open import Cat.Displayed.Univalence.Thin
open import Cat.Displayed.Total
open import Cat.Functor.Adjoint
open import Cat.Prelude

open import Order.Diagram.Lub
open import Order.Base

module Order.Instances.Discrete where

open Functor
open Poset
open _⊣_
open _=>_

Discrete orders🔗

Every set $A$ can be turned into a poset by defining $x\le y$ to be $x=y\text{.}$

Disc : ∀ {ℓ} → Set ℓ → Poset ℓ ℓ
Disc A .Ob = ⌞ A ⌟
Disc A ._≤_ = _≡_
Disc A .≤-thin = A .is-tr _ _
Disc A .≤-refl = refl
Disc A .≤-trans = _∙_
Disc A .≤-antisym p _ = p

We can do that same thing using the inductive identity type.

Discᵢ : ∀ {ℓ} → Set ℓ → Poset ℓ ℓ
Discᵢ A .Ob = ⌞ A ⌟
Discᵢ A ._≤_ = _≡ᵢ_
Discᵢ A .≤-thin = hlevel 
Discᵢ A .≤-refl = reflᵢ
Discᵢ A .≤-trans = _∙ᵢ_
Discᵢ A .≤-antisym reflᵢ _ = refl

This extends to a functor from $\mathbf{Sets}$ into the category of posets.

DiscF : ∀ {ℓ} → Functor (Sets ℓ) (Posets ℓ ℓ)
DiscF .F₀ = Disc
DiscF .F₁ f .hom = f
DiscF .F₁ f .pres-≤ = ap f
DiscF .F-id = trivial!
DiscF .F-∘ f g = trivial!

Furthermore, this functor is a left adjoint to the forgetful functor into $\mathbf{Sets}\text{.}$

DiscF⊣Forget : ∀ {ℓ} → DiscF {ℓ} ⊣ Posets↪Sets
DiscF⊣Forget .unit .η A x = x
DiscF⊣Forget .unit .is-natural _ _ _ = refl
DiscF⊣Forget .counit .η P .hom x  = x
DiscF⊣Forget .counit .η P .pres-≤ = Poset.≤-refl' P
DiscF⊣Forget .counit .is-natural P Q f = trivial!
DiscF⊣Forget .zig = trivial!
DiscF⊣Forget .zag = refl

Least upper bounds🔗

If $f:I\to A$ has a least upper bound in the discrete poset on $A\text{,}$ then $f$ must be a constant family.

disc-is-lub→const
  : ∀ {ℓ iℓ} {Ix : Type iℓ} {A : Set ℓ}
  → {f : Ix → ∣ A ∣} {x : ∣ A ∣}
  → is-lub (Disc A) f x
  → ∀ i → f i ≡ x
disc-is-lub→const x-lub i = is-lub.fam≤lub x-lub i

disc-lub→const
  : ∀ {ℓ iℓ} {Ix : Type iℓ} {A : Set ℓ}
  → {f : Ix → ∣ A ∣}
  → Lub (Disc A) f
  → ∀ i j → f i ≡ f j
disc-lub→const x-lub i j =
  Lub.fam≤lub x-lub i ∙ sym (Lub.fam≤lub x-lub j)