open import Cat.Displayed.Cartesian
open import Cat.Functor.Equivalence
open import Cat.Diagram.Pullback
open import Cat.Displayed.Fibre
open import Cat.Instances.Slice
open import Cat.Displayed.Base
open import Cat.Functor.Base
open import Cat.Prelude

import Cat.Reasoning as CR

module Cat.Displayed.Instances.Slice {o ℓ} (B : Precategory o ℓ) where


# The canonical self-indexing🔗

There is a canonical way of viewing any category $\mathcal{B}$ as displayed over itself, given fibrewise by taking slice categories. Following , we refer to this construction as the canonical-self indexing of $\mathcal{B}$ and denote it $\underline{\mathcal{B}}$. Recall that the objects in the slice over $y$ are pairs consisting of an object $x$ and a map $f : x \to y$. The core idea is that any morphism lets us view an object $x$ as being “structure over” an object $y$; the collection of all possible such structures, then, is the set of morphisms $x \to y$, with domain allowed to vary.

Contrary to the maps in the slice category, the maps in the canonical self-indexing have an extra “adjustment” by a morphism $f : x \to y$ of the base category. Where maps in the ordinary slice are given by commuting triangles, maps in the canonical self-indexing are given by commuting squares, of the form  where the primed objects and dotted arrows are displayed.

record
Slice-hom
{x y} (f : Hom x y)
(px : /-Obj {C = B} x) (py : /-Obj {C = B} y)
: Type (o ⊔ ℓ)
where
constructor slice-hom
field
to      : Hom (px .domain) (py .domain)
commute : f ∘ px .map ≡ py .map ∘ to

open Slice-hom


The intuitive idea for the canonical self-indexing is possibly best obtained by considering the canonical self-indexing of ${{\mathbf{Sets}}}_\kappa$. First, recall that an object $f : {{\mathbf{Sets}}}/X$ is equivalently a $X$-indexed family of sets, with the value of the family at each point $x : X$ being the fibre $f^*(x)$. A function $X \to Y$ of sets then corresponds to a reindexing, which takes an $X$-family of sets to a $Y$-family of sets (in a functorial way). A morphism $X' \to Y'$ in the canonical self-indexing of ${{\mathbf{Sets}}}$ lying over a map $f : X \to Y$ is then a function between the families $X' \to Y'$ which commutes with the reindexing given by $f$.

It’s straightforward to piece together the objects of the (ordinary) slice category and our displayed maps Slice-hom into a category displayed over $\mathcal{B}$.

Slices : Displayed B (o ⊔ ℓ) (o ⊔ ℓ)
Slices .Ob[_] = /-Obj {C = B}
Slices .Hom[_] = Slice-hom
Slices .Hom[_]-set = Slice-is-set
Slices .id′ = slice-hom id id-comm-sym
Slices ._∘′_ {x = x} {y = y} {z = z} {f = f} {g = g} px py =
slice-hom (px .to ∘ py .to) $(f ∘ g) ∘ x .map ≡⟨ pullr (py .commute) ⟩≡ f ∘ (y .map ∘ py .to) ≡⟨ extendl (px .commute) ⟩≡ z .map ∘ (px .to ∘ py .to) ∎ Slices .idr′ {f = f} f′ = Slice-pathp (idr f) (idr (f′ .to)) Slices .idl′ {f = f} f′ = Slice-pathp (idl f) (idl (f′ .to)) Slices .assoc′ {f = f} {g = g} {h = h} f′ g′ h′ = Slice-pathp (assoc f g h) (assoc (f′ .to) (g′ .to) (h′ .to))  It’s only slightly more annoying to show that a vertical map in the canonical self-indexing is a map in the ordinary slice category which, since the objects displayed over $x$ are defined to be those of the slice category $\mathcal{B}/x$, gives an equivalence of categories between the fibre $\underline{\mathcal{B}}^*(x)$ and the slice $\mathcal{B}/x$. Fibre→slice : ∀ {x} → Functor (Fibre Slices x) (Slice B x) Fibre→slice .F₀ x = x Fibre→slice .F₁ f ./-Hom.map = f .to Fibre→slice .F₁ f ./-Hom.commutes = sym (f .commute) ∙ eliml refl Fibre→slice .F-id = /-Hom-path refl Fibre→slice .F-∘ f g = /-Hom-path (transport-refl _) Fibre→slice-is-ff : ∀ {x} → is-fully-faithful (Fibre→slice {x = x}) Fibre→slice-is-ff {_} {x} {y} = is-iso→is-equiv isom where isom : is-iso (Fibre→slice .F₁) isom .is-iso.inv hom = slice-hom (hom ./-Hom.map) (eliml refl ∙ sym (hom ./-Hom.commutes)) isom .is-iso.rinv x = /-Hom-path refl isom .is-iso.linv x = Slice-pathp refl refl Fibre→slice-is-equiv : ∀ {x} → is-equivalence (Fibre→slice {x}) Fibre→slice-is-equiv = is-precat-iso→is-equivalence$
record { has-is-ff = Fibre→slice-is-ff
; has-is-iso = id-equiv
}


## As a fibration🔗

If (and only if) $\mathcal{B}$ has all pullbacks, then the self-indexing $\mathcal{B}$ is a Cartesian fibration. This is almost by definition, and is in fact where the “Cartesian” in “Cartesian fibration” (recall that another term for “pullback square” is “cartesian square”). Since the total space $\int \underline{\mathcal{B}}$ is equivalently the arrow category of $\mathcal{B}$, with the projection functor $\pi : \int \underline{\mathcal{B}} \to \mathcal{B}$ corresponding under this equivalence to the codomain functor, we refer to $\underline{ca{B}}$ regarded as a Cartesian fibration as the codomain fibration.

Codomain-fibration
: (∀ {x y z} (f : Hom x y) (g : Hom z y) → Pullback B f g)
→ Cartesian-fibration Slices
Codomain-fibration pullbacks .has-lift f y′ = lift-f where
open Pullback (pullbacks f (y′ .map))

lift-f : Cartesian-lift Slices f y′
lift-f .x′ = cut p₁
lift-f .lifting .to = p₂
lift-f .lifting .commute = square
lift-f .cartesian .universal m h′ .to = limiting (assoc _ _ _ ∙ h′ .commute)
lift-f .cartesian .universal m h′ .commute = sym p₁∘limiting
lift-f .cartesian .commutes m h′ = Slice-pathp refl p₂∘limiting
lift-f .cartesian .unique m′ x = Slice-pathp refl $Pullback.unique (pullbacks f (y′ .map)) (sym (m′ .commute)) (ap to x)  Since the proof that Slices is a cartesian fibration is given by essentially rearranging the data of pullbacks in $\mathcal{B}$, we also have the converse implication: If $\underline{\mathcal{B}}$ is a Cartesian fibration, then $\mathcal{B}$ has all pullbacks. Codomain-fibration→pullbacks : ∀ {x y z} (f : Hom x y) (g : Hom z y) → Cartesian-fibration Slices → Pullback B f g Codomain-fibration→pullbacks f g lifts = pb where open Pullback open is-pullback the-lift = lifts .has-lift f (cut g) pb : Pullback B f g pb .apex = the-lift .x′ .domain pb .p₁ = the-lift .x′ .map pb .p₂ = the-lift .lifting .to pb .has-is-pb .square = the-lift .lifting .commute pb .has-is-pb .limiting {p₁' = p₁'} {p₂'} p = the-lift .cartesian .universal {u′ = cut id} p₁' (slice-hom p₂' (pullr (idr _) ∙ p)) .to pb .has-is-pb .p₁∘limiting = sym (the-lift .cartesian .universal _ _ .commute) ∙ idr _ pb .has-is-pb .p₂∘limiting = ap to (the-lift .cartesian .commutes _ _) pb .has-is-pb .unique p q = ap to$ the-lift .cartesian .unique
(slice-hom _ (idr _ ∙ sym p)) (Slice-pathp refl q)


Since the fibres of the codomain fibration are given by slice categories, then the interpretation of Cartesian fibrations as “displayed categories whose fibres vary functorially” leads us to reinterpret the above results as, essentially, giving the pullback functors between slice categories.