```
From: Andrew Swan <wakelin.swan@gmail.com>
To: Homotopy Type Theory <HomotopyTypeTheory@googlegroups.com>
Subject: [HoTT] Re: A question about the problem with regularity in CCHM cubical type theory
Date: Sun, 15 Sep 2019 04:55:34 -0700 (PDT)
Message-ID: <16b3b92f-069b-468c-94f8-f3859e152338@googlegroups.com> (raw)
In-Reply-To: <CAGTS-a8fo4FKCLiCVCYm6AGeVc=qB8QNUZ3Weho6kLT8NXtc-Q@mail.gmail.com>
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You might have already seen this, but I have a paper on some related issues
at https://arxiv.org/abs/1808.00920 . In that paper I didn't look at the
original version of regularity, but a more recent version ("all
monomorphisms are cofibrations") that fits better with the general
framework of Orton and Pitts. In that case it is definitely equality of
objects that causes problems.
Best,
Andrew
On Friday, 13 September 2019 08:10:42 UTC+2, Jasper Hugunin wrote:
>
> Hello all,
>
> I've been trying to understand better why composition for the universe
> does not satisfy regularity.
> Since comp^i [ phi |-> E ] A is defined as (roughly) Glue [ phi |->
> equiv^i E ] A, I would expect regularity to follow from two parts:
> 1. That Glue [ phi |-> equivRefl A ] A reduces to A (a sort of regularity
> condition for the Glue type constructor itself)
> 2. That equiv^i (refl A) reduces to equivRefl A
> I'm curious as to which (or both) of these parts was the issue, or if
> regularity for the universe was supposed to follow from a different
> argument.
>
> Context:
> I've been studying and using CCHM cubical type theory recently, and often
> finding myself wishing that J computed strictly.
> If I understand correctly, early implementations of ctt did have strict J
> for Path types, and this was justified by a "regularity" condition on the
> composition operation, but as discussed in this thread on the HoTT
> mailing list
> <https://groups.google.com/d/msg/homotopytypetheory/oXQe5u_Mmtk/3HEDk5g5uq4J>,
> the definition of composition for the universe was found to not satisfy
> regularity.
> I don't remember seeing the regularity condition defined anywhere, but my
> understanding is that it requires that composition in a degenerate line of
> types, with the system of constraints giving the sides of the box also
> degenerate in that direction, reduces to just the bottom of the box. This
> seems to be closed under the usual type formers, plus Glue, but not the
> universe with computation defined as in the CCHM paper
> <http://www.cse.chalmers.se/~coquand/cubicaltt.pdf> (for trivial reasons
> and non-trivial reasons; it gets stuck at the start with Glue [ phi |->
> equiv^i refl ] A not reducing to anything).
>
> Best regards,
> - Jasper Hugunin
>
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<div dir="ltr">You might have already seen this, but I have a paper on some related issues at <a href="https://arxiv.org/abs/1808.00920">https://arxiv.org/abs/1808.00920</a> . In that paper I didn't look at the original version of regularity, but a more recent version ("all monomorphisms are cofibrations") that fits better with the general framework of Orton and Pitts. In that case it is definitely equality of objects that causes problems.<div><br></div><div>Best,</div><div>Andrew<br><br>On Friday, 13 September 2019 08:10:42 UTC+2, Jasper Hugunin wrote:<blockquote class="gmail_quote" style="margin: 0;margin-left: 0.8ex;border-left: 1px #ccc solid;padding-left: 1ex;"><div dir="ltr">Hello all,<div><br></div><div><div>I've been trying to understand better why composition for the universe does not satisfy regularity.</div><div>Since comp^i [ phi |-> E ] A is defined as (roughly) Glue [ phi |-> equiv^i E ] A, I would expect regularity to follow from two parts:</div><div>1. That Glue [ phi |-> equivRefl A ] A reduces to A (a sort of regularity condition for the Glue type constructor itself)</div><div>2. That equiv^i (refl A) reduces to equivRefl A</div><div>I'm curious as to which (or both) of these parts was the issue, or if regularity for the universe was supposed to follow from a different argument.</div><div><br></div><div>Context:</div><div><div>I've been studying and using CCHM cubical type theory recently, and often finding myself wishing that J computed strictly.</div><div>If I understand correctly, early implementations of ctt did have strict J for Path types, and this was justified by a "regularity" condition on the composition operation, but as discussed in <a href="https://groups.google.com/d/msg/homotopytypetheory/oXQe5u_Mmtk/3HEDk5g5uq4J" target="_blank" rel="nofollow" onmousedown="this.href='https://groups.google.com/d/msg/homotopytypetheory/oXQe5u_Mmtk/3HEDk5g5uq4J';return true;" onclick="this.href='https://groups.google.com/d/msg/homotopytypetheory/oXQe5u_Mmtk/3HEDk5g5uq4J';return true;">this thread on the HoTT mailing list</a>, the definition of composition for the universe was found to not satisfy regularity.</div><div>I don't remember seeing the regularity condition defined anywhere, but my understanding is that it requires that composition in a degenerate line of types, with the system of constraints giving the sides of the box also degenerate in that direction, reduces to just the bottom of the box. This seems to be closed under the usual type formers, plus Glue, but not the universe with computation defined as in <a href="http://www.cse.chalmers.se/~coquand/cubicaltt.pdf" target="_blank" rel="nofollow" onmousedown="this.href='http://www.google.com/url?q\x3dhttp%3A%2F%2Fwww.cse.chalmers.se%2F~coquand%2Fcubicaltt.pdf\x26sa\x3dD\x26sntz\x3d1\x26usg\x3dAFQjCNEwGF-VNBcwNjl2ZnvRdrx2CgOJPQ';return true;" onclick="this.href='http://www.google.com/url?q\x3dhttp%3A%2F%2Fwww.cse.chalmers.se%2F~coquand%2Fcubicaltt.pdf\x26sa\x3dD\x26sntz\x3d1\x26usg\x3dAFQjCNEwGF-VNBcwNjl2ZnvRdrx2CgOJPQ';return true;">the CCHM paper</a> (for trivial reasons and non-trivial reasons; it gets stuck at the start with Glue [ phi |-> equiv^i refl ] A not reducing to anything).</div><div><br></div></div></div><div>Best regards,</div><div>- Jasper Hugunin</div></div>
</blockquote></div></div>
<p></p>
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```

next prev parent reply indexThread overview:13+ messages / expand[flat|nested] mbox.gz Atom feed top 2019-09-13 6:10 [HoTT] " Jasper Hugunin 2019-09-15 5:57 ` [HoTT] " Jasper Hugunin [not found] ` <CAMWCppk4PWzfZ1HKNLMdAZ=RBC-ARxtJXR8okvwO3raea5gC8Q@mail.gmail.com> [not found] ` <CAGTS-a-SvWWF+br6sKxGj6ufVY=4m830FH9BDg06QJR1vbNFsw@mail.gmail.com> 2019-09-16 2:18 ` Fwd: " Jasper Hugunin 2019-09-16 16:18 ` [HoTT] " Licata, Dan 2019-09-16 17:09 ` Jasper Hugunin 2019-09-16 19:01 ` Licata, Dan 2019-09-16 20:17 ` Jasper Hugunin 2019-09-18 12:16 ` Anders Mortberg 2019-09-18 12:52 ` Thierry Coquand2019-09-15 11:55 ` Andrew Swan [this message]2019-09-15 22:38 ` [HoTT] " Jasper Hugunin 2019-09-16 1:04 ` Jon Sterling [not found] ` <A605E6EE-0101-4390-B50D-A6AEB36FDCC2@icloud.com> 2019-09-16 1:44 ` Jon Sterling

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