open import Cat.Instances.Presheaf.Exponentials
open import Cat.Instances.Sheaf.Limits.Finite
open import Cat.Functor.Equivalence.Complete
open import Cat.Instances.Sheaf.Exponentials
open import Cat.Functor.Adjoint.Continuous
open import Cat.Functor.Adjoint.Reflective
open import Cat.Instances.Algebras.Limits
open import Cat.Instances.Sets.Cocomplete
open import Cat.Instances.Functor.Limits
open import Cat.Functor.Adjoint.Monadic
open import Cat.Functor.FullSubcategory
open import Cat.Instances.Sets.Complete
open import Cat.Instances.Sheaf.Limits
open import Cat.Diagram.Colimit.Base
open import Cat.Diagram.Exponential
open import Cat.Functor.Equivalence
open import Cat.Site.Sheafification
open import Cat.Diagram.Limit.Base
open import Cat.Instances.Functor
open import Cat.Functor.Adjoint
open import Cat.Functor.Base
open import Cat.Site.Base
open import Cat.Prelude

open Cartesian-closed
open is-exponential
open Exponential

module Cat.Instances.Sheaves where

The topos of sheaves🔗

This module collects a compendium of the nice properties enjoyed by the category of sheaves on a site.

Sh[_,_] : ∀ {ℓ} (C : Precategory ℓ ℓ) (J : Coverage C ℓ) → Precategory (lsuc ℓ) ℓ
Sh[ C , J ] = Sheaves J _

module _ {ℓ} {C : Precategory ℓ ℓ} {J : Coverage C ℓ} where

Monadicity🔗

Since the sheafification construction provides a left adjoint to the fully faithful inclusion of presheaves into sheaves, we can immediately conclude that the category of sheaves on a site is monadic over the presheaves on that same site.

  Sheafification : Functor (PSh ℓ C) Sh[ C , J ]
  Sheafification = free-objects→functor (Small.make-free-sheaf J)

  Sheafification⊣ι : Sheafification ⊣ forget-sheaf J _
  Sheafification⊣ι = free-objects→left-adjoint (Small.make-free-sheaf J)

Note that since the category of $J -sheaves$ is defined to literally have the same $Hom -sets$ as the category of presheaves on $C,$ the action of the forgetful functor on morphisms is definitionally the identity.

  Sheafification-is-reflective : is-reflective Sheafification⊣ι
  Sheafification-is-reflective = id-equiv

  Sheafification-is-monadic : is-monadic Sheafification⊣ι
  Sheafification-is-monadic = is-reflective→is-monadic _ id-equiv

Limits and colimits🔗

By general properties of reflective subcategories, we have that the category of sheaves on a site is complete and cocomplete; Completeness is by an equivalence with the Eilenberg-Moore category of the sheafification monad (which has all limits which $PSh (C)$ does), and cocompleteness follows by computing the colimit in presheaves, then sheafifying the result.

  Sh[]-is-complete : is-complete ℓ ℓ Sh[ C , J ]
  Sh[]-is-complete = equivalence→complete
    (is-equivalence.inverse-equivalence Sheafification-is-monadic)
    (Eilenberg-Moore-is-complete _
      (Functor-cat-is-complete (Sets-is-complete {ι = ℓ} {ℓ} {ℓ})))

  Sh[]-is-cocomplete : is-cocomplete ℓ ℓ Sh[ C , J ]
  Sh[]-is-cocomplete F = done where
    psh-colim : Colimit (forget-sheaf J _ F∘ F)
    psh-colim = Functor-cat-is-cocomplete (Sets-is-cocomplete {ι = ℓ} {ℓ} {ℓ}) _

    sheafified : Colimit ((Sheafification F∘ forget-sheaf J _) F∘ F)
    sheafified = subst Colimit F∘-assoc $
      left-adjoint-colimit Sheafification⊣ι psh-colim

    done = natural-iso→colimit
      (F∘-iso-id-l (is-reflective→counit-iso Sheafification⊣ι id-equiv))
      sheafified

Cartesian closure🔗

Since sheaves are an exponential ideal in presheaves, meaning that $B^{A}$ is a sheaf whenever $B$ is, we can conclude that the category of sheaves on a site is also Cartesian closed.

  Sh[]-cc : Cartesian-closed Sh[ C , J ] (Sh[]-products J) (Sh[]-terminal J)
  Sh[]-cc .has-exp (A , _) (B , bshf) = exp where
    exp' = PSh-closed C .has-exp A B

    exp : Exponential Sh[ C , J ] _ _ _ _
    exp .B^A .fst = exp' .B^A
    exp .B^A .snd = is-sheaf-exponential J A B bshf
    exp .ev       = exp' .ev
    exp .has-is-exp .ƛ        = exp' .ƛ
    exp .has-is-exp .commutes = exp' .commutes
    exp .has-is-exp .unique   = exp' .unique