# Category Archives: Code

## 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

## A Formalized Interpreter

I’d like to announce a result that might be interesting: an interpreter for a very small dependent type system in Coq, assuming uniqueness of identity proofs (UIP). Because it assumes UIP, it’s not immediately compatible with HoTT, but it seems … Continue reading

Posted in Code, Foundations, Programming | 77 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

## 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

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

## 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

Posted in Code, Higher Inductive Types | 42 Comments

## Universe n is not an n-Type

Joint work with Christian Sattler Some time ago, at the UF Program in Princeton, I presented a proof that Universe n is not an n-type. We have now formalized that proof in Agda and want to present it here. One … Continue reading