open import Cat.Instances.Elements.Properties
open import Cat.Instances.Presheaf.Colimits
open import Cat.Functor.Adjoint.Continuous
open import Cat.Functor.Adjoint.Reflective
open import Cat.Diagram.Coproduct.Copower
open import Cat.Diagram.Coproduct.Indexed
open import Cat.Instances.Presheaf.Limits
open import Cat.Diagram.Colimit.Terminal
open import Cat.Instances.Shape.Terminal
open import Cat.Functor.Adjoint.Colimit
open import Cat.Instances.Sets.Complete
open import Cat.Diagram.Colimit.Base
open import Cat.Functor.Hom.Coyoneda
open import Cat.Functor.Equivalence
open import Cat.Functor.Kan.Adjoint
open import Cat.Functor.Hom.Yoneda
open import Cat.Functor.Kan.Unique
open import Cat.Instances.Discrete
open import Cat.Instances.Elements
open import Cat.Instances.Functor
open import Cat.Diagram.Terminal
open import Cat.Functor.Constant
open import Cat.Diagram.Initial
open import Cat.Functor.Adjoint
open import Cat.Functor.Compose
open import Cat.Functor.Final
open import Cat.Functor.Base
open import Cat.Prelude hiding (J)

import Cat.Diagram.Colimit.Coproduct
import Cat.Functor.Reasoning
import Cat.Morphism

module Cat.Total {o ℓ} (C : Precategory o ℓ) where
open import Cat.Functor.Hom C

private
  open module C = Precategory C
  variable
    o' ℓ' : Level
    E : Precategory o' ℓ'
    J : Precategory o' ℓ'

Total precategories🔗

A precategory is total if its Yoneda embedding has a left adjoint. We call this adjoint さ(sa), a reading for 左, which means “left”.

record is-total : Type (o ⊔ lsuc ℓ) where
  field
    {さ} : Functor Cat[ C ^op , Sets ℓ ] C
    has-よ-adj : さ ⊣ よ
  open module さ = Cat.Functor.Reasoning さ using () renaming
    (F₀ to さ₀; F₁ to さ₁) public

Motivation🔗

Total categories represent a particularly nice site in which to do logic, as they are a reflective subcategory of their category of presheaves. In particular, total categories are cocomplete with many useful limits. However, a category being total is a slightly weaker requirement than a category being a topos, as all topoi are both total and cototal.

Free objects relative to よ🔗

Before we investigate the properties of a total category, it’s worth considering the action of such a functor on objects, if it exists. Given some presheaf $F \in PSh (C),$ an object could be $さ (F)$ if it is free with respect to $よ .$

module _ (F : Functor (C ^op) (Sets ℓ)) (c : Free-object よ F) where
  open Free-object c
  private

Since $F$ is a presheaf, it can be written as a colimit of representable functors by the coyoneda lemma.

    F-is-colimit : is-colimit (よ F∘ πₚ C F) F _
    F-is-colimit = coyoneda _ F

As left adjoints preserve colimits, we can imagine that a partial left adjoint (a free object) of a colimit should also be a colimit. Indeed this is true as $よ$ is fully faithful.

We call a free object with respect to $よ$ a realising object for $F .$

  free-is-colimit : is-colimit (πₚ C F) free _
  free-is-colimit =
    free-object→is-colimit よ よ-is-fully-faithful (to-colimit F-is-colimit) c

module Total (C-total : is-total) where
  open module C-total = is-total C-total public
  open Cat.Morphism C public

  さ∘よ≅ⁿid : さ F∘ よ ≅ⁿ Id
  さ∘よ≅ⁿid = is-reflective→counit-iso has-よ-adj よ-is-fully-faithful

  private
    さ∘よ∘F≅ⁿF : ∀ {F : Functor J C} → さ F∘ よ F∘ F ≅ⁿ F
    さ∘よ∘F≅ⁿF = ni-assoc ∙ni (さ∘よ≅ⁿid ◂ni _) ∙ni path→iso F∘-idl

さ on values🔗

For each $F,$ $さ (F)$ yields a free value on $F$ relative to $よ .$ As we have proved, this is a colimit.

  さ₀-is-colimit : (F : ⌞ PSh ℓ C ⌟) → is-colimit (πₚ C F) (さ₀ F) _
  さ₀-is-colimit F = free-is-colimit F $ left-adjoint→free-objects has-よ-adj F

Cocompleteness🔗

Thus, all total categories are cocomplete as their colimits can be computed in $PSh (C) .$

  cocomplete : is-cocomplete ℓ ℓ C
  cocomplete F = natural-iso→colimit さ∘よ∘F≅ⁿF
    $ left-adjoint-colimit has-よ-adj
    $ PSh-cocomplete ℓ C (よ F∘ F)

Terminal object🔗

Total categories also have many limits. For example, the terminal presheaf has a realisation in $C .$ Furthermore, the projection functor from the category of elements of the terminal presheaf is an equivalence. A colimit of an equivalence is equivalent to a colimit of the identity—i.e., a terminal object.

  ★ : C.Ob
  ★ = さ.₀ (⊤PSh _ _)

  private
    ★-is-colimit-id : is-colimit (Id {C = C}) ★ _
    ★-is-colimit-id = extend-is-colimit _
      (right-adjoint-is-final (elements-terminal-is-equivalence.F⁻¹⊣F {s = ℓ})) _
      col' where
        open is-equivalence
        col : is-colimit (πₚ C $ ⊤PSh _ _) ★ _
        col = さ₀-is-colimit _

        col' : is-colimit (Id F∘ πₚ C (⊤PSh _ _)) ★ _
        col' = natural-iso-ext→is-lan
          (left-adjoint-is-cocontinuous (Id-is-equivalence .F⊣F⁻¹) col)
          (!const-isoⁿ id-iso)

  total-terminal : Terminal C
  total-terminal = Id-colimit→Terminal $ to-colimit ★-is-colimit-id
  module total-terminal = Terminal total-terminal

Copowers🔗

As C is cocomplete, it has all set-indexed coproducts and is thus copowered.

  open Cat.Diagram.Colimit.Coproduct C
  has-set-indexed-coproducts : (S : Set ℓ) → has-coproducts-indexed-by C ∣ S ∣
  has-set-indexed-coproducts S F = Colimit→IC F (cocomplete $ Disc-adjunct F)
  open Copowers has-set-indexed-coproducts public
  open Consts total-terminal has-set-indexed-coproducts public