[cwzwarich]

> For this particular question we have a great answer in the form of homotopy type theory. It handles all the complications that the author mentions. This is one of the reasons type theorists were excited about HoTT in the first place.

Arguably, Homotopy type theory still doesn't solve the problem, because while it strengthens the consequences of isomorphism it doesn't distinguish between "isomorphism" and "canonical isomorphism", whereas (some?) mathematicians informally seem to think that there's a meaningful difference.

> The only problem is that the author is working in Lean and apparently* dismissive of non-classical type theory. So now we're back to lamenting that equality in type theory is broken...

In my opinion, Lean made a number pragmatic choices (impredicative Prop, non-classical logic including the axiom of global choice, uniqueness of identity proofs, etc.) that enable the practical formalization of mathematics as actually done by the vast majority of mathematicians who don't research logic, type theory, or category theory.

It's far from established that this is possible at all with homotopy type theory, yet alone whether it would actually be easier or more economical to do so. And even if this state of affairs is permanent, homotopy type theory itself would still be an interesting topic of study like any other field of mathematics.

[lupire]

"Canonical" refers largely to the names we give to things, so that we maximize the extent to which an isomorphism maps the object named A in X to the object named A in Y. And also to choosing an isomorphism that embeds in a different isomorphism (of superstructures) that isn't strictly the topic of the current question.

I don't think anyone thinks canonical isomorphisms are mathematically controversial (except some people having fun with studying scenarios where more than one isomorphism is equally canonical, and other meta studies), they are a convenient communication shorthand for avoiding boring details.

https://en.m.wikipedia.org/wiki/Isomorphism_theorems

[moomin]

I think the original author makes a good case that a) the shorthand approach isn’t going to fly as we formalise b) different mathematicians mean different things by canonical and these details matter when trying to combining results and that therefore 3) it would be better to provide a proper definition of what you’re talking about and give it an unambiguous name.

[euiq]

let me guess: you haven't read the article, have you?

[xigoi]

> Please don't comment on whether someone read an article. "Did you even read the article? It mentions that" can be shortened to "The article mentions that".

https://news.ycombinator.com/newsguidelines.html

[fmap]

> it doesn't distinguish between "isomorphism" and "canonical isomorphism"

You can distinguish these concepts in (higher )category theory, where isomorphisms are morphisms in groupoids and canonical isomorphisms are contractible spaces of morphisms. These sound like complicated concepts, but in HoTT you can discover the same phenomena as paths in types (i.e., equality) and a simple definition of contractibility which looks and works almost exactly like unique existence.

> In my opinion, Lean made a number pragmatic choices (impredicative Prop, non-classical logic including the axiom of global choice, uniqueness of identity proofs, etc.) that enable the practical formalization of mathematics as actually done by the vast majority of mathematicians who don't research logic, type theory, or category theory.

The authors of Lean are brilliant and I'm extremely happy that more mathematicians are looking into formalisations and recognizing the additional challenges that this brings. At the same time, it's a little bit depressing that we had finally gotten a good answer to many (not all!) of these additional challenges only to then retreat back to familiar ground.

Anyway, there were several responses to my original comment and instead of answering each individually, I'm just going to point to the article itself. The big example from section 5 is that of localisations of R-algebras. Here is how this whole discussion changes in HoTT:

1) We have a path R[1/f][1/g] = R[1/fg], therefore the original theorem is applicable in the case that the paper mentions.

2) The statements in terms of "an arbitrary localisation" and "for all particular localisations" are equivalent.

3) ...and this is essentially because in HoTT there is a way to define the localisation of an R-algebra at a multiplicative subset. This is a higher-inductive type and the problematic aspects of the definition in classical mathematics stem from the fact that this is not (automatically) a Set. A higher-inductive definition is a definition by a universal property, yet you can work with this in the same way as you would with a concrete construction and the theorems that the article mentions are provable.

---

There is much more that can be said here and it's not all positive. The only point I want to make is that everybody who ever formalised anything substantial is well aware of the problem the article talks about: you need to pick the correct definitions to formalise, you can't just translate a random math textbook and expect it to work. Usually, you need to pick the correct generalisation of a statement which actually applies in all the cases where you need to use it.

Type theory is actually especially difficult here, because equality in type theory lacks many of the nice properties that you would expect it to have, on top of issues around isomorphisms and equality that are left vague in many textbooks/papers/etc. HoTT really does solve this issue. It presents a host of new questions and it's almost certain that we haven't found the best presentation of these ideas yet, but even the presentation we have solves the problems that the author talks about in this article.

[analytiqhub]

Homotopy type theory is definitely the way to go. This 2016 talk from Vladimir Voevodsky is a good entrypoint for the general audience:

https://www.youtube.com/watch?v=gOBqROtRoPA

[aureianimus]

I'd love to hear a little bit more on what you think the downsides are? (Or a recommendation for a resource to read up on this?)

[zozbot234]

I'd also like to know more about that. My comment points out some things about HoTT that might be described as "downsides" by some but are more like things that people might expect from HoTT given that it 'handles' type equality in a rigorous yet flexible way, but aren't actually feasible in a rigorous treatment, i.e. the system is not going to pick up all the issues that are "brushed aside" in an informal treatment and resolve them on its own. Its behavior will point out to you that these issues exist, but you'll still have to do a lot of work dotting the i's and crossing the t's, as with any formalization.