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Basic properties of the flat modality #1005

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@fredrik-bakke fredrik-bakke commented Jan 18, 2024

Proves a series of basic properties of the flat modality.

Summary

General properties

  • The universal property of flat discrete crisp types
  • The dependent universal property of flat discrete crisp types
  • Functoriality of flat
  • Flat is idempotent
  • A crisp type is crisply flat discrete if its counit has a crisp section

Left exactness of flat

  • Flat distributes over identity types
  • The crisp identity types of flat discrete crisp types are flat discrete
  • Flat distributes over dependent pair types
  • Flat distributes over product types
  • Flat distributes over pullbacks
  • Flat distributes over sequential limits
  • The unit type is flat discrete

Right exactness of flat

  • The empty type is flat discrete
  • The natural numbers are flat discrete
  • Flat distributes over coproduct types
  • Flat distributes over pushouts
  • Flat distributes over coequalizers
  • Flat distributes over sequential colimits

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This PR overlaps with #1021, and since it adds new references it currently conflicts with #1058, so I would appreciate it if that PR could be merged soon.

@fredrik-bakke fredrik-bakke deleted the nitpick-mtt branch March 9, 2024 13:45
@fredrik-bakke fredrik-bakke restored the nitpick-mtt branch March 9, 2024 13:46
@fredrik-bakke fredrik-bakke reopened this Mar 9, 2024
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whoops, turns out renaming branches closes PRs 😅

fredrik-bakke and others added 25 commits March 10, 2024 20:31
Pre-commit tries to run a downloaded NodeJS executable when NodeJS isn't
available in the environment, but that fails to run on NixOS.
…niMath#1052)

### Summary
- Refactors the definition of categories to use the new and more general
strictly involutive identity types for their associativity witnesses.
- Refactor definitions of some instances of large and small
precategories to use a new `make-(Large-)?-Precategory` constructor.
- Defines the underlying large precategory of a large subprecategory.
- Change some prose regarding basic definitions in category theory.
- Refactors all appropriate instances of large precategories to be
(full) large subprecategories.
  - Rename `is-group` to `is-group-Semigroup`.
 
The last item was appropriate to make the handling of
`involutive-eq-associative-comp-hom-***-Large-Precategory` systematic.

Improves on what was implemented in UniMath#945.
Adds citation support using a biblatex file and
[pybtex](https://pybtex.org/), reactors most current citations to use
it, and adds a small guide to explain how to use it.

Resolves UniMath#957.
…h#1066)

I found two instances where the concepts macro wasn't formatted
properly, so I added a check to the preprocessor that fails the website
build.

This means that the preprocessor will run in full for the `linkcheck`
output. We could have it only do some of the work for just checking if
all the tags are well-formatted, but most of the time is still spent in
IO with the `mdbook` process, which we can't speed up.
The changes in this PR are pulled from UniMath#885.
The concept macro looks for Agda definitions when the Agda= component is
specified. Until now the definition name was not escaped, so concepts
like the coproduct type `_+_` could not be found by the macro.

Fixes UniMath#1073
…type-families` (UniMath#1065)

This PR moves the torsoriality of the identity types to
`foundation-core.torsorial-type-families`. Previously it was in
`contractible-types`. I also slightly updated the prose, and fixed
imports wherever they were broken because of this move.
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This PR is succeeded by #1078 due to merging issues.

@fredrik-bakke fredrik-bakke deleted the nitpick-mtt branch March 28, 2024 14:18
EgbertRijke pushed a commit that referenced this pull request Sep 6, 2024
This is the replacement of #1005.

Proves a series of basic properties of the flat modality.

## Summary

### General properties

- [X] The universal property of flat discrete crisp types
- [x] ~The dependent universal property of flat discrete crisp types~
- [X] Functoriality of flat
- [X] Flat is idempotent
- [x] A crisp type is crisply flat discrete if its counit has a crisp
section

### Left exactness of flat

- [X] Flat distributes over identity types
- [X] The crisp identity types of flat discrete crisp types are flat
discrete
- [X] Flat distributes over dependent pair types
- [X] Flat distributes over product types
- [x] Flat distributes over pullbacks
- [x] ~Flat distributes over sequential limits~
- [x] The unit type is flat discrete

### Right exactness of flat

- [X] The empty type is flat discrete
- [X] The natural numbers are flat discrete
- [X] Flat distributes over coproduct types
- [x] ~Flat distributes over pushouts~
- [x] ~Flat distributes over coequalizers~
- [x] ~Flat distributes over sequential colimits~

### Notes
- The constructor for the flat modality is renamed from `cons-flat` to
`intro-flat`. This makes it easier to distinguish from `counit-flat`.
- In the future, we will probably want to prove `crisp-based-ind-Id`
from the existence of the sharp modality rather than postulating it. The
same is true for the modal induction principle of the sharp modality.
- This PR does some ground work with the sharp modality too.
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3 participants