# Category Archives: Higher Inductive Types

## Impredicative Encodings, Part 3

In this post I will argue that, improving on previous work of Awodey-Frey-Speight, (higher) inductive types can be defined using impredicative encodings with their full dependent induction principles — in particular, eliminating into all type families without any truncation hypotheses … Continue reading

## 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

## 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

## 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

## 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

## 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

## 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

## Eilenberg-MacLane Spaces in HoTT

For those of you who have been waiting with bated breath to find out what happened to your favorite characters after the end of Chapter 8 of the HoTT book, there is now a new installment: Eilenberg-MacLane Spaces in Homotopy Type Theory Dan … Continue reading

## Composition is not what you think it is! Why “nearly invertible” isn’t.

A few months ago, Nicolai Kraus posted an interesting and surprising result: the truncation map : ℕ → ‖ℕ‖ is nearly invertible. This post attempts to explain why “nearly invertible” is a misnomer. I, like many others, was very … Continue reading

## The surreals contain the plump ordinals

The very last section of the book presents a constructive definition of Conway’s surreal numbers as a higher inductive-inductive type. Conway’s classical surreals include the ordinal numbers; so it’s natural to wonder whether, or in what sense, this may be … Continue reading

## New writeup of πn(Sn)

I’m giving a talk this week at CPP. While I’m going to talk more broadly about applications of higher inductive types, for the proceedings, Guillaume Brunerie and I put together an “informalization” of πn(Sn), which you can find here. This is … Continue reading

## The Truncation Map |_| : ℕ -> ‖ℕ‖ is nearly Invertible

Magic tricks are often entertaining, sometimes boring, and in some rarer cases astonishing. For me, the following trick belongs to the third type: the magician asks a volunteer in the audience to think of a natural number. The volunteer secretly … Continue reading

## The HoTT Book

This posting is the official announcement of The HoTT Book, or more formally: Homotopy Type Theory: Univalent Foundations of Mathematics The Univalent Foundations Program, Institute for Advanced Study The book is the result of an amazing collaboration between virtually everyone involved … Continue reading

## Homotopy Theory in Type Theory: Progress Report

A little while ago, we gave an overview of the kinds of results in homotopy theory that we might try to prove in homotopy type theory (such as calculating homotopy groups of spheres), and the basic tools used in our synthetic approach … Continue reading

## Covering Spaces

Covering spaces are one of the important topics in classical homotopy theory, and this post summarizes what we have done in HoTT. We have formulated the covering spaces and (re)proved the classification theorem based on (right) -sets, i.e., sets equipped with … Continue reading

## Homotopy Theory in Homotopy Type Theory: Introduction

Many of us working on homotopy type theory believe that it will be a better framework for doing math, and in particular computer-checked math, than set theory or classical higher-order logic or non-univalent type theory. One reason we believe this … Continue reading

## Running Spheres in Agda, Part II

(Sorry for the long delay after the Part I of this post.) This post will summarize my work on defining spheres in arbitrary finite dimensions (Sⁿ) in Agda. I am going to use the tools for higher-order paths (discussed in … Continue reading

## All Modalities are HITs

Last Friday at IAS, Guillaume Brunerie presented a very nice proof that for all . I hope he will write it up and blog about it himself; I want to talk instead about a question regarding modalities that was raised … Continue reading

## Truncations and truncated higher inductive types

Truncation is a homotopy-theoretic construction that given a space and an integer n returns an n-truncated space together with a map in an universal way. More precisely, if i is the inclusion of n-truncated spaces into spaces, then n-truncation is … Continue reading

## Running Spheres in Agda, Part I

Running Spheres in Agda, Part I Introduction Where will we end up with if we generalize circles S¹ to spheres Sⁿ? Here is the first part of my journey to a 95% working definition for arbitrary finite dimension. The 5% … Continue reading

## A Simpler Proof that π₁(S¹) is Z

Last year, Mike Shulman proved that π₁(S¹) is Z in Homotopy Type Theory. While trying to understand Mike’s proof, I came up with a simplification that shortens the proof a bunch (100 lines of Agda as compared to 380 lines … Continue reading

## Reducing all HIT’s to 1-HIT’s

For a while, Mike Shulman and I (and others) have wondered on and off whether it might be possible to represent all higher inductive types (i.e. with constructors of arbitrary dimension) using just 1-HIT’s (0- and 1-cell constructors only), somewhat … Continue reading

## Localization as an Inductive Definition

I’ve been talking a lot about reflective subcategories (or more precisely, reflective subfibrations) in type theory lately (here and here and here), so I started to wonder about general ways to construct them inside type theory. There are some simple … Continue reading

## A formal proof that π₁(S¹)=Z

The idea of higher inductive types, as described here and here, purports (among other things) to give us objects in type theory which represent familiar homotopy types from topology. Perhaps the simplest nontrivial such type is the circle, , which … Continue reading

## Higher Inductive Types via Impredicative Polymorphism

The proof assistant Coq is based on a formal system called the “Predicative Calculus of (Co)Inductive Constructions” (pCiC). But before pCiC, there was the “Calculus of Constructions” (CoC), in which inductive types were not a basic object, but could be … Continue reading

## Higher Inductive Types: a tour of the menagerie

(This was written in inadvertent parallel with Mike’s latest post at the Café, so there’s a little overlap — Mike’s discusses the homotopy-theoretic aspects more, this post the type-theoretic nitty-gritty.) Higher inductive types have been mentioned now in several other … Continue reading

## Running Circles Around (In) Your Proof Assistant; or, Quotients that Compute

Higher-dimensional inductive types are an idea that many people have been kicking around lately; for example, in An Interval Type Implies Function Extensionality. The basic idea is that you define a type by specifying both what it’s elements are (as … Continue reading