open import Cat.Functor.Equivalence
open import Cat.Functor.Naturality
open import Cat.Functor.Univalence
open import Cat.Functor.Adjoint
open import Cat.Functor.Base
open import Cat.Prelude

import Cat.Functor.Reasoning as Fr
import Cat.Reasoning as Cr

module
  Cat.Functor.Adjoint.Unique
  {o ℓ o′ ℓ′} {C : Precategory o ℓ} {D : Precategory o′ ℓ′}
  where

private
  module C = Cr C
  module D = Cr D

Uniqueness of adjoints🔗

Let $F : \mathcal{C} \to \mathcal{D}$ be a functor participating in two adjunctions $F \dashv G$ and $F \dashv G'$. Using the data from both adjunctions, we can exhibit a natural isomorphism $G \cong G'$, which additionally preserves the unit and counit: Letting $\gamma$, $\delta$ be the components of the natural isomorphism, we have $\gamma\eta = \eta'$, idem for $\varepsilon$.

module _ {F : Functor C D} {G G′ : Functor D C} (a : F ⊣ G) (a′ : F ⊣ G′) where
  private
    module F = Fr F
    module G = Fr G
    module G′ = Fr G′
    module a = _⊣_ a
    module a′ = _⊣_ a′
  open a.unit using (η)
  open a.counit using (ε)
  open a′.unit hiding (is-natural) renaming (η to η′)
  open a′.counit hiding (is-natural) renaming (ε to ε′)
  open make-natural-iso

The isomorphism is defined (in components) to be $G'\varepsilon\eta'G$, with inverse $G\varepsilon'\eta{}G$. Here, we show the construction of both directions, cancellation in one directly, and naturality (naturality for the inverse is taken care of by make-natural-iso). Cancellation in the other direction is exactly analogous, and so was omitted.

  private
    make-G≅G′ : make-natural-iso G G′
    make-G≅G′ .eta x = G′.₁ (ε x) C.∘ η′ _
    make-G≅G′ .inv x = G.₁ (ε′ x) C.∘ η _
    make-G≅G′ .inv∘eta x =
      (G.₁ (ε′ x) C.∘ η _) C.∘ G′.₁ (ε _) C.∘ η′ _    ≡⟨ C.extendl (C.pullr (a.unit.is-natural _ _ _) ∙ G.pulll (a′.counit.is-natural _ _ _)) ⟩≡
      G.₁ (ε x D.∘ ε′ _) C.∘ η _ C.∘ η′ _             ≡⟨ C.refl⟩∘⟨ a.unit.is-natural _ _ _ ⟩≡
      G.₁ (ε x D.∘ ε′ _) C.∘ G.₁ (F.₁ (η′ _)) C.∘ η _ ≡⟨ G.pulll (D.cancelr a′.zig) ⟩≡
      G.₁ (ε x) C.∘ η _                               ≡⟨ a.zag ⟩≡
      C.id                                            ∎
    make-G≅G′ .natural x y f =
      G′.₁ f C.∘ G′.₁ (ε x) C.∘ η′ _               ≡⟨ C.pulll (G′.weave (sym (a.counit.is-natural _ _ _))) ⟩≡
      (G′.₁ (ε y) C.∘ G′.₁ (F.₁ (G.₁ f))) C.∘ η′ _ ≡⟨ C.extendr (sym (a′.unit.is-natural _ _ _)) ⟩≡
      (G′.₁ (ε y) C.∘ η′ _) C.∘ G.₁ f              ∎

    make-G≅G′ .eta∘inv x =
          C.extendl (C.pullr (a′.unit.is-natural _ _ _))
        ·· ap₂ C._∘_ refl (C.pushl (sym (a′.unit.is-natural _ _ _)))
        ·· C.extend-inner (a′.unit.is-natural _ _ _)
        ·· G′.extendl (a.counit.is-natural _ _ _)
        ·· ap₂ C._∘_ refl ( ap₂ C._∘_ refl (a′.unit.is-natural _ _ _)
                          ∙ G′.cancell a.zig)
        ∙ a′.zag

The data above is exactly what we need to define a natural isomorphism $G \cong G'$. Showing that this isomorphism commutes with the adjunction natural transformations is a matter of calculating:

  right-adjoint-unique : Cr.Isomorphism Cat[ D , C ] G G′
  right-adjoint-unique = to-natural-iso make-G≅G′

  abstract
    unique-preserves-unit
      : ∀ {x} → make-G≅G′ .eta _ C.∘ η x ≡ η′ x
    unique-preserves-unit =
      make-G≅G′ .eta _ C.∘ η _                 ≡⟨ C.pullr (a′.unit.is-natural _ _ _) ⟩≡
      G′.₁ (ε _) C.∘ G′.₁ (F.₁ (η _)) C.∘ η′ _ ≡⟨ G′.cancell a.zig ⟩≡
      η′ _                                     ∎

    unique-preserves-counit
      : ∀ {x} → ε _ D.∘ F.₁ (make-G≅G′ .inv _) ≡ ε′ x
    unique-preserves-counit =
      ε _ D.∘ F.₁ (make-G≅G′ .inv _)         ≡⟨ D.refl⟩∘⟨ F.F-∘ _ _ ⟩≡
      ε _ D.∘ F.₁ (G.₁ (ε′ _)) D.∘ F.₁ (η _) ≡⟨ D.extendl (a.counit.is-natural _ _ _) ⟩≡
      ε′ _ D.∘ ε _ D.∘ F.₁ (η _)             ≡⟨ D.elimr a.zig ⟩≡
      ε′ _                                   ∎

If the codomain category $\mathcal{C}$ is furthermore univalent, so that natural isomorphisms are an identity system on the functor category $[D, C]$, we can upgrade the isomorphism $G \cong G'$ to an identity $G \equiv G$, and preservation of the adjunction data means this identity can be improved to an identification between pairs of the functors and their respective adjunctions.

is-left-adjoint-is-prop
 : is-category C
 → (F : Functor C D)
 → is-prop $ Σ[ G ∈ Functor D C ] (F ⊣ G)
is-left-adjoint-is-prop cc F (G , a) (G′ , a′) i = G≡G′ cd i , a≡a′ cd i

  where
  G≅G′ = right-adjoint-unique a a′
  cd = Functor-is-category cc
  open _⊣_
  open _=>_
  open Functor
  module F = Fr F

  module _ (d : is-category Cat[ D , C ]) where
    G≡G′ = d .to-path G≅G′

    abstract
      same-eta : PathP (λ i → Id => G≡G′ i F∘ F) (a .unit) (a′ .unit)
      same-eta = Nat-pathp refl (λ i → G≡G′ i F∘ F)
        λ x → Hom-pathp-reflr C $
          ap₂ C._∘_ (whisker-path-left {G = G} {G′} {F = F} d G≅G′) refl
        ∙ unique-preserves-unit a a′

      same-eps : PathP (λ i → F F∘ G≡G′ i => Id) (a .counit) (a′ .counit)
      same-eps = Nat-pathp (λ i → F F∘ G≡G′ i) refl
        λ x → Hom-pathp-refll D $
          ap₂ D._∘_ refl (whisker-path-right {G = F} {F = G} {G′} d G≅G′)
        ∙ unique-preserves-counit a a′

    a≡a′ : PathP (λ i → F ⊣ G≡G′ i) a a′
    a≡a′ i .unit = same-eta i
    a≡a′ i .counit = same-eps i
    a≡a′ i .zig {A} =
      is-set→squarep (λ i j → D.Hom-set (F.₀ A) (F.₀ A))
        (λ i → same-eps i .η (F.₀ A) D.∘ F.₁ (same-eta i .η A))
        (a .zig) (a′ .zig) refl i
    a≡a′ i .zag {A} =
      is-set→squarep (λ i j → C.Hom-set (G≡G′ i .F₀ A) (G≡G′ i .F₀ A))
        (λ i → G≡G′ i .F₁ (same-eps i .η A) C.∘ same-eta i .η (G≡G′ i .F₀ A))
        (a .zag) (a′ .zag) (λ _ → C.id) i

As a corollary, we conclude that, for a functor $F : \mathcal{C} \to \mathcal{D}$ from a univalent category $\mathcal{C}$, “being an equivalence of categories” is a proposition.

open is-equivalence
is-equivalence-is-prop
  : is-category C
  → (F : Functor C D)
  → is-prop (is-equivalence F)
is-equivalence-is-prop ccat F x y = go where
  invs = ap fst $
    is-left-adjoint-is-prop ccat F (x .F⁻¹ , x .F⊣F⁻¹) (y .F⁻¹ , y .F⊣F⁻¹)

  adjs = ap snd $
    is-left-adjoint-is-prop ccat F (x .F⁻¹ , x .F⊣F⁻¹) (y .F⁻¹ , y .F⊣F⁻¹)

  go : x ≡ y
  go i .F⁻¹ = invs i
  go i .F⊣F⁻¹ = adjs i
  go i .unit-iso a =
    is-prop→pathp (λ i → C.is-invertible-is-prop {f = _⊣_.unit.η (adjs i) a})
      (x .unit-iso a)
      (y .unit-iso a) i
  go i .counit-iso a =
    is-prop→pathp (λ i → D.is-invertible-is-prop {f = _⊣_.counit.ε (adjs i) a})
      (x .counit-iso a)
      (y .counit-iso a) i