Hacker News Reader

Comment by spekcular | Hacker News Reader

I find it strange that HoTT and HoTT-adjacent stuff gets posted here so often.

Even in the world of pure mathematics, this is a backwater. As I understand it, the two main selling points are providing a foundation for mathematics and a framework for automated theorem proving, but:

1) We already have a perfectly good foundation of mathematics. It's called ZFC [1], and it does everything we need it to do. If all HoTT does is give a theory that's mutually interpretable with ZFC, that's not interesting (at least to mainstream workers in the foundations of mathematics, who would rather tackle substantive problems).

There are also people like Per-Martin Lof who want to use HoTT as a vehicle for doing mathematics with intuitionistic logic, and in particular denying the law of excluded middle, but at that point you've let the mask slip and you're advocating something that interests approximately zero mainstream mathematicians.

2) It's not at all clear HoTT, or type theory in general, is the best solution for formalizing mathematics. It could be, it couldn't be, but this is ultimately an empirical question . Some comments on the state of the field are given in [2].

[2]: https://xenaproject.wordpress.com/2020/02/09/where-is-the-fa...

View on HN

> vehicle for doing mathematics with intuitionistic logic, and in particular denying the law of excluded middle

I think this is the basis for the enthusiasm, almost entirely. There's a growing perception that a lot of mathematical abstractions that are purely naval-gazing have arisen from ZFC, and the intuitionalists and finitists are gaining followers amongst amateur and upcoming mathematicians. I don't have data to back this, just a feeling that I get.

And I think in particular this is attractive to programmers because this skews closer to what we've learned about computability and program design, and I think there's even some hope of moving forward from obscure notation and single-letter variable names towards a syntax for doing mathematics that can be more expressive and easier to verify.

Couldn’t agree more.

A great example of a mathematics popularizer along these lines is Norman Wildberger, who has an astonishing range of videos on youtube about an amazing range of mathematical topics. Here’s an interview with him: https://youtu.be/E_dGqavx5AU .

My personal feeling is that a lot of beautiful and interesting mathematics will arise as a result of an intuitionist, finitist, computationalist rethinking of the foundations of mathematics.

I am not sure HoTT is your best bet if you want to get away from obscure notation and naval-gazing.

Just kidding. But seriously, ZFC is probably the easiest foundation you can have for proper mathematics. Anything else is more complicated (if you think otherwise, you probably just don't understand it).

You don't need anything like HoTT to start using multi-letter variable names.

E.g. https://mitpress.mit.edu/books/functional-differential-geome...

I just had a flip through this book, yes all the differential geometry is there, but I think its a bit of a let down. If I think "functional" I want to think more abstractly in terms of categories, functors, functions.

The Lisp code in this book is truly not doing anything for me. Seems like a kind of direct numerical computation/translation. I actually think it would be better to do it in Haskell which seems like it would at least let you identify some more generic structures (oh we can actually implement this idea x as a functor/monad blah blah).

> ...a vehicle for doing mathematics with intuitionistic logic, and in particular denying the law of excluded middle...

Intuitionists don't deny the law of excluded middle, they just don't admit it as an axiom. From a classical perspective this looks like denial, since accepting the law means that you have to accept or deny everything, but from an intuitionistic perspective that kind of reasoning looks like begging the question.

> If all HoTT does is give a theory that's mutually interpretable with ZFC, that's not interesting.

"If all python does is give a theory that's mutually interpretable with x86 assembly, that's not interesting."

The point is to have better language support for expressing the things you want to reason about. I don't find it so far-fetched to be interested in better ways of reasoning about equality of types (both as a programmer and as a mathematician).

I don't think this is a good analogy. No one "does mathematics" in ZFC. Mathematicians don't express their reasoning in set-theoretic axioms; they do so in common language proofs. You can see this by opening any graduate textbook or journal.

Mainstream mathematicians are not being prevented from proving theorems because they lack "better ways of reasoning about equality of types."

The importance of ZFC is that it ensures our common language reasoning isn't incoherent (among other things). Once you've done that, you don't need to do it again.

Mathematicians use implicit DSLs approximately embedded in natural language to write proofs. Formalizing these DSLs in ZFC is a nightmare because its axioms are arcane and far from common mathematical discourse (much like assembly). On the other hand, the type formers and rules of (say) MLTT are natural generalizations of the rules of ordinary logic.

> Mainstream mathematicians are not being prevented from proving theorems because they lack "better ways of reasoning about equality of types."

How do you know? I've made the mistake of assuming that the definition of a certain morphism is natural with respect to a certain object when it wasn't (coevaluation in a rigid monoidal category). Better language for equality and naturality are precisely what HoTT provides. More importantly, the hope is that type theory will not only provide better language, but relieve us of the tedium of checking proofs by hand.

> The importance of ZFC is that it ensures our common language reasoning isn't incoherent.

I don't think this is true. AFAIK, no one has written down the definition of (say) a scheme from first principles in ZFC. In fact, if you care about coherence/verification, you should care even more about type theory. IIRC this is what drew Voevodsky to type theory in the first place -- https://mathoverflow.net/questions/234492/what-is-the-mistak...

I don't think you should regard ZFC's purpose as proof verification. If you want verification, by all means use something else, as discussed in the link in my original post (which also addresses your scheme point – that's a problem common to most approaches so far!).

If HoTT helps you prove theorems, great. In particular I could see it being of interest to people working in higher category theory (and such people have responded in other comments here!). My remarks were meant for the other 99% of mathematicians – most of whom go their entire lives without ever writing the word "category" in a paper.

We've hit the comment depth limit, so I'll reply here.

> it's not clear to me why you necessarily want the language you use to rigorously investigate the foundations of mathematics to be the same one you use to do proof verification.

Among other things, I want theorems about foundations to be useful to non-foundational mathematicians. Working on type theory can improve, at the very least, formal verification tools. In my view, traditional ZFC-based mathematical logic has a much smaller likelihood of being useful for regular mathematicians. For example, the vast majority of mathematicians will never explicitly assume the existence of a large cardinal in a paper. It seems to me that the difference in usefulness is probably due to their relative distance from common discourse.

Thanks for the interesting discussion. My main point of disagreement re:foundations is that in my view a central criterion for a foundational system is easy translation of common mathematical discourse into formal proofs. Otherwise, it seems that the coherence of ordinary mathematical discourse must be an article of faith.

I don't have time to explain in detail at the moment, but I recommend reading about the history of set theory and the problems that the introduction of ZFC was meant to solve. I think you'll agree that they were both important and have nothing to do with proof verification.

More generally, it's not clear to me why you necessarily want the language you use to rigorously investigate the foundations of mathematics to be the same one you use to do proof verification. ZFC – and set-theoretic approaches more broadly – facilitate proving things about mathematics (meta-mathematical theorems), and for that they are very useful. E.g., the strength of a theory is equivalent to the size of the cardinals it assumes, loosely speaking [1].

[1] https://en.wikipedia.org/wiki/Large_cardinal

> No one "does mathematics" in ZFC.

How is this not partial evidence that ZFC is trash?

> Mainstream mathematicians are not being prevented from proving theorems because they lack "better ways of reasoning about equality of types."

Mainstream mathematicians do run the risk of being siloed in narrow specialties because abstractions are needed to distill the all definitions of remote areas of mathematics.

Saying HoTT is a failure because people do informal proofs is rather moving the goal posts; people won't do formal proofs until the tooling is really good. But none of the up-and-comming tools are classic first order logic + ZFC (Please don't mention TLA+) and there's a reason for that.

> The importance of ZFC is that it ensures our common language reasoning isn't incoherent (among other things). Once you've done that, you don't need to do it again.

No you need formal proofs to check that there aren't errors along the way. And lack of formal proofs slows down new weird stuff like the ABC conjecture proof candididate.

I don't expect this to be solved now (tooling, as per above), but mathematicians should learn more category theory now as that works just fine pencil paper and brain. When the type theoretic tooling is ready they will be ready.

> How is this not partial evidence that ZFC is trash?

ZFC was never intended as a language for practically doing mathematics in, so that's a weird criticism.

> Mainstream mathematicians do run the risk of being siloed in narrow specialties because abstractions are needed to distill the all definitions of remote areas of mathematics.

Mathematicians are siloed in narrow specialties because they focus on solving hard problems that require a great deal of specific subject matter expertise. I strongly doubt that "distill[ing] the all definitions of remote areas of mathematics" is even possible, or a fruitful project to attempt. Could you give an example of how definitions from, say, probability theory and algebraic geometry might be merged in a useful way?

> No you need formal proofs to check that there aren't errors along the way. And lack of formal proofs slows down new weird stuff like the ABC conjecture proof candididate.

I think this comment misunderstands the problem with the ABC "proof," which is (speaking roughly) that a certain lemma asserted something that was crucial to the argument, but which was never actually proved. Peter Scholze and others spotted this pretty quickly. Formalization wouldn't really have helped at all.

> ZFC was never intended as a language for practically doing mathematics in, so that's a weird criticism.

Isn't that just the point? Currently we do metamathematics in an esoteric language where your normal mathematical tools won't always have the meaning you expect them to have - HoTT is about enabling us to do it in something much closer to normal mathematics.

> Could you give an example of how definitions from, say, probability theory and algebraic geometry might be merged in a useful way?

A lot of category theory is "this theorem from this context can be translated to this theorem in this other context". Do I know any specific equivalences between probability distributions and multivariate polynomials? No. But I wouldn't be at all surprised if we found some. At one point the idea that arithmetic could tell us something about geometry was a surprising and novel one.

> I think this comment misunderstands the problem with the ABC "proof," which is (speaking roughly) that a certain lemma asserted something that was crucial to the argument, but which was never actually proved. Peter Scholze and others spotted this pretty quickly. Formalization wouldn't really have helped at all.

The fact that it was disputed for at least two years whether that lemma was or wasn't valid is the fact that formalisation would have been helpful.

> No one "does mathematics" in ZFC. How is this not partial evidence that ZFC is trash?

What percentage of coders do programming on a turing tape? Is this partial evidence that turing tapes are trash? Does that question even make sense?

> I don't expect this to be solved now (tooling, as per above), but mathematicians should learn more category theory now as that works just fine pencil paper and brain. When the type theoretic tooling is ready they will be ready.

The abstractions of category theory are useless in many areas of mathematics. Prime example: PDEs.

Laypeople think that category theory is the 'ultimate math' because they hear that it provides bridges or analogies between different areas of math. Perhaps programmers are especially prone to this because category theory does have some applications to programming.

The thing is, almost all of pure math is itself is a bridge between different areas of math. Some of these areas are bridged by category theory, some are bridged by other kinds of math, which have less catchy names.

> What percentage of coders do programming on a turing tape? Is this partial evidence that turing tapes are trash? Does that question even make sense?

Yes it is. Turing machine models are very limited, and a programme to let us achieve the things we can do with Turing machines (mainly runtime analysis) with a better model (i.e. a lambda-calculus style model) is a very good idea.

> The thing is, almost all of pure math is itself is a bridge between different areas of math. Some of these areas are bridged by category theory, some are bridged by other kinds of math, which have less catchy names.

I'd be equally interested in a programme of doing metamathematics in some non-category-theoretic model that was still "normal" mathematics in the same way that category theory is (and ZFC isn't). But I'm not aware of any such competing effort.

> What percentage of coders do programming on a turing tape? Is this partial evidence that turing tapes are trash? Does that question even make sense? Yes it is. Turing machine models are very limited, and a programme to let us achieve the things we can do with Turing machines (mainly runtime analysis) with a better model (i.e. a lambda-calculus style model) is a very good idea.

What you wrote is a different justification for why turing tapes are worse than lambda calculus. It has nothing to do with the number of people programming on turing tapes, which is the argument that I was responding to. I could easily have used 'lambda calculus' instead of 'turing tape' above. Most people do not code in the lambda calculus. They write haskell or javascript or whatever.

It doesn't mean that the lambda calculus is trash. Likewise, most mathematicians don't work directly with ZFC. Doesn't mean ZFC is trash.

> I'd be equally interested in a programme of doing metamathematics in some non-category-theoretic model that was still "normal" mathematics in the same way that category theory is (and ZFC isn't).

My point is that almost all pure math (e.g: linear algebra, topology, differential geometry, category theory, group theory) is already metamathematics. Of course, there is a spectrum of 'meta-ness' but I think this is a continuous spectrum. I do not think there is a well-defined division between 'mathematics' and 'metamathematics'.

For example, can you give an argument for why, say, the irrationality of sqrt(2) is not 'metamath', yet godel's incompleteness theorem is 'metamath'?

> What you wrote is a different justification for why turing tapes are worse than lambda calculus. It has nothing to do with the number of people programming on turing tapes, which is the argument that I was responding to.

It has everything to do with it: the reason for wanting a lambda-calculus-like model is that lambda-calculus-like models are what working programmers actually program in. If programmers actually used languages that looked like turing tapes then turing tapes would be a good model for talking about programming in.

> I could easily have used 'lambda calculus' instead of 'turing tape' above. Most people do not code in the lambda calculus. They write haskell or javascript or whatever.

Haskell has been described as essentially a typed lambda calculus. You're treating this as a binary distinction when it isn't: there's a lot of value in the model that we can do formal program analysis with being close to the models that we like to program in, whether the models are exactly identical is a lot less significant than the degree of similarity. Likewise the problem that "mathematicians don't work in ZFC" isn't just that mathematicians are doing something slightly different day-to-day, it's that it's a very different paradigm from normal mathematics.

> My point is that almost all pure math (e.g: linear algebra, topology, differential geometry, category theory, group theory) is already metamathematics. Of course, there is a spectrum of 'meta-ness' but I think this is a continuous spectrum. I do not think there is a well-defined division between 'mathematics' and 'metamathematics'.

> For example, can you give an argument for why, say, the irrationality of sqrt(2) is not 'metamath', yet godel's incompleteness theorem is 'metamath'?

I'd argue that irrationality of sqrt(2) is applicable outside of a mathematical context - it's a fact about something we're modelling rather than solely a fact about our models ("2" and "sqrt" are of course abstract models, but they can be applied to model a variety of concrete things that we care about, and you can carry over the irrationality of sqrt(2) into at least some of those contexts, where it will translate into something meaningful and useful). Whereas godel's incompleteness theorem is a map for which there is no territory; it's a fact about abstract models that could never correspond to anything that wasn't an abstract model.

But if you want to regard number theory as a subset of metamathematics then I don't mind. When I say I want to be able to do metamathematics, I mean I want to be able to do all metamathematics; in particular I want to be able to talk about proofs in general. You could argue that the irrationality of sqrt(2) is a statement about proofs, but it's certainly not in a context that allows you to reason about general proofs, and number theory does not give you a first-class way to work with proofs in general (of course the Godel encoding exists, but it's extremely tedious and not useful for practical work). Likewise, as far as I know, there's no way to really talk about (general) proofs directly in terms of group theory or linear algebra.

> What percentage of coders do programming on a turing tape? Is this partial evidence that turing tapes are trash? Does that question even make sense?

ZFC should be more like machine code we actually use than turn tape concept which we don't. The fact that no one uses something higher level that compiles down to ZFC is disheartening.

As the FOM mailing list demonstrates, it's really about goal posts here.

For one camp, the goal posts are such that ZFC or many other things are equally good. ZFC by share age has the "large cardinal" advantage in that people have been grinding away at it longer.

For the other camp, large-cardinal-type research agendas aren't very interesting, and the goal posts are dramatically different.

I still think ZFC is trash, but I will admit my mistake in thinking other share my goal posts.

> The abstractions of category theory are useless in many areas of mathematics. For example, PDEs.

At the moment that is true.

But I think this is more due to the human concerns than the actually Math. Until Statistics overtook it, differential equalization were the most-applied branch of mathematics, which definitely influenced the culture around it. There is also the general algebraist---analyst cultural divergence.

I look forward to the day when the computer tools are so good they are used in those fields too. That should bridge the culture gap, and then we shall see what the math holds.

What's the point of this comment? It's useful to some people therefore people study it. Why assume it needs to be useful to all mathematicians?

It is useful for my own work, and thus I study it, why isn't this as simple as that?

I don't know what your work is, but for the vast majority of people learning HoTT is a complete waste of time.

There are many intelligent and curious people reading HN, who might be tempted to spend time understanding it because its somewhat frequent appearance here gives the (false) impression that it plays some important role in modern mathematics.

I regret spending the (large) amount of time I did learning what it is about, only to realize that's not much actual content there, and I would hope my post saves other people that trouble. There is a lot of far more interesting, elegant, and useful mathematics out there, imo.

People find it interesting and fun so IMO that’s the only bar that needs to be cleared to justify the attention it gets. If your pet areas of mathematics are more exciting to you, then share that with people and/or post here and maybe it will get more attention. Or not! I wouldn’t dissuade people from following their interests.

My background - I’m a programmer and like building programming languages. I’m interested in HoTT because I’m interested in type theory generally and also formalization of mathematics.

Most mathematicians don’t care much about these things, which is fine. Like most weird stuff, it’s good that some folks are doing it.

Lastly you might enjoy this thread about why formalization is interesting to another mathematician:

https://mobile.twitter.com/andrejbauer?prefetchtimestamp=160...

Your link does not work for me. Maybe there's a bug with Twitter, or my browser is blocking too many trackers?

I want to clarify that I think formalizing mathematics is interesting, too! It's just entirely unclear to me that HoTT has any special advantages here (see link from my original post).

Here’s a link that works I think: https://mobile.twitter.com/andrejbauer/status/12599019719910...

The supposed selling point of HoTT wrt to formalizing mathematics is higher level proofs that are more general and are less work to express, due to having this richer notion of equivalence in the type theory.

I think the post you linked is pretty good... it makes me wonder - at this point, what is holding back more formalization? Is it a cultural thing? (None of the people who are doing “interesting” math care about formalizing It) Is it a technical thing? (Proof assistants and their type theories aren’t good enough?) Are we just missing a big enough “standard library” of mathematics so mathematicians can get to the interesting parts sooner? It’s probably all of the above.

What I think might happen is it reaches a tipping point where enough of the pieces are in place to where it rapidly becomes normal or expected to formalize mathematics and we have awesome tools and “standard libraries” to make it good.

> I don't know what your work is, but for the vast majority of people learning HoTT is a complete waste of time.

This is a bizarre thing to say. Learning any mathematics is a complete waste of time.

> gives the (false) impression that it plays some important role in modern mathematics.

What are you talking about?

HoTT is broadly regarded as an exciting and important new area of mathematics; your view is absolutely the minority view. I mean I don't think I'm going to be able to convince you otherwise if you genuinely think that there's "not much content" in HoTT, but there are loads of well-regarded and prominent mathematicians who hold the opposite view to yours.

Learning mathematics had been incredibly useful to me. If that hasn't been the case for you, I'm sorry.

Your claim that HoTT is "broadly regarded as an exciting and important new area of mathematics" is simply wrong. I suggest talking to a wider variety of mathematicians.

In particular, concerning "well-regarded and prominent mathematicians," Steve Simpson and Harvey Friedman are on record on the Foundations of Mathematics mailing list saying, essentially, that HoTT is a waste of time [0]. The other set theory experts I know agree with them but haven't said so publicly. My impression is that this is a fairly common view in the set theory and foundations community.

Further, Jacob Lurie, who is perhaps the most talented mathematician currently working in higher category theory, has also written he doesn't think highly of it [1].

[0] I can provide links if it you want, but if you search the archive these arguments are easy to find.

[1] View the comments on this blog post: https://mathematicswithoutapologies.wordpress.com/2015/05/13...

> Steve Simpson and Harvey Friedman are on record on the Foundations of Mathematics mailing list saying, essentially, that HoTT is a waste of time [0]. The other set theory experts I know agree with them but haven't said so publicly. My impression is that this is a fairly common view in the set theory and foundations community.

The set theory and foundations community, i.e. the small minority of mathematicians who manage to do effective work on mathematical foundations in ZFC, are precisely the people who we'd expect to be most anti HoTT. It's not necessarily wrong, but it's like concluding that OOP is pointless because you've talked to leading C programming experts.

> Learning mathematics had been incredibly useful to me.

You have missed my point. We generally don't study only the "useful" things; if we did most of the most important mathematics we have today would have gone unstudied.

> Your claim that HoTT is "broadly regarded as an exciting and important new area of mathematics" is simply wrong.

According to whom? You? You're making an argument about consensus regarding the importance of a given area, but I have seen nothing to indicate that the consensus among mathematicians is in line with your opinion.

> Steve Simpson and Harvey Friedman

They are in the minority. Again, if you're arguing about consensus you can't just point to one or two individuals and say "look! They agree with me!" For goodness' sake, the thread your referring to is well-known partially because it was controversial.

> Jacob Lurie [...] doesn't think highly of it

The content of Lurie's comment and yours are quite different.

You wrote that "Learning any mathematics is a complete waste of time." There are a variety of ways math could not be a waste of time; practical applications are only one. I don't mean to imply pure mathematics shouldn't be studied, only that I don't believe math was a time waster for me.

> According to whom?

I've talked to a lot of very good mathematicians, including people who've studied and collaborated with people in the HoTT orbit, and people from many other subfields. I'm sorry I don't have exact quantitative data for you.

> They are in the minority. Again, if you're arguing about consensus you can't just point to one or two individuals and say "look! They agree with me!" For goodness' sake, the thread your referring to is well-known partially because it was controversial.

Again, I don't think they're in the minority. Also, there's way more than one relevant thread. That particular fight was litigated over years.

> The content of Lurie's comment and yours are quite different.

He makes many comments on that post. I invite you to read them all. Certainly, I'm not just regurgitating what he says there, but it does provide an example of a top mathematician expressing skepticism about the entire enterprise.

> You wrote that "Learning any mathematics is a complete waste of time."

Yes, I was being facetious. My point is that by whatever measure pure maths is worth studying (i.e. those measure beyond "practical" applications) also obviously applies to HoTT.

> I've talked to a lot of very good mathematicians

You're an anonymous person on the internet, this is such a strange thing to fall back on. There are pretty standard ways of talking about how well-regarded/important certain subfields are in mathematics, "I've asked around" isn't one of them.

> I'm not just regurgitating what he says there

His comments express a substantially different point to yours. He does not say that HoTT is a waste of time, or that it "doesn't have much content".

> My point is that by whatever measure pure maths is worth studying (i.e. those measure beyond "practical" applications) also obviously applies to HoTT.

Not all branches of pure mathematics are equally worthy of study. A few are essentially dead for various (good) reasons, for example.

> There are pretty standard ways of talking about how well-regarded/important certain subfields are in mathematics, "I've asked around" isn't one of them.

How should one figure out how well-regarded a subfield of mathematics is if not by asking members of the mathematical community?

> His comments express a substantially different point to yours. He does not say that HoTT is a waste of time, or that it "doesn't have much content".

His comments support the point I intended them to support, which is that Lurie is not a huge fan of HoTT. You made a remark about well-regarded mathematicians, and I gave you comments from a well-regarded mathematician.

> Not all branches of pure mathematics are equally worthy of study.

Ok? What does this have to do with the point I was making?

My point was, pretty simply, that "usefulness" is not the only thing we consider when deciding whether something is worthy of study. (If it was we wouldn't have touched most of modern pure mathematics.) As a result, your notion that studying HoTT would be a waste of time because it's not "useful" is nonsense.

> How should one figure out how well-regarded a subfield of mathematics is if not by asking members of the mathematical community?

Oh come on, you really don't understand the point I'm making?

When talking about consensus it's not good enough to say "I've talked to a few people and they agree with me". Work on HoTT, measured by any kind of actual metric rather than "people I've spoken to (who I can't name by the way) say it's bad", is broadly well-regarded and seen as important. In terms of research funding, citation/publication metrics, or the opinion of most mathematicians.

> His comments support the point I intended them to support

No they do not. My objection is to you saying:

* That HoTT is a waste of time.

* That HoTT doesn't have much content.

* That HoTT is not broadly regarded by mathematicians as important or exciting.

Please do not move the goalposts.

> You made a remark about well-regarded mathematicians, and I gave you comments from a well-regarded mathematician.

What? I "made a remark" about well-regarded mathematicians? I said, specifically, "there are loads of well-regarded and prominent mathematicians who hold the opposite view to yours".

I also tried to stress that:

> if you're arguing about consensus you can't just point to one or two individuals

Please bear that in mind.

I think to develop this conversation further would require me explaining why mathematicians are interested in certain subfields of mathematics and not others (e.g. the dead ones), but that's far too big a project for a comment. Buzzard says a little bit about this in the blog post I linked, which is a highly compressed version of what I'd write; if you're curious, maybe read that.

I've talked to more than "a few people," about this, and I think it's clear I've thought more about the sociology of this issue than just chatting with Tom Scanlon or whoever over a beer one night. I stand by what I said about that earlier, along with my comment that you ought to talk to a wider variety of mathematicians if you think there's some broad consensus that HoTT is important. (Start with the PDE and numerical analysis people, for example.)

Anyway, sticking purely to objective metrics, how many young mathematicians (postdocs) who work primarily on HoTT have ever been hired by R1 universities to tenure-track positions? Surely if this were seen as an important area, departments would want to snatch people in that field up, right? (I'd even accept examples of postdocs at these schools working on HoTT, though this represents a far less serious commitment by the department.)

This is (in principle – you'd have to collect it) publicly available information, which anyone reading this exchange can check on. (Look for assistant profs at schools listed as R1 on Wikipedia.) Maybe I'm forgetting someone, or not knowledgeable enough, but I can't really think of any.

(ER at Johns Hopkins was the first name that popped into my head, but she doesn't count because she has only a single HoTT paper among dozens, and it was written after she was hired.)

Another metric: how many HoTT papers are there in top mathematics journals? Annals, Inventiones, etc.?

What an extraordinary comment. You seem to be missing various points:

- Mathematics is a very large field.

- For beginners, it is necessary to have wiser people than yourself put up some signposts.

I found the comment you're referring to very illuminating. I'm not naively assuming the author's view is the only one, or "correct", but s/he certainly like they have some familiarity with graduate/research level mathematics in relevant areas. So it was very helpful to see, at least from the point of view of some mathematicians, that this is not considered an important area.

I was left wondering whether the author of that comment understood the importance of this stuff in computer science (i.e. in academic approaches to programming languages, might be the right term, I don't know, I'm pretty ignorant of these areas myself), or whether they were only able to criticize it from the point of view of pure mathematics.

> s/he certainly like they have some familiarity with graduate/research level mathematics in relevant areas

You should not assume this about any anonymous person on the internet.

> There are also people like Per-Martin Lof who want to use HoTT…

Sorry, but this is plainly false. Per does not want to use HoTT. My impression is that he finds HoTT mildly interesting, and has not really used/advocated it. And, you are implicitly calling him and Voedvodsky "not a mainstream mathematician", which is rather offensive considering their contributions to statistics (Per) and homotopy theory (Vladimir).

Your comment comes out as very reactionary. There is room for a lot of both HoTT (weather classical logic is used or not) and set theory in modern mathematics. In fact, doing set theory in HoTT is a very pleasant experience!

Martin-Lof went to at least one early HoTT conference and his interest is clearly in intuitionistic type theory, so I don't think my statement is inaccurate. Anyway, the broader point is that there's a community of people whose interest comes from that viewpoint.

I also don't think it's inaccurate or offensive to say Martin-Lof no longer works in mainstream mathematics. He hasn't done statistics since the 70s.

Voevodsky was of course a great mathematician, but it seems quite clear that his work in type theory was not mainstream mathematics and not of interest to most mathematicians (quantifying over, say, people who hold jobs as postdocs or professors in math departments worldwide). Whether the work is valuable is a separate question, but the sociological point is, imo, pretty inarguable.

I wouldn't be surprised to see anything related to type-theory on here. And I disagree that many people doing HoTT today are proposing it as an alternative foundation in anything except (categorical) homotopy theory, and I'd like to know where you see anyone doing that.

Intuitionism in HoTT isn't about constructivity, which I agree is rejected by mainstream mathematicians. It's about the models of the theory. For example, LEM doesn't hold in the category of bundles over the circle (see my other comment on here). If you think of the axiom of choice as "every surjection has a section", then this doesn't hold for topological spaces. (Take the double cover of the circle; there's no continuous section.) I disagree that these examples are uninteresting to mainstream mathematicians.

1. ZFC does everything you need it to do. It's a really hard set of axioms for me to justify compared to MLTT, and it's important to me how "obvious" some set of axioms is. MLTT also has the advantage that it's very easy to use without encoding everything into well-founded, untyped trees, which is nice.

2. What are these "substantive problems"? I don't have any experience in foundations, so I'm genuinely curious.

3. Sure, intuitionistic mathematics has very little popularity, but a lot of that popularity is among computer scientists, for fairly obvious reasons.

4. The HoTT book and the nLab are two of the most out-in-the-open developments in math I've seen --- the HoTT book was even written in a public GitHub repo iirc. Naturally that has a lot of appeal for HN. If you have anything similar (blogs, wikis, etc.) for mainstream foundations, I'd love to read them.

5. Yeah, personally, I'm not really convinced that HoTT is a good idea, but I guess we'll just have to wait and see.

1. ZFC is fairly "obvious" when taught the right way. Unfortunately, most people do have not the privilege of a good set theory instructor.

2. Whether V equals Ultimate L, for example. There's "pop math" coverage available [1], as well as more technical introductions by Woodin if you Google for them.

3. I'm not sure what reasons you're referring to; can you enlighten me? I can guess, but the ones I'm guessing are not very good, and I'd rather not attribute weak arguments to you or them.

4. The Stacks Project, for one [2]. It covers most of the foundations of modern algebraic geometry, is of extremely high quality, and is regularly cited in research papers.

[1]: https://www.quantamagazine.org/to-settle-infinity-question-a...

[2]: https://stacks.math.columbia.edu/

3. I am not sure what GP meant with this, but I do know that in type theory, intuitionistic logic is very important. The propositions of intuitionistic logic correspond exactly to the types of types programs in certain models of programming languages, and the proofs of intuitionistic logic to well-typed programs in those models. This is called the Curry-Howard isomorphism, and has been a very fruitful source of inspiration for type theory. For example, it was originally noted for intuitionistic propositional logic only, but adding universal quantification gives a type theory with so-called parametric polymorphism, which was later developed into generics in Java and other languages. I have seen modal logic used to model distributed computation, and linear logic inspired work which eventually led to Rust's borrow checker.

As an amateur who nowadays works a lot with ZFC (but who doesn't know HoTT) and who is very interested in foundations, I don't think that learning HoTT is a waste of time, I'm sure HoTT is great and I'm glad there are people working on it. However, my time is limited, so I can't really afford to spend months going down a rabbit hole if it's not going to help me with my work. So I appreciate that you shared your experience, it was helpful in making me decide to stay focused on what I'm already doing.

You should really learn type theory even if you aren't interested in HoTT specifically. ZFC is a pretty ugly formalization.

My "empirical" evidence for this is most working mathematicians today do not care about foundations at all, whereas with something better, they might.

Most mathematicians still wouldn't care about foundations if you came up with better foundations. It's a separate area of research from most other fields of mathematics, it's not relevant to what most of them are researching, and they just don't have any interest in it. And unlike category theory more broadly, they don't particularly benefit by studying foundations deeply. It doesn't provide any improvement to their own area of study.

Everything I've just said is completely uncontroversial among mathematicians, for what it's worth. The comment I just wrote has probably been expressed on r/math dozens of times at this point.

Yes I agree with that. Let me fix my phrasing: they still wouldn't all suddenly start researching mathematical foundations, but they might apply foundational tools to what they are doing.

Really "foundations" is a word that emphasizes the ZFC "learn and ignore" approach. The type theory / category theory approach might better be called "framing", in that it begins in the pilings below the basement but also goes all the way to the roof.

I think people are excited about the connection to type theory (tangentially connected to tech) as well as the idea that this field and the book are being developed in such an open-source way.

My (educated layman) understanding is that there actually is a useful thing that HoTT gives us as a foundation for mathematics. that ZFC does not. This is the translation of proofs from one structure to an isomorphic one.

For example, if I have, say, a group, and I prove something about it, you would think that the proof would hold for all isomorphic groups. Under ZFC this isn't true, or at least we don't get it for free.

Under HoTT, we get this for free.

> We already have a perfectly good foundation of mathematics. It's called ZFC [1]

This is completely missing a core point of category theory and type theory (which is about categories equipped with certain extra structure), which is to work is settings which can be flexibly tightened or loosened in order to do constructions which are as general as possible. A theorem in HoTT is far more widely applicable than a theorem in ZFC; HoTT can be interpreted in any higher Gröthendieck topos, ZFC can be only intepreted in a certain class of 1-toposes. In HoTT, we can simply assume choice and LEM to internally reproduce ZFC. In ZFC, univalence is false and cannot be assumed.

> It's not at all clear HoTT, or type theory in general, is the best solution for formalizing mathematics

Xena project isn't too hot on HoTT indeed, but they will tell you that type theory in general is absolutely the best solution for formalizing mathematics.

> in particular denying the law of excluded middle

MLTT does not deny LEM, nor HoTT. The point is not to deny LEM, but to work in a more general system which does not necessarily assume it.

There are numerous advantages to having a complete categorical or type-theoretic foundations of mathematics, especially one as constructive as HoTT. The main advantages in my areas of research would be that proofs in "higher-category theory" would become less cumbersome. Many set-theoretic proofs in this area are non-informative because they are are messy and opaque. This is the real promise offered by any alternative theory or explanation: insight.

Besides the immediate usefulness: if "all HoTT does is give a theory that's mutually interpretable with ZFC", then holy shit what an accomplishment!

You seem to have a fundamental misunderstanding about the nature of mathematical research. Most fields of research do nothing but "give a theory that's mutually interpretable" with some other field, or at least did so initially.

The vast majority of people that design and write rigorously formal systems to express complex real world ideas and behaviours are programmers.

They are rigorous, not in the sense that anything is ever proven about their formal systems, but in the sense that they are interpreted rigorously as to their formal consequences by computers. This is really different to how I as a physicist might use equations to express ideas to other physicists, and if I can't exactly formalize what the equation means that isn't always relevant.

This is of course an unorthodox way to frame what programmers do, but I think it's relevant to your last comment. What is a good foundation for formal systems is an empirical question, and empirically, type theory is extremely important to programmers. Set theory not at all.

HoTT is very interesting to people working on programming language theory. I, for one, expect we will see a handful of very interesting research programming languages coming out in the near-future with HoTT-based features, which will allow a lot of stuff which is currently inefficient to do in functional languages to be implemented in very efficient (and provably safe) ways.

Sure, that's very much a niche and we can't be sure anything practical will come out of it, but for some of us it's interesting in its own right. :)

I understand how this isn't interesting to folks working on pure math, just as category theory likely isn't, but it's the fact that these mathematical abstractions map so well onto (certain kinds of) programming that makes them such recurrent topics on HN.

I would guess most mathematicians do not truly fully use ZFC at all. I think there is a lot of mathematics that is done which is actually independent of ZFC, its more expressive than needed (if we look at reverse mathematics).

I think Cubical Type Theory (derived from HoTT) is a part of the future for general-purpose dependently-typed programming -- it has canonicity, which I'd argue is necessary for a general-purpose programming language, and it being suitable for a foundation of mathematics nigh-guarantees it's powerful enough to express whatever properties are useful in the domain.

> it has canonicity, which I'd argue is necessary for a general-purpose programming language

MLTT, even without HoTT/CTT, already features canonicity, no? (At least for positive types.)

I thought the main advantage of HoTT is that you can define non-trivial equalities, and the advantage of CTT in particular is that univalence can be derived in it (rather than introducing it as a non-computational axiom).

Right, I think I'd put univalence in my list of "I want this for my dream general-purpose dependently-typed programming language" features; being able to prove properties on simple representations of data and transport those properties to more optimized forms is pretty important, imo.

So you're saying that cubical type theory will be a good fit for cubical programming?

It's popular at CMU? It got a huge-for-pure-math grant 5 years ago (https://homotopytypetheory.org/2014/04/29/hott-awarded-a-mur...)? Idk.

> I find it strange that HoTT and HoTT-adjacent stuff gets posted here so often.

I don't have any dog in this fight, but I also find it super interesting that this topic gets to the 1st page so often.

TBH I think answering that question is more interesting that litigating foundations.

Your first point is hysterical considering we as computer scientists use many many different programming languages.

Nah the math people are fine with assembly, even though they don't actually use it in their work.

That's a flawed analogy. Mathematicians do use 'higher level languages', that's precisely why most of them don't care about HoTT vs set theory. Just like a web dev usually does not care about the instruction set of the processor.

Its not flawed because the correct analogy is: pen and paper Turing machines versus Python.

Now I'm not sure what your point is. Are you suggesting that HoTT is to python as ZFC is to turing machine tape?

Nitpick, but I think what you meant to say what "HoTT is to ZFC as Python is to Turing machine tape". Hope it helps.

the audience here tends to have more of a Computer Science background rather than a traditional Mathematics background, and HoTT feels more like a CS theory than like traditional math. When I read about the kind of reasoning that lead to ZFC, I get frustrated because that style of thinking feels unnatural to me. HoTT feels more like how I think, when I’m trying to solve problems. Not that I could productively do math research in either system - I’m not a mathematician at all! It’s just an aesthetic judgement.

You don't sound like you've heard of the Curry-Howard isomorphism. We want practical dependently typed programming languages, ZFC is useless for that. Furthermore, LEM is never denied, it's just not adopted as an axion and thus requires proof per case.

Reply on news.ycombinator.com