# Category Archives: Higher Inductive Types

## Combinatorial Species and Finite Sets in HoTT

(Post by Brent Yorgey) My dissertation was on the topic of combinatorial species, and specifically on the idea of using species as a foundation for thinking about generalized notions of algebraic data types. (Species are sort of dual to containers; … Continue reading

## Colimits in HoTT

In this post, I would want to present you two things: the small library about colimits that I formalized in Coq, a construction of the image of a function as a colimit, which is essentially a sliced version of the … Continue reading

Posted in Code, Higher Inductive Types | 14 Comments

## The Lean Theorem Prover

Lean is a new player in the field of proof assistants for Homotopy Type Theory. It is being developed by Leonardo de Moura working at Microsoft Research, and it is still under active development for the foreseeable future. The code … Continue reading

Posted in Code, Higher Inductive Types, Programming | 47 Comments

## Constructing the Propositional Truncation using Nonrecursive HITs

In this post, I want to talk about a construction of the propositional truncation of an arbitrary type using only non-recursive HITs. The whole construction is formalized in the new proof assistant Lean, and available on Github. I’ll write another … Continue reading

Posted in Code, Higher Inductive Types | 25 Comments

## The torus is the product of two circles, cubically

Back in the summer of 2012, emboldened by how nicely the calculation π₁(S¹) had gone, I asked a summer research intern, Joseph Lee, to work on formalizing a proof that the higher-inductive definition of the torus (see Section 6.6 of the HoTT book) … Continue reading

Posted in Higher Inductive Types, Homotopy Theory | 31 Comments

## Universal properties without function extensionality

A universal property, in the sense of category theory, generally expresses that a map involving hom-sets, induced by composition with some canonical map(s), is an isomorphism. In type theory we express this using equivalences of hom-types. For instance, the universal … Continue reading

Posted in Code, Foundations, Higher Inductive Types | 16 Comments

## Higher inductive-recursive univalence and type-directed definitions

In chapter 2 of the HoTT book, we prove theorems (or, in a couple of cases, assert axioms) characterizing the equality types of all the standard type formers. For instance, we have and (that’s function extensionality) and (that’s univalence). However, … Continue reading

Posted in Code, Foundations, Higher Inductive Types, Univalence | 7 Comments