Outputs (22)

Internal Parametricity, without an Interval (2024)
Journal Article
Altenkirch, T., Chamoun, Y., Kaposi, A., & Shulman, M. (2024). Internal Parametricity, without an Interval. Proceedings of the ACM on Programming Languages, 8(POPL), 2340-2369. https://doi.org/10.1145/3632920

Parametricity is a property of the syntax of type theory implying, e.g., that there is only one function having the type of the polymorphic identity function. Parametricity is usually proven externally, and does not hold internally. Internalising it... Read More about Internal Parametricity, without an Interval.

The Münchhausen Method in Type Theory (2023)
Journal Article
Altenkirch, T., Kaposi, A., Šinkarovs, A., & Végh, T. (2023). The Münchhausen Method in Type Theory. LIPIcs, 269, https://doi.org/10.4230/LIPIcs.TYPES.2022.10

In one of his long tales, after falling into a swamp, Baron Münchhausen salvaged himself and the horse by lifting them both up by his hair. Inspired by this, the paper presents a technique to justify very dependent types. Such types reference the ter... Read More about The Münchhausen Method in Type Theory.

Combinatory logic and lambda calculus are equal, algebraically (2023)
Presentation / Conference Contribution
Altenkirch, T., Kaposi, A., Šinkarovs, A., & Végh, T. Combinatory logic and lambda calculus are equal, algebraically. Presented at FSCD, Rome, Italy

It is well-known that extensional lambda calculus is equivalent to extensional combinatory logic. In this paper we describe a formalisation of this fact in Cubical Agda. The distinguishing features of our formalisation are the following: (i) Both lan... Read More about Combinatory logic and lambda calculus are equal, algebraically.

Should Type Theory Replace Set Theory as the Foundation of Mathematics? (2023)
Journal Article
Altenkirch, T. (2023). Should Type Theory Replace Set Theory as the Foundation of Mathematics?. Global Philosophy, 33(1), Article 21. https://doi.org/10.1007/s10516-023-09676-0

Mathematicians often consider Zermelo-Fraenkel Set Theory with Choice (ZFC) as the only foundation of Mathematics, and frequently don’t actually want to think much about foundations. We argue here that modern Type Theory, i.e. Homotopy Type Theory (H... Read More about Should Type Theory Replace Set Theory as the Foundation of Mathematics?.

The Integers as a Higher Inductive Type (2020)
Presentation / Conference Contribution
Altenkirch, T., & Scoccola, L. (2020, July). The Integers as a Higher Inductive Type. Presented at LICS '20: 35th Annual ACM/IEEE Symposium on Logic in Computer Science, Saarbrücken Germany

We consider the problem of defining the integers in Homotopy Type Theory (HoTT). We can define the type of integers as signed natural numbers (i.e., using a coproduct), but its induction principle is very inconvenient to work with, since it leads to... Read More about The Integers as a Higher Inductive Type.

Naive Type Theory (2019)
Book Chapter
Altenkirch, T. (2019). Naive Type Theory. In S. Centrone, D. Kant, & D. Sarikaya (Eds.), Reflections on the Foundations of Mathematics: Univalent Foundations, Set Theory and General Thoughts (101-136). Springer

We introduce Type Theory, including Homotopy Type Theory, as an alternative to set theory as a foundation of Mathematics emphasising the intuitive and naive understanding of its concepts.

Free Higher Groups in Homotopy Type Theory (2018)
Presentation / Conference Contribution
Kraus, N., & Altenkirch, T. (2018, July). Free Higher Groups in Homotopy Type Theory. Presented at LICS '18: 33rd Annual ACM/IEEE Symposium on Logic in Computer Science, Oxford United Kingdom

© 2018 ACM. Given a type A in homotopy type theory (HoTT), we can define the free∞-group onA as the loop space of the suspension ofA+1. Equivalently, this free higher group can be defined as a higher inductive type F(A) with constructors unit : F(A),... Read More about Free Higher Groups in Homotopy Type Theory.

Quotient inductive-inductive types (2018)
Book Chapter
Altenkirch, T., Capriotti, P., Dijkstra, G., Kraus, N., & Nordvall Forsberg, F. (2018). Quotient inductive-inductive types. In C. Baier, & U. Dal Lago (Eds.), FoSSaCS 2018: Foundations of Software Science and Computation Structures (293-310). Springer Publishing Company. https://doi.org/10.1007/978-3-319-89366-2_16

Higher inductive types (HITs) in Homotopy Type Theory (HoTT) allow the definition of datatypes which have constructors for equalities over the defined type. HITs generalise quotient types and allow to define types which are not sets in the sense of H... Read More about Quotient inductive-inductive types.

Towards a cubical type theory without an interval (2018)
Journal Article
Altenkirch, T., & Kaposi, A. (2018). Towards a cubical type theory without an interval. LIPIcs, 3:1-3:27. https://doi.org/10.4230/LIPIcs.TYPES.2015.3

Following the cubical set model of type theory which validates the univalence axiom, cubical type theories have been developed that interpret the identity type using an interval pretype. These theories start from a geometric view of equality. A proof... Read More about Towards a cubical type theory without an interval.

Normalisation by evaluation for type theory, in type theory (2017)
Journal Article
Altenkirch, T., & Kaposi, A. (2017). Normalisation by evaluation for type theory, in type theory. Logical Methods in Computer Science, 13(4), https://doi.org/10.23638/LMCS-13%284%3A1%292017

© Altenkirch and Kaposi. We develop normalisation by evaluation (NBE) for dependent types based on presheaf categories. Our construction is formulated in the metalanguage of type theory using quotient inductive types. We use a typed presentation henc... Read More about Normalisation by evaluation for type theory, in type theory.