module Cat.Instances.InternalFunctor
  {o ℓ} (C : Precategory o ℓ)
  where

Internal functor categories🔗

If C\mathcal{C} and D\mathcal{D} are categories internal to B\mathcal{B}, we can define the internal functor category C→D\mathcal{C} \to \mathcal{D}, mirroring the construction of the ordinary functor categories. This relativisation plays a similar role in the theory of internal categories.

Before we define the category, we need some simple constructions on internal natural transformations. First, we note that there is an internal identity natural transformation.

  idnti : ∀ {F : Internal-functor ℂ 𝔻} → F =>i F
  idnti .ηi x = 𝔻.idi _
  idnti .is-naturali x y f =
    𝔻.idli _ ∙ sym (𝔻.idri _)
  idnti {F = F} .ηi-nat x σ = 𝔻.casti $
    𝔻.idi-nat σ 𝔻.∙i ap 𝔻.idi (F .Fi₀-nat x σ)

Next, we show that we can compose internal natural transformations α:G⇒H\alpha : G \Rightarrow H and β:F⇒G\beta : F \Rightarrow G to obtain an internal transformation α∘β:F⇒H\alpha \circ \beta : F \Rightarrow H.

  _∘nti_ : ∀ {F G H : Internal-functor ℂ 𝔻} → G =>i H → F =>i G → F =>i H
  (α ∘nti β) .ηi x = α .ηi x 𝔻.∘i β .ηi x
  (α ∘nti β) .is-naturali x y f =
    𝔻.pullri (β .is-naturali x y f)
    ∙ 𝔻.extendli (α .is-naturali x y f)
  (α ∘nti β) .ηi-nat x σ = 𝔻.casti $
    (α .ηi x 𝔻.∘i β .ηi x) [ σ ]     𝔻.≡i⟨ 𝔻.∘i-nat (α .ηi x) (β .ηi x) σ ⟩𝔻.≡i
    α .ηi x [ σ ] 𝔻.∘i β .ηi x [ σ ] 𝔻.≡i⟨ (λ i → α .ηi-nat x σ i 𝔻.∘i β .ηi-nat x σ i) ⟩𝔻.≡i
    α .ηi (x ∘ σ) 𝔻.∘i β .ηi (x ∘ σ) ∎

Armed with these facts, we proceed to construct the internal functor category. Objects are internal functors C→D\mathcal{C} \to \mathcal{D}, morphisms are internal natural transformations F⇒GF \Rightarrow G.

module _ (ℂ 𝔻 : Internal-cat) where
  private
    module ℂ = Internal-cat ℂ
    module 𝔻 = Internal-cat 𝔻

  Internal-functors : Precategory (o ⊔ ℓ) (o ⊔ ℓ)
  Internal-functors .Precategory.Ob      = Internal-functor ℂ 𝔻
  Internal-functors .Precategory.Hom F G = F =>i G

  Internal-functors .Precategory.id  = idnti
  Internal-functors .Precategory._∘_ = _∘nti_

The category equations all follow from the fact that equality of internal natural transformations is given by componentwise equality.

  Internal-functors .Precategory.Hom-set _ _ = Internal-nat-set
  Internal-functors .Precategory.idr α =
    Internal-nat-path λ x → 𝔻.idri _
  Internal-functors .Precategory.idl α =
    Internal-nat-path λ x → 𝔻.idli _
  Internal-functors .Precategory.assoc α β γ =
    Internal-nat-path λ x → 𝔻.associ _ _ _

Internal natural isomorphisms🔗

To continue our relativisation project, we can define the internal natural isomorphisms to be the natural isomorphisms in an internal functor category.

  Internal-Inversesⁿ
    : {F G : Internal-functor ℂ 𝔻}
    → F =>i G → G =>i F
    → Type _
  Internal-Inversesⁿ = ℂ𝔻.Inverses

  is-internal-natural-invertible
    : {F G : Internal-functor ℂ 𝔻}
    → F =>i G
    → Type _
  is-internal-natural-invertible = ℂ𝔻.is-invertible

  Internal-natural-iso : (F G : Internal-functor ℂ 𝔻) → Type _
  Internal-natural-iso F G = F ℂ𝔻.≅ G
  module Internal-Inversesⁿ
    {F G : Internal-functor ℂ 𝔻}
    {α : F =>i G} {β : G =>i F}
    (inv : Internal-Inversesⁿ α β) = ℂ𝔻.Inverses inv
  module is-internal-natural-invertible
    {F G : Internal-functor ℂ 𝔻}
    {α : F =>i G}
    (inv : is-internal-natural-invertible α) = ℂ𝔻.is-invertible inv
  module Internal-natural-iso
    {F G : Internal-functor ℂ 𝔻}
    (eta : Internal-natural-iso F G) = ℂ𝔻._≅_ eta

  record make-internal-natural-iso (F G : Internal-functor ℂ 𝔻) : Type (o ⊔ ℓ) where
    field
      etai : ∀ {Γ} (x : Hom Γ ℂ.C₀) → 𝔻.Homi (F .Fi₀ x) (G .Fi₀ x)
      invi : ∀ {Γ} (x : Hom Γ ℂ.C₀) → 𝔻.Homi (G .Fi₀ x) (F .Fi₀ x)
      etai∘invi : ∀ {Γ} (x : Hom Γ ℂ.C₀) → etai x 𝔻.∘i invi x ≡ 𝔻.idi _
      invi∘etai : ∀ {Γ} (x : Hom Γ ℂ.C₀) → invi x 𝔻.∘i etai x ≡ 𝔻.idi _
      naturali : ∀ {Γ} (x y : Hom Γ ℂ.C₀) (f : ℂ.Homi x y)
               → etai y 𝔻.∘i F .Fi₁ f ≡ G .Fi₁ f 𝔻.∘i etai x
      etai-nat : ∀ {Γ Δ} (x : Hom Δ ℂ.C₀)
               → (σ : Hom Γ Δ)
               → PathP (λ i → 𝔻.Homi (F .Fi₀-nat x σ i) (G .Fi₀-nat x σ i))
                   (etai x [ σ ]) (etai (x ∘ σ))
      invi-nat : ∀ {Γ Δ} (x : Hom Δ ℂ.C₀)
               → (σ : Hom Γ Δ)
               → PathP (λ i → 𝔻.Homi (G .Fi₀-nat x σ i) (F .Fi₀-nat x σ i))
                   (invi x [ σ ]) (invi (x ∘ σ))

  to-internal-natural-iso
    : {F G : Internal-functor ℂ 𝔻}
    → make-internal-natural-iso F G
    → Internal-natural-iso F G
  to-internal-natural-iso {F = F} {G = G} mk = ni where
    open make-internal-natural-iso mk
    open Internal-natural-iso {F} {G}
    open Internal-Inversesⁿ {F} {G}

    ni : Internal-natural-iso F G
    ni .to .ηi = etai
    ni .to .is-naturali = naturali
    ni .to .ηi-nat = etai-nat
    ni .from .ηi = invi
    ni .from .is-naturali x y f =
      invi y 𝔻.∘i G .Fi₁ f                         ≡⟨ ap (invi y 𝔻.∘i_) (sym (𝔻.idri _) ∙ ap (G .Fi₁ _ 𝔻.∘i_) (sym (etai∘invi x))) ⟩≡
      invi y 𝔻.∘i G .Fi₁ f 𝔻.∘i etai x 𝔻.∘i invi x ≡⟨ ap (invi y 𝔻.∘i_) (𝔻.extendli (sym (naturali _ _ _))) ⟩≡
      invi y 𝔻.∘i etai y 𝔻.∘i F .Fi₁ f 𝔻.∘i invi x ≡⟨ 𝔻.cancelli (invi∘etai y) ⟩≡
      F .Fi₁ f 𝔻.∘i invi x                         ∎
    ni .from .ηi-nat = invi-nat
    ni .inverses .invl = Internal-nat-path etai∘invi
    ni .inverses .invr = Internal-nat-path invi∘etai