[ refactor ] (more) decidable `Data.Fin.Properties` #2744

jamesmckinna · 2025-06-23T09:34:14Z

This PR refactors some proofs about Decidable predicates on Fin n, to clarify some, and to add others, including some additional syntax, around a independently interesting minimisation principle from which certain others follow.

A downstream refactoring might reconsider these things in terms of specialisation of lemmas in Induction.InfiniteDescent.

Outstanding issue: could be more systematic about use of variables to clean the whole module up?

src/Data/Fin/Properties.agda

jamesmckinna · 2025-06-25T10:31:47Z

Thanks @MatthewDaggitt for the feedback:

regarding lifting out these properties to a new module, that does seem like a good idea
regarding naming/(use of) syntax, indeed there are lots of stylistic questions about the current version.

I'll move to DRAFT for the time being, and remove from v2.3, while I reconsider things.

… module

JacquesCarette

All the code seems fine. Only issues are that things seem to have moved without a good reason to.

src/Data/Fin/Relation/Unary/Decidable.agda

jamesmckinna · 2025-06-26T18:57:18Z

Hmmm... @JacquesCarette the code motion was in response to earlier review comments from @MatthewDaggitt , but maybe I misunderstood his intention. Nevertheless, the Quantification section is necessary for the subsequent decFinSubset and all?/any? definitions... so it seemed cognate to keep them together (esp. given the public re-export in Data.Fin.Properties...)

JacquesCarette · 2025-06-26T19:08:17Z

I had seen some of the conversation. This is why maybe a larger break up (Data.Fin.Properties.Quantification too?) might make more sense. Same with the Effectful stuff, it most definitely does not belong in .Properties.Decidable. The main code motion does make a lot of sense, it just has knock-on consequences.

jamesmckinna · 2025-06-27T07:53:02Z

Last addition: dual version of searchMin so that client use doesn't have to figure out the DNE...

jamesmckinna · 2025-06-29T07:59:13Z

@JacquesCarette If you are happy with the revisions, then we could add this to the v2.3 docket?

JacquesCarette

Not thrilled with where the Effectful have ended up, but not a hill I'm willing to die on.

MatthewDaggitt · 2025-07-02T07:43:45Z

src/Data/Fin/Relation/Unary/Decidable.agda

+
+{-# OPTIONS --cubical-compatible --safe #-}
+
+module Data.Fin.Relation.Unary.Decidable where


Why do we need Decidable in the name? Isn't everything over Fin going to be decidable?

MatthewDaggitt · 2025-07-02T07:45:15Z

src/Data/Fin/Relation/Unary/Base.agda

+
+{-# OPTIONS --cubical-compatible --safe #-}
+
+module Data.Fin.Relation.Unary.Base where


Why do we need this Base module?

MatthewDaggitt · 2025-07-02T07:48:45Z

src/Data/Fin/Relation/Unary/Decidable.agda

+
+------------------------------------------------------------------------
+-- search
+-- a Decidable predicate is either always true, or has a smallest counterexample


Capitalisation and formatting of this header could be better?

MatthewDaggitt · 2025-07-02T07:56:18Z

src/Data/Fin/Relation/Unary/Decidable.agda

+syntax Universal< (λ j → P) i = ∀[ j < i ] P
+
+ExistsMinimalCounterexample : (P : Pred (Fin n) p) → Set p
+ExistsMinimalCounterexample P = ∃[ i ] ¬ P i × ∀[ j < i ] P j


We could just call this MinimalCounterexample? Making this a record with named fields would make it easier to read as well?

MatthewDaggitt · 2025-07-02T07:58:50Z

src/Data/Fin/Relation/Unary/Decidable.agda

+ExistsMinimalCounterexample : (P : Pred (Fin n) p) → Set p
+ExistsMinimalCounterexample P = ∃[ i ] ¬ P i × ∀[ j < i ] P j
+
+syntax ExistsMinimalCounterexample P = μ⟨¬ P ⟩


I'm still kind of against this for readability reasons, but why does this have to be a syntax declaration rather than an ordinary infix declaration?

MatthewDaggitt · 2025-07-02T08:01:34Z

src/Data/Fin/Properties.agda

+
+------------------------------------------------------------------------
+-- Fin
+------------------------------------------------------------------------


Is there a reason this and Bundles have been moved? To be they logically make sense to live at the top of the file as they related to the type itself, rather than functions or relations over it.

MatthewDaggitt · 2025-07-02T08:01:50Z

src/Data/Fin/Properties.agda

+------------------------------------------------------------------------
+
+open import Data.Fin.Relation.Unary.Base public
+open import Data.Fin.Relation.Unary.Decidable public


I'm not convinced about this direction of dependencies.

MatthewDaggitt · 2025-07-02T08:03:39Z

src/Data/Fin/Relation/Unary/Decidable.agda

+
+
+Universal< : Pred (Fin n) p → Pred (Fin n) p
+Universal< P i = (j : Fin′ i) → P (inject j)


Why do we use Fin' and inject here rather than an explicit j < i proof? Generally Fin' is quite difficult to work with.

MatthewDaggitt · 2025-07-02T08:06:21Z

src/Data/Fin/Relation/Unary/Decidable.agda

+
+syntax ExistsMinimalCounterexample P = μ⟨¬ P ⟩
+
+searchMin : Decidable P → (∀ i → P i) ⊎ ∃[ i ] ¬ P i × ∀[ j < i ] P j


How about calling this searchForCounterexample and the one below searchForWitness?

MatthewDaggitt · 2025-07-03T06:04:40Z

Again has merge conflicts, no one waiting on it particularly and @jamesmckinna is away so bumping to v2.4

jamesmckinna · 2025-07-03T07:16:17Z

Happy to bump! There was enough feedback/pushback on some of the choices I'd made that this one deserves a bit more time/energy/thought.

jamesmckinna added 6 commits June 23, 2025 10:00

refactor: avoid indirection via decFinSubset

906c0f6

cosmetic: clarify type

fde3f60

refactor: clarify proof

6832038

add: new definitions

b92e1a9

cosmetic: clarify type

498db73

refactor: clarify proof

53a8d00

jamesmckinna added this to the v2.3 milestone Jun 23, 2025

jamesmckinna added addition refactoring cosmetic labels Jun 23, 2025

jamesmckinna changed the title ~~[ refactor ] decidable Data.Fin.Properties~~ [ refactor ] (more) decidable Data.Fin.Properties Jun 24, 2025

MatthewDaggitt reviewed Jun 25, 2025

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jamesmckinna marked this pull request as draft June 25, 2025 11:21

add: whitespace

824ccbf

MatthewDaggitt removed this from the v2.3 milestone Jun 25, 2025

jamesmckinna mentioned this pull request Jun 26, 2025

Proved sorted permutations are equal #2748

Merged

jamesmckinna added 6 commits June 26, 2025 12:08

refactor: code motion

c7961fc

refactor: lift out Core properties, ahead of adding new Decidable…

38245f7

… module

refactor: Core not actually needed!

47ccbf5

refactor: lift out properties of Decidable predicates

ce5efba

refactor: code motion and new module

d7b30f2

refactor: update CHANGELOG

c5aebff

JacquesCarette requested changes Jun 26, 2025

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src/Data/Fin/Relation/Unary/Decidable.agda Outdated Show resolved Hide resolved

src/Data/Fin/Relation/Unary/Decidable.agda Outdated Show resolved Hide resolved

fix: cosmetic tweaks

2a77e81

jamesmckinna added 3 commits June 27, 2025 06:09

refactor: better module structure?

23d252d

refactor: turn with into let

56f3b78

refactor: clarify types

e8e4772

add: dual version of searchMin

355eb56

jamesmckinna marked this pull request as ready for review June 27, 2025 07:53

JacquesCarette approved these changes Jun 30, 2025

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JacquesCarette added this to the v2.3 milestone Jun 30, 2025

Merge branch 'master' into refactor-decidable-Fin

1470a95

MatthewDaggitt reviewed Jul 2, 2025

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MatthewDaggitt modified the milestones: v2.3, v2.4 Jul 3, 2025

jamesmckinna marked this pull request as draft July 3, 2025 07:16


		{-# OPTIONS --cubical-compatible --safe #-}

		module Data.Fin.Relation.Unary.Decidable where


		{-# OPTIONS --cubical-compatible --safe #-}

		module Data.Fin.Relation.Unary.Base where



		Universal< : Pred (Fin n) p → Pred (Fin n) p
		Universal< P i = (j : Fin′ i) → P (inject j)


		syntax ExistsMinimalCounterexample P = μ⟨¬ P ⟩

		searchMin : Decidable P → (∀ i → P i) ⊎ ∃[ i ] ¬ P i × ∀[ j < i ] P j

[ refactor ] (more) decidable Data.Fin.Properties #2744

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[ refactor ] (more) decidable Data.Fin.Properties #2744

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jamesmckinna commented Jun 25, 2025

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jamesmckinna commented Jun 26, 2025

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MatthewDaggitt commented Jul 3, 2025

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[ refactor ] (more) decidable `Data.Fin.Properties` #2744

[ refactor ] (more) decidable `Data.Fin.Properties` #2744

jamesmckinna commented Jun 23, 2025 •

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