After a productive year at the Institute for Advanced Study, the Univalent Foundations Program has written a book on Homotopy Type Theory (HoTT). The foreword gives a succinct description of the purpose of this book:

We did not set out to write a book. The present work has its origins in our collective attempts to develop a new style of "informal type theory" that can be read and understood by a human being, as a complement to a formal proof that can be checked by a machine. Univalent foundations is closely tied to the idea of a foundation of mathematics that can be implemented in a computer proof assistant. Although such a formalization is not part of this book, much of the material presented here was actually done first in the fully formalized setting inside a proof assistant, and only later "unformalized" to arrive at the presentation you find before you — a remarkable inversion of the usual state of affairs in formalized mathematics.

The danger in writing such a book is to fall into the minefields of logic wars. The authors successfully avoided much of these traps, so logicians from other perspectives can read the book without too much cringing. To avoid unnecessary confusion, I recommend mentally substituting most instances of "set theory" by the more apropos "classical mathematics." Readers from strongly opposing points of view should be prepared for a certain amount of propaganda, which is to be expected in a book written to promote one point of view. Barring these caveats, you will find an enjoyable and well-written book on a very interesting subject. Readers should not be too concerned with the word "homotopy" in the title, homotopy theory is not required background for the book though some basic knowledge of the ideas of topology and homotopy theory helps to understand the motivation behind certain concepts.

Having addressed all the necessary caveats, let’s talk about why this book is interesting and why you should read it…

### What is so hot about HoTT?

The most interesting aspect from my point of view is that HoTT fully supports proof-relevant mathematics, a way of doing mathematics where proofs are real objects that are manipulated on the same level as numbers, sets, functions and all the usual objects of classical mathematics. This is not a brand new idea, logicians have been playing with proofs in this way for a long while, but HoTT brings this idea into the realm of everyday mathematics and that is a major step forward in mathematics.

The key difference with first-order logic is that equality is not primitive. To define a type $A$ one must also define what equality means for $A$. Formally if $x,y:A$ then $x =_A y$ (or $\mathsf{Id}_A(x,y)$) is another type, an identity type, which can be thought of as made of reasons to identify $x$ and $y$. Elements $p:x =_A y$ are often called "paths" by analogy with topolgy. Indeed, these paths can be inverted and concatenated to realize symmetry and transitivity of equality, respectively; reflexivity is realized by a path $\mathsf{refl}_x:x =_A x$. Thus each type $A$ is actually a groupoid rather than a plain set. In fact, since each $x =_A y$ is itself a type with its own identity types and so forth, the type $A$ is actually a kind of $\infty$-groupoid.

It is this rich structure associated with each type is what permits HoTT to support proof relevant mathematics. To get a basic feel of how this works, the statement "$x =_A y$ and $y =_A z$" is interpreted via the product type $(x =_A y)\times(y =_A z)$, whose elements are pairs of paths that explain why $x$ is to be identified with $y$ and why $y$ is to be identified with $z$. Similarly, "$x =_A y$ or $y =_A z$" is interpreted via the coproduct type $(x =_A y) + (y =_A z)$, whose elements are either paths that explain why $x$ is to be identified with $y$ or paths that explain why $y$ is to be identified with $z$. The catch, as you may have guessed from the last example, is that this form of constructive reasoning is intuitionistic and thus not as familiar to mathematicians.

Interestingly, the learning curve for constructive reasoning appears to be much less steep with HoTT than with other constructive frameworks. One of the reasons is that the topological interpretation of the key concepts is very intuitive but more significantly HoTT provides many tools to revert to more familiar territory. The analogue of a plain set in HoTT is a $0$-type: a type $A$ where the identity types $x =_A y$ always contain at most one path. In other words, these are types where the groupoid structure is trivial and contains no other information than how to handle equality of elements. It is consistent with HoTT that the $0$-types form a model of ETCS, a classical theory of sets and functions. Thus, by "truncating" thoughts to $0$-types, one can revert to a more familiar classical setting.

### What is the big deal with univalence?

It is natural to identify things that are not significantly different. For example, the axiom of extensionality in set theory identifies sets that have the same elements since the elements of a set are all that matter in this context. Extensionality for functions identifies functions that agree on all inputs. Univalence is an indiscernibility axiom in the same spirit: it identifies types that are not significantly different.

To make sense of equality for types, we first need to put them in an ambient type, a universe, with its associated identity types. We can’t have a truly universal type since that directly leads to the usual paradoxes of self-reference. Instead, we have a bunch of universes such that each type belongs to some universe and each universe is closed under the basic type formation rules. Once we have a universe $\mathcal{U}$ we can talk about equality of types in $\mathcal{U}$, and because $\mathcal{U}$ is a type we have a lot of freedom in defining what equality means for types in $\mathcal{U}$.

This is exactly what the univalence axiom does. It can be stated elegantly: $(A \simeq B) \simeq (A =_\mathcal{U} B).$ The equivalence relation ${\simeq}$ is similar to isomorphism but it is slightly more permissive. To say $A \simeq B$ requires the existence of a function $f:A \to B$ together with $\ell,r:B \to A$ such that $\ell \circ f$ is homotopic to $\mathsf{id}_A$ and $f \circ r$ is homotopic to $\mathsf{id}_B$. Given two functions $f$ and $g$ of the same dependent product type $\prod_{x:A} B(x)$, a homotopy from $f$ to $g$ is an element of $\prod_{x:A} (f(x) =_{B(x)} g(x))$. So $f$ and $g$ are homotopic if they agree on all inputs, which does not mean that $f = g$ in the absence of function extensionality.

In general type theory, $A \simeq B$ is definitely a good way to say "$A$ and $B$ are not siginificantly different" and thus univalence arguably captures the right indiscernibility axiom for type theory. The surprising fact is that such a strong axiom does not appear to collapse more than it should. The benefits of univalence are interesting and need to be explored further.

I still need to digest the book so this is probably only the first of many posts on HoTT. The next posts will undoubtedly wander deeper into the technical levels of HoTT. There are a few interesting leads but nothing definite yet.

There is only one thing that bugs me right now, which is the way universes are handled in the book. However, since these assumptions do not appear to be crucial for the development of HoTT and there are plenty of alternatives out there, I’m not overly concerned about this at the moment.

I will eventually need to talk about higher inductive types. These are really interesting and I’m happy to see that the book devotes an entire chapter to them. This is a very interesting outgrowth of this project and which deserves study even independently of HoTT.

## 7 thoughts on “Homotopy Type Theory”

1. Ah, but Univalence does itself imply Function Extensionality (more or less…)! (Just because it’s worth mentioning, I’m not supposing you didn’t know)

2. A nice post! Of course, I’m most curious to hear more about your quibbles, in the interests of improving future versions of the book. Can you say any more about why you think our uses of “set theory” should instead be “classical mathematics”? I just searched through the introduction for the phrase “set theory” and I couldn’t find any instances which I could imagine replacing with “classical mathematics”.

Also, can you explain what it is you don’t like about our universes? We did spend a while discussing how to treat universes, and the current choice is a compromise between competing goals. So there’s certainly lots to dislike about it, but I’d like to hear if you have any specific suggestions.

1. There weren’t any serious problems with the uses of “set theory.” The suggestion was for more sensitive readers to tame things down by mentally replacing “set theory” by something less “polarized” since there were a few times where I felt uncomfortable in my reading. I didn’t bother taking notes about this specifically though I did make a note that in chapter 3, “paradoxes of Cantorian set theory” should be “paradoxes of self-reference” since the issue is not inherent to set theory. I will let you know if I find anything that is particularly bad.

As for universes, I’m not a fan of typical ambiguity but I agree that there is probably no better way to do this. My main concern is that you fix an ascending sequence of them. That makes sense for implementation but I would prefer just requiring that any two types belong to some common universe, which gives a lot more freedom how the universes are organized. This doesn’t change much as far as the book’s contents are concerned but I think such freedom is necessary to interpret very strong theories in extensions of HoTT.

1. Ah, a good point about chapter 3. I’ll fix that; thanks!

And I do see what you mean about universes. Ultimately I think we wanted to say the simplest possible correct thing so that we could immediately start ignoring it. (-: Perhaps it would be appropriate to mention in the Notes to chapter 1, though, that a fixed ascending sequence of universes is not the only possible choice. (In fact, even in implementations it may not be the universal choice — I think Coq’s universe handling, and maybe even Agda’s, actually does operate on the system of max/successor rather than indexing by natural numbers.)

2. It’s also not far to attribute those paradoxes to Cantor. Cantor knew that some multiplicities were inconsistent (in modern terminology, that some classes are large), and his set theory had no antinomies. The worst that one could say is that Cantor’s set theory was too imprecise to have the paradoxes. It was Frege’s attempt to make Cantor’s set theory more precise in a too naive way that introduced the paradoxes.

3. I don’t understand what you mean by saying that equality is not primitive. It’s not an independent primitive concept, because it’s just one instance of the general notion of inductive type (and the same could be said of disjunction, conjunction, etc). But it’s not a user-defined concept either (as it is in Bishop’s set theory); you don’t define equality on A in a separate step (any more than you define A + A or other inductive types derivable from A).

1. True. I didn’t put a huge amount of thought in the choice of word there. A more accurate statement is that equality is intrinsic rather than extrinsic.