on predicative mathematics by exploring what cannot be done predicatively in univalent foundations. Our first main result is that nontrivial (directed or bounded) complete posets are necessarily large. That is, if such a nontrivial poset is small, then weak propositional resizing holds. It is possible to derive full propositional resizing if we strengthen nontriviality to positivity. The distinction between nontriviality and positivity is analogous to the distinction between nonemptiness and inhabitedness. We prove our results for a general class of posets, which includes directed complete posets, bounded complete posets and sup-lattices, using a technical notion of a δ_V-complete poset. We also show that nontrivial locally small δ_V-complete posets necessarily lack decidable equality. Specifically, we derive weak excluded middle from assuming a nontrivial locally small δ_V-complete poset with decidable equality. Moreover, if we assume positivity instead of nontriviality, then we can derive full excluded middle. Secondly, we show that each of Zorn’s lemma, Tarski’s greatest fixed point theorem and Pataraia’s lemma implies propositional resizing. Hence, these principles are inherently impredicative and a predicative development of order theory must therefore do without them. Finally, we clarify, in our predicative setting, the relation between the traditional definition of sup-lattice that requires suprema for all subsets and our definition that asks for suprema of all small families.
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Tom de Jong and Martín Hötzel Escardó. Predicative Aspects of Order Theory in Univalent Foundations. In 6th International Conference on Formal Structures for Computation and Deduction (FSCD 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 195, pp. 8:1-8:18, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)

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@InProceedings{dejong_et_al:LIPIcs.FSCD.2021.8,
  author =	{de Jong, Tom and Escard\'{o}, Mart{\'\i}n H\"{o}tzel},
  title =	{{Predicative Aspects of Order Theory in Univalent Foundations}},
  booktitle =	{6th International Conference on Formal Structures for Computation and Deduction (FSCD 2021)},
  pages =	{8:1--8:18},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-191-7},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{195},
  editor =	{Kobayashi, Naoki},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.FSCD.2021.8},
  URN =		{urn:nbn:de:0030-drops-142461},
  doi =		{10.4230/LIPIcs.FSCD.2021.8},
  annote =	{Keywords: order theory, constructivity, predicativity, univalent foundations}
}
Document
DOI: 10.4230/LIPIcs.CSL.2021.28
Domain Theory in Constructive and Predicative Univalent Foundations

Authors: Tom de Jong and Martín Hötzel Escardó

Published in: LIPIcs, Volume 183, 29th EACSL Annual Conference on Computer Science Logic (CSL 2021)

Abstract
We develop domain theory in constructive univalent foundations without Voevodsky’s resizing axioms. In previous work in this direction, we constructed the Scott model of PCF and proved its computational adequacy, based on directed complete posets (dcpos). Here we further consider algebraic and continuous dcpos, and construct Scott’s D_∞ model of the untyped λ-calculus. A common approach to deal with size issues in a predicative foundation is to work with information systems or abstract bases or formal topologies rather than dcpos, and approximable relations rather than Scott continuous functions. Here we instead accept that dcpos may be large and work with type universes to account for this. For instance, in the Scott model of PCF, the dcpos have carriers in the second universe U₁ and suprema of directed families with indexing type in the first universe U₀. Seeing a poset as a category in the usual way, we can say that these dcpos are large, but locally small, and have small filtered colimits. In the case of algebraic dcpos, in order to deal with size issues, we proceed mimicking the definition of accessible category. With such a definition, our construction of Scott’s D_∞ again gives a large, locally small, algebraic dcpo with small directed suprema.
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Tom de Jong and Martín Hötzel Escardó. Domain Theory in Constructive and Predicative Univalent Foundations. In 29th EACSL Annual Conference on Computer Science Logic (CSL 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 183, pp. 28:1-28:18, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)

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@InProceedings{dejong_et_al:LIPIcs.CSL.2021.28,
  author =	{de Jong, Tom and Escard\'{o}, Mart{\'\i}n H\"{o}tzel},
  title =	{{Domain Theory in Constructive and Predicative Univalent Foundations}},
  booktitle =	{29th EACSL Annual Conference on Computer Science Logic (CSL 2021)},
  pages =	{28:1--28:18},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-175-7},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{183},
  editor =	{Baier, Christel and Goubault-Larrecq, Jean},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.CSL.2021.28},
  URN =		{urn:nbn:de:0030-drops-134625},
  doi =		{10.4230/LIPIcs.CSL.2021.28},
  annote =	{Keywords: domain theory, constructivity, predicativity, univalent foundations}
}
Document
DOI: 10.4230/LIPIcs.TYPES.2016.7
Parametricity, Automorphisms of the Universe, and Excluded Middle

Authors: Auke B. Booij, Martín H. Escardó, Peter LeFanu Lumsdaine, and Michael Shulman

Published in: LIPIcs, Volume 97, 22nd International Conference on Types for Proofs and Programs (TYPES 2016)

Abstract
It is known that one can construct non-parametric functions by assuming classical axioms. Our work is a converse to that: we prove classical axioms in dependent type theory assuming specific instances of non-parametricity. We also address the interaction between classical axioms and the existence of automorphisms of a type universe. We work over intensional Martin-Löf dependent type theory, and for some results assume further principles including function extensionality, propositional extensionality, propositional truncation, and the univalence axiom.
Cite as

Auke B. Booij, Martín H. Escardó, Peter LeFanu Lumsdaine, and Michael Shulman. Parametricity, Automorphisms of the Universe, and Excluded Middle. In 22nd International Conference on Types for Proofs and Programs (TYPES 2016). Leibniz International Proceedings in Informatics (LIPIcs), Volume 97, pp. 7:1-7:14, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2018)

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@InProceedings{booij_et_al:LIPIcs.TYPES.2016.7,
  author =	{Booij, Auke B. and Escard\'{o}, Mart{\'\i}n H. and Lumsdaine, Peter LeFanu and Shulman, Michael},
  title =	{{Parametricity, Automorphisms of the Universe, and Excluded Middle}},
  booktitle =	{22nd International Conference on Types for Proofs and Programs (TYPES 2016)},
  pages =	{7:1--7:14},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-065-1},
  ISSN =	{1868-8969},
  year =	{2018},
  volume =	{97},
  editor =	{Ghilezan, Silvia and Geuvers, Herman and Ivetic, Jelena},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.TYPES.2016.7},
  URN =		{urn:nbn:de:0030-drops-98554},
  doi =		{10.4230/LIPIcs.TYPES.2016.7},
  annote =	{Keywords: relational parametricity, dependent type theory, univalent foundations, homotopy type theory, excluded middle, classical mathematics, constructive mat}
}
Document
DOI: 10.4230/LIPIcs.CSL.2017.21
Partial Elements and Recursion via Dominances in Univalent Type Theory

Authors: Martín H. Escardó and Cory M. Knapp

Published in: LIPIcs, Volume 82, 26th EACSL Annual Conference on Computer Science Logic (CSL 2017)

Abstract
We begin by revisiting partiality in univalent type theory via the notion of dominance. We then perform first steps in constructive computability theory, discussing the consequences of working with computability as property or structure, without assuming countable choice or Markov’s principle. A guiding question is what, if any, notion of partial function allows the proposition “all partial functions on natural numbers are Turing computable” to be consistent.
Cite as

Martín H. Escardó and Cory M. Knapp. Partial Elements and Recursion via Dominances in Univalent Type Theory. In 26th EACSL Annual Conference on Computer Science Logic (CSL 2017). Leibniz International Proceedings in Informatics (LIPIcs), Volume 82, pp. 21:1-21:16, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2017)

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@InProceedings{escardo_et_al:LIPIcs.CSL.2017.21,
  author =	{Escard\'{o}, Mart{\'\i}n H. and Knapp, Cory M.},
  title =	{{Partial Elements and Recursion via Dominances in Univalent Type Theory}},
  booktitle =	{26th EACSL Annual Conference on Computer Science Logic (CSL 2017)},
  pages =	{21:1--21:16},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-045-3},
  ISSN =	{1868-8969},
  year =	{2017},
  volume =	{82},
  editor =	{Goranko, Valentin and Dam, Mads},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.CSL.2017.21},
  URN =		{urn:nbn:de:0030-drops-76822},
  doi =		{10.4230/LIPIcs.CSL.2017.21},
  annote =	{Keywords: univalent type theory, homotopy type theory, partial function, dominance, recursion theory, computability theory}
}
Document
DOI: 10.4230/LIPIcs.TLCA.2015.153
The Inconsistency of a Brouwerian Continuity Principle with the Curry–Howard Interpretation

Authors: Martín Hötzel Escardó and Chuangjie Xu

Published in: LIPIcs, Volume 38, 13th International Conference on Typed Lambda Calculi and Applications (TLCA 2015)

Abstract
If all functions (N -> N) -> N are continuous then 0 = 1. We establish this in intensional (and hence also in extensional) intuitionistic dependent-type theories, with existence in the formulation of continuity expressed as a Sigma type via the Curry-Howard interpretation. But with an intuitionistic notion of anonymous existence, defined as the propositional truncation of Sigma, it is consistent that all such functions are continuous. A model is Johnstone’s topological topos. On the other hand, any of these two intuitionistic conceptions of existence give the same, consistent, notion of uniform continuity for functions (N -> 2) -> N, again valid in the topological topos. It is open whether the consistency of (uniform) continuity extends to homotopy type theory. The theorems of type theory informally proved here are also formally proved in Agda, but the development presented here is self-contained and doesn't show Agda code.
Cite as

Martín Hötzel Escardó and Chuangjie Xu. The Inconsistency of a Brouwerian Continuity Principle with the Curry–Howard Interpretation. In 13th International Conference on Typed Lambda Calculi and Applications (TLCA 2015). Leibniz International Proceedings in Informatics (LIPIcs), Volume 38, pp. 153-164, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2015)

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@InProceedings{hotzelescardo_et_al:LIPIcs.TLCA.2015.153,
  author =	{H\"{o}tzel Escard\'{o}, Mart{\'\i}n and Xu, Chuangjie},
  title =	{{The Inconsistency of a Brouwerian Continuity Principle with the Curry–Howard Interpretation}},
  booktitle =	{13th International Conference on Typed Lambda Calculi and Applications (TLCA 2015)},
  pages =	{153--164},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-939897-87-3},
  ISSN =	{1868-8969},
  year =	{2015},
  volume =	{38},
  editor =	{Altenkirch, Thorsten},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.TLCA.2015.153},
  URN =		{urn:nbn:de:0030-drops-51618},
  doi =		{10.4230/LIPIcs.TLCA.2015.153},
  annote =	{Keywords: Dependent type, intensional Martin-L\"{o}f type theory, Curry-Howard interpretation, constructive mathematics, Brouwerian continuity axioms, anonymous exi}
}
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