------------------------------------------------------------------------
-- The Agda standard library
--
-- Properties satisfied by Heyting Algebra
------------------------------------------------------------------------
{-# OPTIONS --cubical-compatible --safe #-}
open import Relation.Binary.Lattice
module Relation.Binary.Lattice.Properties.HeytingAlgebra
{c ℓ₁ ℓ₂} (L : HeytingAlgebra c ℓ₁ ℓ₂) where
open import Algebra.Core using (Op₁)
open import Data.Product.Base using (_,_)
open import Function.Base using (_$_; flip; _∘_)
open import Level using (_⊔_)
open import Relation.Binary.Core using (_Preserves_⟶_; _Preserves₂_⟶_⟶_)
import Relation.Binary.Reasoning.PartialOrder as ≤-Reasoning
import Relation.Binary.Reasoning.Setoid as ≈-Reasoning
open HeytingAlgebra L
open import Algebra.Definitions _≈_ using (_DistributesOverˡ_; _DistributesOverʳ_)
open import Relation.Binary.Lattice.Properties.MeetSemilattice meetSemilattice
using (∧-comm; ∧-idempotent; ∧-assoc; ∧-monotonic; ∧-cong)
open import Relation.Binary.Lattice.Properties.JoinSemilattice joinSemilattice
using (∨-idempotent; ∨-monotonic)
import Relation.Binary.Lattice.Properties.BoundedMeetSemilattice boundedMeetSemilattice as BM
using (identityˡ)
open import Relation.Binary.Lattice.Properties.BoundedLattice boundedLattice
using ( ∧-zeroˡ; ∧-zeroʳ; ∨-zeroˡ; ∨-zeroʳ )
------------------------------------------------------------------------
-- Useful lemmas
⇨-eval : ∀ {x y} → (x ⇨ y) ∧ x ≤ y
⇨-eval {x} {y} = transpose-∧ refl
swap-transpose-⇨ : ∀ {x y w} → x ∧ w ≤ y → w ≤ x ⇨ y
swap-transpose-⇨ x∧w≤y = transpose-⇨ $ trans (reflexive $ ∧-comm _ _) x∧w≤y
------------------------------------------------------------------------
-- Properties of exponential
⇨-unit : ∀ {x} → x ⇨ x ≈ ⊤
⇨-unit = antisym (maximum _) (transpose-⇨ $ reflexive $ BM.identityˡ _)
y≤x⇨y : ∀ {x y} → y ≤ x ⇨ y
y≤x⇨y = transpose-⇨ (x∧y≤x _ _)
⇨-drop : ∀ {x y} → (x ⇨ y) ∧ y ≈ y
⇨-drop = antisym (x∧y≤y _ _) (∧-greatest y≤x⇨y refl)
⇨-app : ∀ {x y} → (x ⇨ y) ∧ x ≈ y ∧ x
⇨-app = antisym (∧-greatest ⇨-eval (x∧y≤y _ _)) (∧-monotonic y≤x⇨y refl)
⇨ʳ-covariant : ∀ {x} → (x ⇨_) Preserves _≤_ ⟶ _≤_
⇨ʳ-covariant y≤z = transpose-⇨ (trans ⇨-eval y≤z)
⇨ˡ-contravariant : ∀ {x} → (_⇨ x) Preserves (flip _≤_) ⟶ _≤_
⇨ˡ-contravariant z≤y = transpose-⇨ (trans (∧-monotonic refl z≤y) ⇨-eval)
⇨-relax : _⇨_ Preserves₂ (flip _≤_) ⟶ _≤_ ⟶ _≤_
⇨-relax {x} {y} {u} {v} y≤x u≤v = begin
x ⇨ u ≤⟨ ⇨ʳ-covariant u≤v ⟩
x ⇨ v ≤⟨ ⇨ˡ-contravariant y≤x ⟩
y ⇨ v ∎
where open ≤-Reasoning poset
⇨-cong : _⇨_ Preserves₂ _≈_ ⟶ _≈_ ⟶ _≈_
⇨-cong x≈y u≈v = antisym (⇨-relax (reflexive $ Eq.sym x≈y) (reflexive u≈v))
(⇨-relax (reflexive x≈y) (reflexive $ Eq.sym u≈v))
⇨-applyˡ : ∀ {w x y} → w ≤ x → (x ⇨ y) ∧ w ≤ y
⇨-applyˡ = transpose-∧ ∘ ⇨ˡ-contravariant
⇨-applyʳ : ∀ {w x y} → w ≤ x → w ∧ (x ⇨ y) ≤ y
⇨-applyʳ w≤x = trans (reflexive (∧-comm _ _)) (⇨-applyˡ w≤x)
⇨-curry : ∀ {x y z} → x ∧ y ⇨ z ≈ x ⇨ y ⇨ z
⇨-curry = antisym (transpose-⇨ $ transpose-⇨ $ trans (reflexive $ ∧-assoc _ _ _) ⇨-eval)
(transpose-⇨ $ trans (reflexive $ Eq.sym $ ∧-assoc _ _ _)
(transpose-∧ $ ⇨-applyˡ refl))
------------------------------------------------------------------------
-- Various proofs of distributivity
∧-distribˡ-∨-≤ : ∀ x y z → x ∧ (y ∨ z) ≤ x ∧ y ∨ x ∧ z
∧-distribˡ-∨-≤ x y z = trans (reflexive $ ∧-comm _ _)
(transpose-∧ $ ∨-least (swap-transpose-⇨ (x≤x∨y _ _)) $ swap-transpose-⇨ (y≤x∨y _ _))
∧-distribˡ-∨-≥ : ∀ x y z → x ∧ y ∨ x ∧ z ≤ x ∧ (y ∨ z)
∧-distribˡ-∨-≥ x y z = let
x∧y≤x , x∧y≤y , _ = infimum x y
x∧z≤x , x∧z≤z , _ = infimum x z
y≤y∨z , z≤y∨z , _ = supremum y z
in ∧-greatest (∨-least x∧y≤x x∧z≤x)
(∨-least (trans x∧y≤y y≤y∨z) (trans x∧z≤z z≤y∨z))
∧-distribˡ-∨ : _∧_ DistributesOverˡ _∨_
∧-distribˡ-∨ x y z = antisym (∧-distribˡ-∨-≤ x y z) (∧-distribˡ-∨-≥ x y z)
⇨-distribˡ-∧-≤ : ∀ x y z → x ⇨ y ∧ z ≤ (x ⇨ y) ∧ (x ⇨ z)
⇨-distribˡ-∧-≤ x y z = let
y∧z≤y , y∧z≤z , _ = infimum y z
in ∧-greatest (transpose-⇨ $ trans ⇨-eval y∧z≤y)
(transpose-⇨ $ trans ⇨-eval y∧z≤z)
⇨-distribˡ-∧-≥ : ∀ x y z → (x ⇨ y) ∧ (x ⇨ z) ≤ x ⇨ y ∧ z
⇨-distribˡ-∧-≥ x y z = transpose-⇨ (begin
(((x ⇨ y) ∧ (x ⇨ z)) ∧ x) ≈⟨ ∧-cong Eq.refl $ Eq.sym $ ∧-idempotent _ ⟩
(((x ⇨ y) ∧ (x ⇨ z)) ∧ x ∧ x) ≈⟨ Eq.sym $ ∧-assoc _ _ _ ⟩
(((x ⇨ y) ∧ (x ⇨ z)) ∧ x) ∧ x ≈⟨ ∧-cong (∧-assoc _ _ _) Eq.refl ⟩
(((x ⇨ y) ∧ (x ⇨ z) ∧ x) ∧ x) ≈⟨ ∧-cong (∧-cong Eq.refl $ ∧-comm _ _) Eq.refl ⟩
(((x ⇨ y) ∧ x ∧ (x ⇨ z)) ∧ x) ≈⟨ ∧-cong (Eq.sym $ ∧-assoc _ _ _) Eq.refl ⟩
(((x ⇨ y) ∧ x) ∧ (x ⇨ z)) ∧ x ≈⟨ ∧-assoc _ _ _ ⟩
(((x ⇨ y) ∧ x) ∧ (x ⇨ z) ∧ x) ≤⟨ ∧-monotonic ⇨-eval ⇨-eval ⟩
y ∧ z ∎)
where open ≤-Reasoning poset
⇨-distribˡ-∧ : _⇨_ DistributesOverˡ _∧_
⇨-distribˡ-∧ x y z = antisym (⇨-distribˡ-∧-≤ x y z) (⇨-distribˡ-∧-≥ x y z)
⇨-distribˡ-∨-∧-≤ : ∀ x y z → x ∨ y ⇨ z ≤ (x ⇨ z) ∧ (y ⇨ z)
⇨-distribˡ-∨-∧-≤ x y z = let x≤x∨y , y≤x∨y , _ = supremum x y
in ∧-greatest (transpose-⇨ $ trans (∧-monotonic refl x≤x∨y) ⇨-eval)
(transpose-⇨ $ trans (∧-monotonic refl y≤x∨y) ⇨-eval)
⇨-distribˡ-∨-∧-≥ : ∀ x y z → (x ⇨ z) ∧ (y ⇨ z) ≤ x ∨ y ⇨ z
⇨-distribˡ-∨-∧-≥ x y z = transpose-⇨ (trans (reflexive $ ∧-distribˡ-∨ _ _ _)
(∨-least (trans (transpose-∧ (x∧y≤x _ _)) refl)
(trans (transpose-∧ (x∧y≤y _ _)) refl)))
⇨-distribˡ-∨-∧ : ∀ x y z → x ∨ y ⇨ z ≈ (x ⇨ z) ∧ (y ⇨ z)
⇨-distribˡ-∨-∧ x y z = antisym (⇨-distribˡ-∨-∧-≤ x y z) (⇨-distribˡ-∨-∧-≥ x y z)
------------------------------------------------------------------------
-- Heyting algebras are distributive lattices
isDistributiveLattice : IsDistributiveLattice _≈_ _≤_ _∨_ _∧_
isDistributiveLattice = record
{ isLattice = isLattice
; ∧-distribˡ-∨ = ∧-distribˡ-∨
}
distributiveLattice : DistributiveLattice _ _ _
distributiveLattice = record
{ isDistributiveLattice = isDistributiveLattice
}
------------------------------------------------------------------------
-- Heyting algebras can define pseudo-complement
infix 8 ¬_
¬_ : Op₁ Carrier
¬ x = x ⇨ ⊥
x≤¬¬x : ∀ x → x ≤ ¬ ¬ x
x≤¬¬x x = transpose-⇨ (trans (reflexive (∧-comm _ _)) ⇨-eval)
------------------------------------------------------------------------
-- De-Morgan laws
de-morgan₁ : ∀ x y → ¬ (x ∨ y) ≈ ¬ x ∧ ¬ y
de-morgan₁ x y = ⇨-distribˡ-∨-∧ _ _ _
de-morgan₂-≤ : ∀ x y → ¬ (x ∧ y) ≤ ¬ ¬ (¬ x ∨ ¬ y)
de-morgan₂-≤ x y = transpose-⇨ $ begin
¬ (x ∧ y) ∧ ¬ (¬ x ∨ ¬ y) ≈⟨ ∧-cong ⇨-curry (de-morgan₁ _ _) ⟩
(x ⇨ ¬ y) ∧ ¬ ¬ x ∧ ¬ ¬ y ≈⟨ ∧-cong Eq.refl (∧-comm _ _) ⟩
(x ⇨ ¬ y) ∧ ¬ ¬ y ∧ ¬ ¬ x ≈⟨ Eq.sym $ ∧-assoc _ _ _ ⟩
((x ⇨ ¬ y) ∧ ¬ ¬ y) ∧ ¬ ¬ x ≤⟨ ⇨-applyʳ $ transpose-⇨ $
begin
((x ⇨ ¬ y) ∧ ¬ ¬ y) ∧ x ≈⟨ ∧-cong (∧-comm _ _) Eq.refl ⟩
((¬ ¬ y) ∧ (x ⇨ ¬ y)) ∧ x ≈⟨ ∧-assoc _ _ _ ⟩
(¬ ¬ y) ∧ (x ⇨ ¬ y) ∧ x ≤⟨ ∧-monotonic refl ⇨-eval ⟩
¬ ¬ y ∧ ¬ y ≤⟨ ⇨-eval ⟩
⊥ ∎ ⟩
⊥ ∎
where open ≤-Reasoning poset
de-morgan₂-≥ : ∀ x y → ¬ ¬ (¬ x ∨ ¬ y) ≤ ¬ (x ∧ y)
de-morgan₂-≥ x y = transpose-⇨ $ ⇨-applyˡ $ transpose-⇨ $ begin
(x ∧ y) ∧ (¬ x ∨ ¬ y) ≈⟨ ∧-distribˡ-∨ _ _ _ ⟩
(x ∧ y) ∧ ¬ x ∨ (x ∧ y) ∧ ¬ y ≤⟨ ∨-monotonic (⇨-applyʳ (x∧y≤x _ _))
(⇨-applyʳ (x∧y≤y _ _)) ⟩
⊥ ∨ ⊥ ≈⟨ ∨-idempotent _ ⟩
⊥ ∎
where open ≤-Reasoning poset
de-morgan₂ : ∀ x y → ¬ (x ∧ y) ≈ ¬ ¬ (¬ x ∨ ¬ y)
de-morgan₂ x y = antisym (de-morgan₂-≤ x y) (de-morgan₂-≥ x y)
weak-lem : ∀ {x} → ¬ ¬ (¬ x ∨ x) ≈ ⊤
weak-lem {x} = begin
¬ ¬ (¬ x ∨ x) ≈⟨ ⇨-cong (de-morgan₁ _ _) Eq.refl ⟩
¬ (¬ ¬ x ∧ ¬ x) ≈⟨ ⇨-cong ⇨-app Eq.refl ⟩
⊥ ∧ (x ⇨ ⊥) ⇨ ⊥ ≈⟨ ⇨-cong (∧-zeroˡ _) Eq.refl ⟩
⊥ ⇨ ⊥ ≈⟨ ⇨-unit ⟩
⊤ ∎
where open ≈-Reasoning setoid
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