Codiscrete categories

We define Codiscrete A as an alias for the type A , and use this type alias to provide a Category instance whose Hom type are Unit types.

Codiscrete.functor promotes a function f : C → A (for any category C) to a functor f : C ⥤ Codiscrete A.

Similarly, Codiscrete.natTrans and Codiscrete.natIso promote I-indexed families of morphisms, or I-indexed families of isomorphisms to natural transformations or natural isomorphism.

We define functorToCat : Type u ⥤ Cat.{0,u} which sends a type to the codiscrete category and show it is right adjoint to Cat.objects.

structure CategoryTheory.Codiscrete (α : Type u) : Type u

A wrapper for promoting any type to a category, with a unique morphisms between any two objects of the category.

• as : α

  A wrapper for promoting any type to a category, with a unique morphisms between any two objects of the category.

theorem CategoryTheory.Codiscrete.ext_iff {α : Type u} {x y : Codiscrete α} : x = y ↔ x.as = y.as

theorem CategoryTheory.Codiscrete.ext {α : Type u} {x y : Codiscrete α} (as : x.as = y.as) : x = y

@[simp]

theorem CategoryTheory.Codiscrete.mk_as {α : Type u} (X : Codiscrete α) : { as := X.as } = X

def CategoryTheory.codiscreteEquiv {α : Type u} : Codiscrete α ≃ α

Codiscrete α is equivalent to the original type α.

@[simp]

theorem CategoryTheory.codiscreteEquiv_apply {α : Type u} (self : Codiscrete α) : codiscreteEquiv self = self.as

@[simp]

theorem CategoryTheory.codiscreteEquiv_symm_apply_as {α : Type u} (as : α) : (codiscreteEquiv.symm as).as = as

instance CategoryTheory.instDecidableEqCodiscrete {α : Type u} [DecidableEq α] : DecidableEq (Codiscrete α)

instance CategoryTheory.Codiscrete.instCategory (A : Type u_1) : Category.{0, u_1} (Codiscrete A)

def CategoryTheory.Codiscrete.functor {C : Type u} [Category.{v, u} C] {A : Type w} (F : C → A) : Functor C (Codiscrete A)

Any function C → A lifts to a functor C ⥤ Codiscrete A.

def CategoryTheory.Codiscrete.invFunctor {C : Type u} [Category.{v, u} C] {A : Type w} (F : Functor C (Codiscrete A)) : C → A

The underlying function C → A of a functor C ⥤ Codiscrete A.

def CategoryTheory.Codiscrete.natTrans {C : Type u} [Category.{v, u} C] {A : Type w} {F G : Functor C (Codiscrete A)} : F ⟶ G

Given two functors to a codiscrete category, there is a trivial natural transformation.

def CategoryTheory.Codiscrete.natIso {C : Type u} [Category.{v, u} C] {A : Type w} {F G : Functor C (Codiscrete A)} : F ≅ G

Given two functors into a codiscrete category, the trivial natural transformation is an natural isomorphism.

def CategoryTheory.Codiscrete.natIsoFunctor {C : Type u} [Category.{v, u} C] {A : Type w} {F : Functor C (Codiscrete A)} : F ≅ functor (as ∘ F.obj)

Every functor F to a codiscrete category is naturally isomorphic {(actually, equal)} to Codiscrete.as ∘ F.obj.

@[simp]

theorem CategoryTheory.Codiscrete.natIsoFunctor_hom_app {C : Type u} [Category.{v, u} C] {A : Type w} {F : Functor C (Codiscrete A)} (X : C) : natIsoFunctor.hom.app X = CategoryStruct.id (F.obj X)

@[simp]

theorem CategoryTheory.Codiscrete.natIsoFunctor_inv_app {C : Type u} [Category.{v, u} C] {A : Type w} {F : Functor C (Codiscrete A)} (X : C) : natIsoFunctor.inv.app X = CategoryStruct.id (F.obj X)

def CategoryTheory.Codiscrete.functorOfFun {A : Type u_1} {B : Type u_2} (f : A → B) : Functor (Codiscrete A) (Codiscrete B)

A function induces a functor between codiscrete categories.

def CategoryTheory.Codiscrete.oppositeEquivalence (A : Type u_1) : (Codiscrete A)ᵒᵖ ≌ Codiscrete A

A codiscrete category is equivalent to its opposite category.

def CategoryTheory.Codiscrete.functorToCat : Functor (Type u) Cat

Codiscrete.functorToCat turns a type into a codiscrete category.

def CategoryTheory.Codiscrete.equivFunctorToCodiscrete {C : Type u} [Category.{v, u} C] {A : Type w} : (C → A) ≃ Functor C (Codiscrete A)

For a category C and type A, there is an equivalence between functions objects.obj C ⟶ A and functors C ⥤ Codiscrete A.

def CategoryTheory.Codiscrete.adj : Cat.objects ⊣ functorToCat

The functor that turns a type into a codiscrete category is right adjoint to the objects functor.

def CategoryTheory.Codiscrete.unitApp (C : Type u) [Category.{v, u} C] : Functor C (Codiscrete C)

Components of the unit of the adjunction Cat.objects ⊣ Codiscrete.functorToCat.

def CategoryTheory.Codiscrete.counitApp (A : Type u) : Codiscrete A → A

Components of the counit of the adjunction Cat.objects ⊣ Codiscrete.functorToCat

theorem CategoryTheory.Codiscrete.adj_unit_app (X : Cat) : adj.unit.app X = unitApp ↑X

theorem CategoryTheory.Codiscrete.adj_counit_app (A : Type u) : adj.counit.app A = counitApp A

theorem CategoryTheory.Codiscrete.left_triangle_components (C : Type u) [Category.{v, u} C] : counitApp C ∘ (unitApp C).obj = id

Left triangle equality of the adjunction Cat.objects ⊣ Codiscrete.functorToCat, as a universe polymorphic statement.

theorem CategoryTheory.Codiscrete.right_triangle_components (X : Type u) : (unitApp (Codiscrete X)).comp (functorOfFun (counitApp X)) = Functor.id (Codiscrete X)

Right triangle equality of the adjunction Cat.objects ⊣ Codiscrete.functorToCat, stated using a composition of functors.