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now I have finally the time to come back to this, as announced, and so I am now starting an entry:
relation between type theory and category theory .
So far there is just some literature collected. I now plan to extract the essence of Seely’s artice into the entry in some technical detail.
it won’t come as a surprise to you that I am creating a table.
Currently it looks like this:
flavor of type theory | $\;$equivalent to$\;$ | flavor of category theory | |
---|---|---|---|
first-order logic | hyperdoctrine | (Seely 1984a) | |
dependent type theory | locally cartesian closed category | (Seely 1984b) | |
intensional type theory with identity types | locally cartesian closed (∞,1)-category | (conjectural) | |
homotopy type theory with higher inductive types | elementary (∞,1)-topos | (conjectural) |
But that made me wonder: what’s the best terminology for the entries on the left? What should be the canonical term for that part of homotopy type theory which does not assume univalence and hence is supposedly the language of locally cartesian closed $(\infty,1)$-categories?
I dunno. Just “homotopy type theory” maybe?
I think there’s also an equivalence between some kind of type theory and elementary 1-toposes somewhere in the literature…
One could add in the entry that the morphisms in the categories in the left column of the table are what Seely calls ”derivations”.
have been working on HoTT methods for homotopy theorists. Mainly added a bunch of translation tables, also reorganized a bit. Still far from what it should eventually be, but at least it is beginning to look like an entry.
A point to be amplified more, also with respect to the $n$Café-discussion, is how in the internal HoTT language statements about fibers simplify, in that the naive external statements which are generally wrong become true internally: “A morphism is $n$-truncated/$n$-connected if all fibers are so.”
the morphisms in the categories in the left column of the table are what Seely calls ”derivations”.
That seems like a weird name to me; I’d be more inclined to call them something like “translations”.
it won’t come as a surprise to you that I am creating a table.
I like the encyclopedic flavor of your tables!
have been working on HoTT methods for homotopy theorists. Mainly added a bunch of translation tables,
In the basic dictionary table I missed that a morphism $f:X\to Y$ which is not necessarily a fibration corresponds to a typing judgement $x:X\vdash f(x):Y$. Is this omission intended?
I’d be more inclined to call them something like “translations”.
This sounds fairly descriptive. Can you explain why this is a good name?
Mike writes:
I dunno. Just “homotopy type theory” maybe?
It’s just that eventually we need two terms, one for the internal language of locally cartesian closed $(\infty,1)$-catgegories in general, and one for that of elementary $(\infty,1)$-toposes.
I think there’s also an equivalence between some kind of type theory and elementary 1-toposes somewhere in the literature…
Scott and Lambek in their section II have an adjunction between toposes with NNO and type theories with product types and natural numbers. Is that maybe what you have in mind? If not, I’d be very interested in finding the reference that you do have in mind.
Concerning “derivations” and “translations”: I think you are maybe talking about different tables each?
Seely calls a term of a type a “derivation”, because it can be regarded a proof of a proposition. What should be called “translations” are the morphisms in the category of type theories, since they translate one internal language into another)
Stephan writes:
I like the encyclopedic flavor of your tables!
Thanks for letting me know. I am certainly benefitting from making such tables explicit. It helps me organize my thoughts and see clearly.
In the basic dictionary table I missed that a morphism f:X→Y which is not necessarily a fibration corresponds to a typing judgement x:X⊢f(x):Y. Is this omission intended?
It’s not really omitted, as this is a special case of the clause for a term (forth item in the table). There also the type $Y$ may depend on $x$. Your typing judgement is obtained from this by assuming that $Y$ does not in fact depend on $X$, hence by assuming that the fibration in question is the projection $Y \times X \to X$.
Maybe I should add a remark about that (as soon as the $n$Lab reacts again).
Is the relationship between the hyperdoctrine approach and the syntactic category approach to logic known? We were talking about it here.
We were talking about it here.
Hm, interesting that slide 96 of Hirschowitz.
I guess the devil is in the details somewhere, but Seely seems to have made this kind of adjunction an equivalence
$FirstOrderTheories \simeq Hyperdoctrines$already in 1987 (references at relation between type theory and category theory).
I haven’t looked through all the details. One thing that seems noteworthy is that in his proof Seely in fact considers hyperdoctrines whose fibers are not required to be posets. Maybe that makes the difference. But I’d say the equivalence proven justifies whatever fine-tuning on $Hyperdoctrines$ made it work.
Hm, Seely’s article contains a mistake, in that it is not dealing with the non-uniqueness of pullbacks in the category correctly.
In
there is at least a partial fix. Though I need to figure out if that fix retains the equivalence of categories $DependentTypeTheories \simeq LocallyCartesianClosedCategories$ that Seely claimed.
What about this paper?
I think you are maybe talking about different tables each?
Ah, indeed we were! I misinterpreted Stephan’s remark #4. But in that case, I think the morphisms should just be called “terms”.
It’s just that eventually we need two terms, one for the internal language of locally cartesian closed (∞,1)-catgegories in general, and one for that of elementary (∞,1)-toposes.
“homotopy type theory” and “univalent (homotopy) type theory”?
“homotopy type theory” and “univalent (homotopy) type theory”?
But what about the higher inductive types?
Currently in the entry I have (I think) “homotopy type theory” for the language of LCC $\infty$-categories and “homotopy type theory with higher inductive types and univalence” for the language of elementary $\infty$-toposes.
What about this paper?
Ah, great! Thanks, Mike. And what a coincidence that we posted #13 and #14 in the very same minute! :-)
@Mike #15: Whether morphisms should be called ‘terms’ depends on whether objects are called ‘types’. Are the objects here types or contexts?
Toby: types.
The article under discussion is this one.
re 18: I have worked this and related references into the entry.
When I started this quest, I had no idea that what I am after was still not settled by December last year! I thought this is ancient stuff. Interesting to see that it is not only in physics that some statements become so much folk lore that people forget that they haven’t proven them yet ;-)
I think we can be flexible with terminology and not demand a concise term for every possible variation on a theme.
Is it really a good idea to include the detailed constructions of the functors $Con$ and $Lan$ in the particular case of DTT vs LCCCs on the page relation between type theory and category theory? The construction of $Con$ is already described at (now) syntactic category; why duplicate it elsewhere? If we want to include the proof that $Con(T)$ is LCCC when $T$ is a DTT, it seems more natural to me to put it at syntactic category or at dependent type theory. Similarly for $Lan$.
That sounds reasonable. I didn’t finish what had planned to write there anyway.
Right now I don’t have the time to look into this. But if you have a second to do so, feel free. Otherwise I can try to look into it later.
I think there is a lot of duplication and confusing organization right now. (Nobody’s fault, surely, it just happened organically.) The page internal logic discusses, for first-order theories, exactly the sort of thing that categorical semantics currently does for dependent type theories. A lot of the same thing is also at the introductory section on categorical semantics at type theory, as well as at relation between type theory and category theory.
I suppose it makes sense to keep an intuitive discussion at type theory for people who are just stumbling on that page. But maybe the rest of it can be unified and better organized. What about merging relation between type theory and category theory with categorical semantics? Maybe a descriptive pagename would be categorical semantics of type theory. Then we can try to centralize constructions of the functor $Lang$ on the page internal logic, construction of the functor $Syn$ on the page syntactic category, and discussion of the adjunction/equivalence between them at the merged page.
Yes, I feel precisely the same way. The current status of the pages is the result of several attempts to start with the same topic, each of them unfinished.
I’d certainly have no objections against merging pages.
Does anyone know if the Clairambault-Dybjer article on “The Biequivalence of Locally Cartesian Closed Categories and Martin-Löf Type Theories” has meanwhile been published in a journal?
From MathSciNet:
@incollection {MR2830789,
AUTHOR = {Clairambault, Pierre and Dybjer, Peter},
TITLE = {The biequivalence of locally {C}artesian closed categories and
{M}artin-{L}\"of type theories},
BOOKTITLE = {Typed lambda calculi and applications},
SERIES = {Lecture Notes in Comput. Sci.},
VOLUME = {6690},
PAGES = {91--106},
PUBLISHER = {Springer},
ADDRESS = {Heidelberg},
YEAR = {2011},
MRCLASS = {03B15 (18B99)},
MRNUMBER = {2830789 (2012i:03029)},
DOI = {10.1007/978-3-642-21691-6_10},
URL = {http://dx.doi.org/10.1007/978-3-642-21691-6_10},
}
Thanks! I have added that to the entry now.
At relation between type theory and category theory I made the table entry on linear logic come out right and link to
(made the pdf appear here)
Also highlighted this more at star-autonomous category and at linear logic.
Added a pointed to Joyal’s recent talk on categorical semantics of HoTT, and also added Mike’s notes that precede this by two years, so in total I added this to the References section at relation between type theory and category theory:
Details on this higher categorical semantics of homotopy type theory are in
with lecture notes in
Mike Shulman, Categorical models of homotopy type theory, April 13, 2012 (pdf)
Andre Joyal, Categorical homotopy type theory, March 17, 2014 (pdf)
The Joyal link seems to point to Voevodsky’s slides.
Woops.
Could somebody fix it for me, as i am just on my phone now.
The link is: http://ncatlab.org/homotopytypetheory/files/Joyal.pdf
(Unwise choice of file name, eh.)
Fixed.
Thanks!
The entry long has a placeholder section On HoTT with Univalence interpreted in infinity-Toposes.
Is there a precise version of the statement that involves mumbling “weakly Tarski universes”?
I imagine at that point in the entry it could say the following, but I don’t know what the formal status of the second half of the statement is:
Homotopy type theory with the univalence axiom added has interpretation in locally presentable and locally Cartesian closed $\infty$-categories which also satisfy the Giraud-Rezk-Lurie axioms. This is strictly so for those which have presentation by model structures of simplicial presheaves over elegant Reedy categories (Shulman), and is true generally if the univalent universes are interpreted “weakly a la Tarski”.
I wrote a bit leading up to a more formal meaning of the statement in quotes at universe (homotopytypetheory).
Thanks for that.
I have adapted the text that I found at that link to expand the $n$Lab entry type of types. There is now a section "Formalization" with essentially your text (enriched by some more hyperlinks), and then I have added a brief paragraph Properties – Categorical semantics with pointers to the articles that I am aware of.
I tried to vaguely indicate that the discussion in Gepner-Kock would fit under the "weakly Tarskian" umbrella. But you may feel wanting to touch the text here and there.
Then, finally, I have added a corresponding paragraph at relation between TT and CT – Univalent HoTT and Elementary infinity-toposes.
Again I pointed to the pertinent references (or tried to). The paragraph currently ends with a brief remark on how, therefore, HoTT+UV+HIT is analogous to a homotopy theoretic version of ETCS.
It makes me kind of sad to copy text from one wiki to another rather than linking, since then if it gets improved in one place, the improvement won’t be visible in the other place. I also wasn’t sure that it was really reasonable to call a Tarski universe a “type of types” rather than a “type of codes for types”.
I tried to clarify some statements at “relation” that I thought were misleading or confusing.
Mike,
regarding the “relations”-entry: thanks for the edits.
Regarding duplicating material: that is in the end due to your decision, not mine. I am not supposed to edit over at homotopytheory, not to link back to the nLab, and you are duplicating keywords over there which we already have on the nLab. So that makes that wiki be an external resource from the point of view of the nLab.
If I just linked to your entry over at homotopytypetheory then all the supplementary links that the nLab does offer are not abvailable, I can’t add links etc. That makes just linking to the entry less preferred than adapting its content to the nLab.
I never said you weren’t supposed to edit at the HoTT wiki; just that the edits should be in its spirit, which is different from the nLab’s. Linking to the nLab is also fine from there if it’s for a topic that we probably won’t want a page on there, and otherwise one can just make a stub there which links to here, just as we sometimes do here linking to wikipedia.
I also don’t think I agree that copying content from an external resource into the nLab is always preferable to linking to it. When the external resource is static, that makes some sense (apart from the duplication of effort involved), but when the external resource is liable to change and improve itself, I think it’s better to link. By analogy, I administer a database system for a nonprofit that I volunteer for, and if you want to see only some of the information in a database table, you don’t copy that data into a smaller table, but you make a “stored view” that essentially links to the original data.
Belated reply: I had had added to the copied material a link to the original source!
Added a pointer to Curien-Garner-Hofmann
Linking to the original source is better than not linking, but linking without copying is still better.
Actually, here’s a question: can we do [[!include]]
in between webs? I.e. could we include a page from the HoTT wiki into an nLab page? I just tried it at the Sandbox and it doesn’t seem to work, but it seems like this oughtn’t to be too hard to implement even if Instiki doesn’t do it yet.
Mike, we both easily agree on what would be optimal here. But maybe we disagree on what the most practical compromise is. From my point of view currently if I send a reader from an $n$Lab page on type theory to the corresponding entry on the homotopytypetheory then I deprive him/her of a whole lot of type theoretic cross-link information that the nLab has, while homotoptypetheory does not. Readers who are already expert on the multitude of type theoretic verbiage may be happy with this, but my feeling is that the readers I care most about are not. Therefore from my point of view the best compromise is that I don’t mandatorily send readers to homotopytypetheory. Instead I copy from there the relevant information and then provide a pointer to it for those readers who wish to check for more information. This is much like we treat any other nLab-external source.
added a pointer to Lumsdaine-Warren 13
One comment: on the nLab there easily is room for every entry to have a section providing the “HoTT-specific perpective”. Many enries already try to provide exactly that (though of course, as usual on the nLab, the current state may be found wanting). Given it’s very nature, HoTT is very much on-topic for the nLab.
My understanding is that the reason not to try to merge the content of the homotopytyetheory web into the nLab alltogether is more a sociological one, that people potentially contributing to the homotopytypetheory web may prefer the “more quiet” place that it may feel like, undisturbed by what weird ideas other people may have about editing wiki pages.
Yes, the point is for the HoTT wiki to feel “owned” by the HoTT community.
Also, I would not recommend trying to learn about HoTT from the HoTT wiki! It’s nowhere near complete enough for that.
Hi Alexis,
first up, welcome to the nForum! Regarding ’fibrant’, my understanding is that it is analogous to fibrant objects in a model category. Since Homotopy Type Theory is a synthetic homotopy theory, there is likely some cross-pollination that lead to this term.
re #50, option B is the most “sustainable”, I think. This is essentially what I usually do on the nLab itself: when we need a keyword but have no content for it yet, I create a stub entry on the nLab that essentially just points to Wikipedia or some other standard source, and then link to that stub. This way the link exists right away, needs not to be remembered to be changed in the future, and the moment somebody comes and fills in actual details all is good.
Oh, I had missed that there was a quetion on what “fibrant type” means. Am adding a brief explanation now, just a moment…
Okay, I have written something at fibrant type and then also had to write something at Homotopy Type System.
@Alexis:
Urs is right: fibrant types seem to be a recent development centred on HTS, and are special types that in a sense can ’see’ the correct maps into or out of them (I’m not sure which, because like you I don’t follow the details of the discussions on the HoTT list very well).
(Well, “fibrant” $\leftrightarrow$ “into”, sort of by definition.)
Yes, it’s not only a recent development, but also a somewhat ideosyncratic one, in that it is really mainly a development that Vladimir Voevosky did, when he ran into technical issues with defining semisimplicial types in “default” HoTT.
One thing has finally become clear to me after lots of discussion (personal and on the HoTT list), namely: Vladimir Voevodsky himself is not actually after “synthetic homotopy theory”. For him univalent HoTT is a way to fix intensional ML-type theory and hence make it available for formalizing mathematics. This formalization he imagines to proceed very much along the classical lines.
For instance when he says that his long-term goal is to have his work on motivic cohomology fully formalized, he is emphatically NOT thinking of using HoTT as the internal language for the $\infty$-topos over the Nisnevich site (maybe to find the “synthetic” axioms needed to be added to HoTT such that it speaks about $\infty$-toposes close enough to that over the Nisnevich site for $\mathbb{A}^1$-homotopy theory to be expressible), instead he is aiming for a formalization that faithfully follows the classical ways these things are set up via model categories of simplicial presheaves. Indeed, he is (and has been) thinking about axioms to be added to HoTT that will exclude all models other than that in plain simplicial sets.
If you are interested in “synthetic homotopy theory” proper, you need to follow the recent developements by Shulman, Lumsdaine et al. Voevodsky has been pushing much in a different direction, and HTS with its “non-fibrant types” is the latest incarnation of that.
I vote for option B. I think creating stubs that link to relevant nLab pages is a great idea; then people can fill in more HoTT-specific info as they have time.
Re: the book, when a concept has multiple pages listed in the index, the page where it is defined should be in bold. If you find entries that don’t conform to this, please bring them to our attention. But it does make sense that it is easier to search a wiki than a book.
Re: fibrancy, the most important thing to understand is that this is only relevant (and meaningful) in two situations: (1) when constructing models of type theory in a model category, and (2) when working in an experimental type system such as HTS. The book never mentions it because the concept doesn’t exist in the type theory used in the book and because the book doesn’t discuss constructing models.3
Are you aware of the hott-amateurs mailing list? It was created specifically for newcomers to ask questions that would feel uncomfortable on the more research/expert-focused list.
hott-amateurs sounds like a porn site! But I’m glad to learn of it.
@urs I know fibrancy $\leftrightarrow$ mapping into, but the discussion has been around constructing new type by various means that took to imply mapping out of things. But now I read some recent posts, I realise that I’d misread, and people are talking about mapping out of the constructions into fibrant types.
arg flaky connection. I’ll fix the previous anon. (Done)
@Todd: Yeah, I know. I didn’t name it…
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